Giant AGN clouds

The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black-hole accretion events


Some active galactic nuclei (AGN) are surrounded by extended emission-line regions (EELRs), which trace both the illumination pattern of escaping radiation and its history over the light-travel time from the AGN to the gas. From a new set of such EELRs, we present evidence that the AGN in many Seyfert galaxies undergo luminous episodes 0.2–2 years in duration. Motivated by the discovery of the spectacular nebula known as Hanny’s Voorwerp, ionized by a powerful AGN which has apparently faded dramatically within years, Galaxy Zoo volunteers have carried out both targeted and serendipitous searches for similar emission-line clouds around low-redshift galaxies. We present the resulting list of candidates and describe spectroscopy identifying 19 galaxies with AGN-ionized regions at projected radii kpc. This search recovered known EELRs (such as Mkn 78, Mkn 266, and NGC 5252) and identified additional previously unknown cases, one with detected emission to kpc. One new Sy 2 was identified. At least 14/19 are in interacting or merging systems, suggesting that tidal tails are a prime source of distant gas out of the galaxy plane to be ionized by an AGN. We see a mix of one- and two-sided structures, with observed cone angles from 23–112. We consider the energy balance in the ionized clouds, with lower and upper bounds on ionizing luminosity from recombination and ionization-parameter arguments, and estimate the luminosity of the core from the far-infrared data. The implied ratio of ionizing radiation seen by the clouds to that emitted by the nucleus, on the assumption of a nonvariable nuclear source, ranges from 0.02 to ; 7/19 exceed unity. Small values fit well with a heavily obscured AGN in which only a small fraction of the ionizing output escapes to be traced by surrounding gas. However, large values may require that the AGN has faded over tens of thousands of years, giving us several examples of systems in which such dramatic long-period variation has occurred; this is the only current technique for addressing these timescales in AGN history. The relative numbers of faded and non-faded objects we infer, and the projected extents of the ionized regions, give our estimate (0.2–2 years ) for the length of individual bright phases.

galaxies: Seyfert — galaxies: ISM — galaxies: active

1 Introduction

The compact sizes of the central engines of active galactic nuclei (AGN) have long driven study of their distant surroundings for clues to their geometry and interaction with the surrounding galaxy. Observations of gas seen many kpc from the AGN itself have proven fruitful in offering views of the core from different angles, and implicitly at different times.

Narrowband images revealed extended emission-line regions (EELRs) around some luminous AGN, particularly radio-loud QSOs as well as radio galaxies, as reviewed by Stockton, Fu, & Canalizo (2006). Similar structures in lower-luminosity Seyfert galaxies often appear as single or double triangles in projection (Unger et al. 1987, Tadhunter & Tsvetanov 1989), generally interpreted as ionization cones. When small-scale radio jets are present, they lie within the ionization cones. However, in many cases, the gas must be ionized by radiation from the nucleus rather than direct interaction with a jet or outflow, as seen from narrow linewidths and (particularly diagnostic) modest electron temperatures, both of which would be much larger in the presence of shocks fast enough to match the observed ionization levels. This is particularly true for very large EELRs, where interaction with the radio jet or an origin in outflows alone become less and less likely. In fact, the best-defined ionization cones are seen in radio-quiet objects (Wilson, 1996).

This is one line of evidence linking large-scale structures to the small-scale obscuring regions (“tori”) implied by other arguments for a unification scheme (Antonucci, 1993), in which Seyferts of types 1 and 2 are part of a single parent population, appearing different based on how our line of sight passes this torus. The emission-line structures can be large and well-resolved, offering a way to measure the opening angle over which ionizing radiation escapes. Some previous studies have also noted that these emission-line clouds provide a view to the immediate past of the AGN, via light-travel time to the cloud and then toward us (Dadina et al., 2010).

Using extended emission-line clouds as probes of AGN history came of age with the discovery of Hanny’s Voorwerp, a high-ionization region extending 45 kpc in projection from the low-ionization nuclear emission-line region (LINER) galaxy IC 2497 at (Lintott et al., 2009). Linewidths and electron temperature indicate that the gas is photoionized rather than shock-excited, while a combination of ionization-parameter and recombination arguments bound the required nuclear ionizing luminosity to be erg s. However, X-ray spectroscopy shows the nucleus of IC 2497 to be only modestly absorbed, with ionizing luminosity only erg s (Schawinski et al., 2010a). It is difficult to avoid the conclusion that the nucleus of IC 2497 was in fact a QSO (the nearest known luminous QSO) until roughly years before our current view, and has faded dramatically in the interim; radio and HST observations offer hints that some of its energy output may have switched to kinetic forms over this timespan (Josza et al. 2009, Rampadarath et al. 2010, Schawinski et al. 2010a, Keel et al. in preparation). The unlikeliness of the nearest QSO showing highly unusual behaviour suggests that such variations may be common among AGN, prompting us to re-examine the incidence and properties of extended ionized clouds around nearby AGN. Such an examination should not be confined to catalogued AGN, since the most interesting objects - those which have faded dramatically - may no longer appear as spectroscopically classified AGN.

Hanny’s Voorwerp was first noted by Dutch teacher Hanny van Arkel in the course of the Galaxy Zoo project (Lintott et al., 2008), on the basis of its unusual structure and colour. In view of the interest of similar ionized clouds for study of both the history and obscuration of AGN, participants in the Galaxy Zoo project have carried out a wide search for such clouds using data from the Sloan Digital Sky Survey (SDSS). They examined both known AGN hosts and galaxies not known to have AGN, using the distinctive colour of highly-ionized regions across the SDSS filters as a first selection criterion. We present the results of further analysis of the SDSS images, narrow-band imaging, and spectroscopy, yielding a list of 19 galaxies with AGN-photoionized clouds detected to beyond 10 kpc from the nuclei (many of which are newly identified). We consider constraints on changes in ionizing luminosity for these, and identify several as the most likely candidates for the kind of long-term fading seen in IC 2497 and Hanny’s Voorwerp.

2 Searches for emission-line clouds

The Galaxy Zoo search for giant AGN-ionized clouds combined both targeted and serendipitous approaches, to combine a complete examination of known AGN hosts with the possibility of finding ionized clouds around AGN which are yet unknown or in fact optically unseen. In the targeted search, we formed a sample of potential AGN at . This combined all galaxies whose SDSS pipeline emission-line ratios put them in either the AGN or composite regions of the Baldwin-Phillips-Terlevich (BPT) diagram (Baldwin, Phillips, & Terlevich 1981, as revised by Kewley et al. 2001 and Kauffmann et al. 2003) using [O III]/H and [N II]/H, and all additional objects listed in the Veron-Cetty & Veron catalog (Veron-Cetty & Veron, 2010) at falling within the SDSS data release 7 (DR7) area. This addition accounted for AGN with no SDSS nuclear spectrum, either because they are relatively bright or, more often, because fibre collisions or sampling rules prevented their selection for spectra, and type 1 AGN where the pipeline spectroscopic classification is less reliable than for narrow-line objects. The merged AGN sample, designed to err on the side of inclusion in borderline cases, included 18,116 objects. With a web interface designed by RP, 199 participants examined all of these within a 6-week period in 2009, marking each as certain, possible, or lacking an extended emission region. These emission regions have distinctive signatures in both morphology and colour from the SDSS data. They do not follow the usual spiral or annular distributions of star formation in disc galaxies. Such regions show unusual colours in the SDSS composite images, which map bands to blue, green, and red (Lupton et al. 2004). Hence strong [O III] at low redshift is rendered as a pure blue, as in the discovery of Hanny’s Voorwerp. A combination of strong [O III] and significant H+[N II] appears purple; beyond about , [O III] falls in the gap between and filters, so our search technique loses utility until [O III] is well within the band, when the galaxies have much smaller angular sizes. This subproject was known as the “voorwerpje hunt”, using the Dutch diminutive form of Voorwerp.

Each galaxy was examined by at least ten participants; 199 Zoo volunteers participated in this program, seven of whom examined the entire sample. The final average number of votes was 11.2 per object. After this screening process, a straightforward ranking was by relative numbers of “yes” (weight=1), “maybe” (weight=0.5), and “no” (weighted zero) votes.

The most interesting results of such a search would be galaxies with prominent AGN-ionized clouds in which we don’t see the AGN, either because of strong obscuration or dramatic variability during the light-travel time from the nucleus to the clouds. These would not be found by targeting known AGN, and neither would clouds around AGN which do not have catalogued spectral information. Accordingly, we also posted a request on the Galaxy Zoo discussion forum, with examples of confirmed AGN clouds and various kinds of similar-appearing image artifacts. Participants were invited to post possible cases from among the galaxies they saw in the ordinary course of the Galaxy Zoo classification programs (Lintott et al., 2008), and some active users reposted examples from other discussion threads. The resulting followups3 provided an additional sample for investigation; WCK also checked all the threads with image discussion for more such objects, early examples of which instigated this search on the first place. To reduce the number of false positives caused by extended star-forming regions or starburst winds, objects were removed from consideration if an SDSS spectrum shows emission lines characteristic of a starburst. Remaining candidates were examined first on the SDSS composite images for appropriate colour and geometry, and the most promising ones were carried forward for further analysis.

Both targeted and serendipitous lists overlap for many objects with bright emission-line structures, and recover such well-studied cases from the literature as Mkn 266, NGC 5252 and Mkn 78; we observed these so as to have a consistent set of spectra for comparison. The entire list of candidates is given in Table 1. In the Search column, S or T denotes whether the object was found in the serendipitous survey, the targeted survey of known AGN, or both. The type of nuclear optical spectrum is listed as Sy 1/1.5/1.9/2, LINER, SB for starburst, or nonAGN for an ordinary stellar population. The final column indicates which Galaxy Zoo participant (by user name) first posted objects in the serendipitous survey.

2.1 SDSS image analysis and new images

For both subsamples, further winnowing had the same steps. Most importantly, we reanalyzed the SDSS images, to verify that the features do not have continuum counterparts, and eliminate artifacts caused by imperfect registration of the images when forming the colour composites. This effect is of particular concern for Seyfert 1 nuclei, where the PSF of the bright nucleus can produce a decentered colour signature if one of the constituent images is slightly misregistered; Sy 1 galaxy image are more vulnerable to this artifact than normal galactic nuclei. Since many candidates (including some with spectroscopic confirmation) have “purple haze” on the SDSS images, which could either be genuinely extended and somewhat amorphous [O III] and H or an artifact, this was a helpful step. We adopted a tomographic approach, taking one of the SDSS bands free of strong emission lines ( or , depending on redshift) as an estimate of the structure of starlight in the galaxy. This was scaled to match the largest part of the structure, iteratively when necessary. This is illustrated in Fig. 1, isolating the emission-line loop in SDSS 1430+13 (nicknamed the Teacup AGN because of this structure). Chojnowski and Keel inspected the best subtraction among various scalings (often a compromise, due to colour gradients within the galaxy) to assess the reality of extended emission-line features not associated with clear spiral arms or stellar rings. These results let us rank the candidate lists from both targeted and serendipitous searches in order of significance of the emission-line structures based on the SDSS images themselves. We used these results to limit the number of candidates from the targeted search to the top 50; below this there were no convincing candidates based on more detailed analysis of the SDSS images.

Figure 1: Linear combination of SDSS images to isolate candidate [O III] emission regions, shown with SDSS J143029.88+133912.0 (the Teacup AGN). The region shown spans SDSS pixels, or ”, with north at the top. Each image is displayed with a logarithmic intensity mapping, with a small zero point offset to reduce the effects of noise around zero. At the redshift of this galaxy, H and [N II] emission fall redward of the filter band (response 0.2% of peak) so the image is used as a continuum estimate.

Where appropriate filters were available for [O III] or H at a galaxy’s redshift, some candidates were imaged at the remote SARA 1m (Kitt Peak) and 0.6m (Cerro Tololo) telescopes. For [O III], we used a filter centered at 5100 Å with half-transmission width 100Å , useable for the redshift range . At H, both telescopes have stepped sets of filters 75 Å apart with FWHM Å . Continuum was taken from or , appropriately scaled for subtraction to show net emission-line structures. These data are particularly helpful in tracing the emission-line structures of UGC 7342 (Fig. 2) and SDSS 2201+11 (Fig. 3).

Figure 2: The extended clouds in UGC 7342. Left, the starlight continuum in a band at 6450 Å from the SARA 1m telescope. Center, an estimated [O III] image from the SDSS data as in Fig. 1. Right, continuum-subtracted H image from the SARA 1m. North is at the top and east to the left; the field shown spans arcseconds.
Figure 3: The extended clouds in SDSS 2201+11. Left, a image from the SARA-S 0.6m telescope, showing the dusty disk. Right, continuum-subtracted [O III] image from the SARA-N 1m telescope, smoothed by a Gaussian of 2.0” FWHM. North is at the top and east to the left; the field shown spans arcseconds.

2.2 Spectroscopy

To confirm that regions are in fact ionized by AGN, and derive diagnostic emission-line properties, we carried out long-slit spectroscopy for the highest-priority candidates. Observations used the GoldCam spectrograph at the 2.1m telescope of Kitt Peak National Observatory and the Kast double spectrograph at the 3m Shane telescope of Lick Observatory. Table 5 compares the setups used for each session. The slit width was set at 2” for all these observations, and the spectrographs were rotated to sample the most extended known structures of each galaxy. Scheduling allowed us to reduce the Kitt Peak data before the first Lick observing run, so that the 3m spectra could be concentrated on the most interesting galaxies. Total exposures ranged from 30 minutes, for initial reconnaissance to see whether an object might host AGN clouds, to 2 hours for weaker lines in confirmed targets. Either night-sky line or interspersed lamp observations were used to track flexure, as needed. Reduction used the longslit package in IRAF4 (Tody, 1986), and included rebinning to a linear wavelength scale, sky subtraction, and flux calibration. Spectrophotometric standard stars were observed to set the flux scale; in a few cases where passing clouds were an issue, the spectra were scaled so that the nucleus within a ” region matched the flux of the SDSS spectra.

Our identification of these extended regions as being photoionized by AGN rests on three results - location in the strong-line BPT diagram, strength of the high-ionization species He II and [Ne V], and electron temperature consistent with photoionization but not with shock ionization. We classify emission regions based on the “BPT” line-ratio diagrams pioneered for galactic nuclei by Baldwin, Phillips, & Terlevich (1981) and refined by Veilleux & Osterbrock (1987), with caution based on the possibility that some of the external gas could have much lower metal abundances than found in galactic nuclei (as seen in Hanny’s Voorwerp; Lintott et al. 2009). Abundance effects in gas photoionized by AGN, as manifested in the BPT diagrams, have been considered in calculations by Bennert et al. (2006a). The largest effect is higher equilibrium temperature at lower O abundance, since it is an important coolant, which drives stronger forbidden lines and higher ionization levels until very low levels (0.1 solar) are reached. In any case, the abundance changes are not large enough to move these clouds across the empirical AGN/starburst ionization boundary. Furthermore, in the galaxies where we have data covering the red emission lines, the clouds’ locations in the (essentially abundance-independent) auxiliary BPT diagram of [O III]/H versus [O I]/H also indicated photoionization by an AGN continuum. The various BPT diagrams are compared for Points along the slit in each of the clouds we classify as AGN-ionized in Fig. 4. This classification is examined more closely in the context of its radial behavior in the next section.

Figure 4: Summary Baldwin-Phillips-Terlevich (BPT) diagrams for the Lick spectra, where we measured the requisite red emission lines. Circles indicate points along the slit for extended clouds classified as AGN-ionized and the host nuclei. Gray-scale background shows the density of points from all low-redshift SDSS galactic nuclei, as in Schawinski et al. (2010b). The dividing lines between regions photoionized by AGN and by hot stars are shown as given by Kewley et al. (2001; Ke01) and by Kauffmann et al. (2003; Ka03).

Independent of these line ratios, strong He II or [Ne V] indicate photoionization by a harder continuum than provided by young stars, and resolved emission from these species is immediately diagnostic of AGN photoionization in this context. For some objects, we do not have red data; in these, we classify the cloud as AGN-ionized based on the presence of the high-ionization lines or continuity of line ratios with the nucleus. Line ratios in the extreme blue may be affected in subtle ways by atmospheric dispersion (Filippenko, 1982); the scheduling of our observations forced us to observe most targets at hour angles which did not allow us to put the slit simultaneously along the structures of interest and close to the parallactic angle. The extended regions we observe are generally wider than the slit; to first order line intensities are not affected by atmospheric refraction, since we calibrate with standard stars at low airmass. Some of the Lick blue spectra have atmospheric dispersion contributing as much as 3” of offset along the slit from red to blue ends of the spectrum, important only for the nuclei and corrected in extracting their spectra.

The BPT diagrams are designed to separate common sources of photoionization in galaxies; temperature and kinematic data are also important to understand whether shocks pay a significant role. In a few cases, the [O III] line was measured in the extended clouds with sufficient signal-to-noise ratio for a measurement of the electron temperature via its ratio to the strong lines. Using the IRAF task temden, which implements the algorithm of Shaw & Dufour (2007), and considering cm, we find values of in the SDSS 2201+11, for Mkn 266, and for the Teacup system. These confirm that the gas is photoionized rather than shocked; for comparison, temperatures in the [O III] zone of supernova remnants (including some with lower ionization levels than in these clouds) range from 20,000-69,000 K (e.g., Fesen et al. 1982, Wallerstein & Balick 1990, Morse et al. 1995). In addition, very high shock velocities km s are needed to produce significant [Ne V] emission Dopita & Sutherland (1996). This is far in excess of the local velocity ranges we observe (section 6); even though we would not necessarily observe material on both sides of a shock in the same ion, it is difficult to envision a situation with large-scale shocks of this velocity without observable velocity widths or structures exceeding 100 k s.

While not the main thrust of our survey, it is worth noting that we find a few instances of either double AGN in interacting systems, or AGN in the fainter member of a close pair (Mkn 177, Was 49, possibly SDSS J111100.60-005334.9 and SDSS J142522.28+141126.5). These may be worth deeper spectroscopy in the context of mapping AGN obscuration; if a high-ionization component can be isolated in the gas of the other galaxy, its distribution could show where ionizing radiation escapes any circumnuclear absorbing structure. This offers a distinct way of tracing the ionizing radiation even in the absence of extensive gaseous tidal features, in an approach that has been discussed for Was 49ab by Moran et al. (1992).

Table 6 lists the results of our spectroscopy. Confirmed, resolved clouds ionized by the AGN are separated from other results (unresolved AGN emission, extended star-forming regions denoted as H II, and so on). The instruments used are denoted by GCam (Kitt Peak GoldCam) and Lick (Lick 3m with Kast spectrograph). New redshifts and spectral classifications are marked with asterisks. We separate the AGN clouds of most interest based on the detected extent of [O III] ; our spectra have a lower detection threshold than our images for this, roughly erg cm s arcsec for emission regions a few arcseconds in size. Spectra of the nuclei and representative cloud regions are shown in Figs. 5 and 7. Table 7 lists emission-line ratios and selected fluxes for the same regions plotted in these figures. Fluxes are given both for [O III] and H, since these were usually measure with different gratings and detectors. For some of the nuclei, correction of the H flux for underlying absorption in the stellar population was significant; we have applied an approximate correction based on typical values for synthetic stellar populations from Keel (1983).

Figure 5: Lick spectra for nuclei and associated AGN-ionized clouds. Small insets at left show the H+[O III] region for nuclei and clouds, scaled to the peak of emission. Panels on the right show the [O I] - [S II] region at the same flux scale as the blue spectra. He II and [Ne V] emission, especially important as indicators of a hard ionizing radiation field, are indicated by red dotted lines when clearly detected in clouds. Nuclear spectra represent -arcsecond areas, and cloud spectra are summed over -arcsecond areas. Distances and directions of cloud relative to nuclei are indicated as shown. Three spectra have gaps in the blue region, since they were taken with the dichroic splitting red and blue optical trains near 4600 Å .
Figure 6: continued
Figure 7: KPNO GoldCam spectra sampling nuclei and associated AGN-ionized clouds. Spectra are scaled to show [O III] , with nuclear spectra representing -arcsecond areas and cloud spectra representing -arcsecond areas. He II and [Ne V] emission, especially important as indicators of a hard ionizing radiation field, are indicated by red dotted lines when clearly detected in clouds. Distances and directions of cloud relative to nuclei are indicated as shown.

The upper part in Table 6, with AGN-ionized gas detected more than 10 kpc from the nucleus, forms the sample for our subsequent analysis. As a sign of completeness, of these 19, 14 were found in both targeted and serendipitous searches. SDSS J095559.88+395446.9 was newly identified as a type 2 Seyfert in our spectrum, after having been found in the “blind” search of galaxies independent of prior classification as an AGN host (so it was not included in the targeted sample). Of the remainder, Mkn 78 and Mkn 463 were selected in the targeted AGN sample, while Mkn 1498 and UGC 11185 were recognized only in the serendipitous survey. It may be relevant that both Mkn 78 and Mkn 463 have ionized regions with relatively small projected extent, easily lost against the galaxy starlight (which in Mkn 463 is morphologically complex).

The [S II] lines are particularly important, tracing electron densities and thereby providing one estimate of the intensity of the impinging ionizing radiation. Since the densities in these extended clouds are low, and the ratio is generally near its low-density limit, where the mapping from line ratio to density is highly nonlinear, we have examined the errors in measuring the line ratio closely. We generated multiple realizations of pixel-to-pixel noise, and each was scaled to four representative fractions of the stronger line peak. This was added to line pairs, modeled to match the line widths and pixel separation of the red Lick data. Gaussian fitting of the lines with added noise gave a relation between the peak signal-to-noise and error of the fitted ratio which we adopted; we use error bounds to derive bounds on the density. Density values were calculated using the IRAF task temden.

3 Energy budget in extended clouds: obscuration versus variability

Seeing the effects of radiation from an AGN on gas tens of kpc from the nucleus allows us the possibility of tracing dramatic changes in core luminosity. One straightforward way to approach this question is a simple energy balance. The spectra give us upper and lower bounds on the required ionizing luminosity. To probe the most extreme conditions, we analyze galaxies in which we detect ionized gas at projected distances kpc. For all distances and luminosities, we use the WMAP “consensus” cosmological parameters, with H km s Mpc (Spergel et al., 2007).

The lower bound comes from the highest recombination-line surface brightness we observe; the central source must provide at least enough ionizing photons to sustain this over periods longer than the recombination timescale (which may be as long as years at these low densities). This is a lower limit, since the actual emission-line surface brightness of some regions may be smeared out by seeing, and we do not know that a given feature is optically thick at the Lyman limit. This limit depends only very weakly on the slope of the ionizing continuum, since helium will generally absorb most of the radiation shortward of its ionization edge leaving only the 13.6-54.4 eV range to consider for hydrogen ionization. We base our bounds on the highest implied luminosity among structures at various projected radii in a given system, with no correction for projection effects. This makes our limits conservative, since a given cloud will always lie farther from the nucleus than our projected measurement. In essence, this argument is based on the surface brightness in a recombination line; we use H since we have these data for the whole sample. In a simple approximation, we take the surface brightness in the brightest portion of a cloud, assuming this to be constant across the slit. We take the region sampled in this way to be circular in cross-section as seen from the nucleus, so its solid angle is derived from the region subtended by the slit. We then see this region occupying a small angle as seen projected at angular distance from the AGN, the required ionizing luminosity is given from observed quantities as for in degrees. The derived values are listed in Table 8, along with complementary quantities related to the nuclear luminosity (as collected below). The derived ionizing luminosities are lower limits, since there may be unresolved regions of higher surface brightness, and we do not know whether a given cloud is optically thick in the Lyman continuum. Higher-resolution imaging in the emission lines could this increase these values.

Upper limits to the incident ionizing flux come from a complementary analysis using the ionization parameter (, the ratio of ionizing photons to particles), since these emission-line features all have [S II] line ratios near the low-density limit. Our density results from the [S II] line ratio are given in Table 9. Values are listed only for objects with useful measures far from the core. In each case, we evaluated the density at a typical temperature of K, and at the higher temperature K found in Hanny’s Voorwerp (Lintott et al., 2009) and in our data for Mkn 266 and SDSS 2201+11, where the higher temperature is set by thermal equilibrium for substantially subsolar oxygen abundance. We quote the extreme range of density values between these two cases (allowing in the Teacup an upper bound on the electron density as high as 240 cm, and in some cases limits cm), since the temperature-sensitive [O III] line ratio is not well-enough measured in most of these objects to use individual values. We derive from the [O II] /[O III] ratio using the power-law continuum models from Komossa & Schulz (1997), and the analytic fits from Bennert (2005) as interpolation tools. For fully ionized hydrogen at a distance from the AGN, the photon flux in the ionizing continuum is . For objects with red spectra, giving densities from the [S II] lines, limits to the luminosity are given in Table 9. It is reassuring that the upper limits to ionizing luminosity derived from and always fall above the lower limits from recombination balance.

The lower limits from recombination-balance are independent of assumptions about the local density , making it more robust than ionization-parameter arguments when we have no independent tracer at these low densities. Fig. 8 shows several of our objects in one of the “BPT” diagrams, going beyond their initial use to classify the gas as AGN-ionized to examine changes with projected distance from the nuclei. Some of these, such as Mkn 1498 and the Teacup 1430+13, show a phenomenon remarked earlier in, for example, NGC 5252 (Dadina et al., 2010) - the ionization balance stays roughly constant with radius, which is naturally explained if the characteristic density . This might occur naturally for gas in the host galaxy; tidal streams of gas would not be likely to match the extrapolated behavior of gas within the galaxy and indeed we see some cases (Mkn 266, NGC 5972, SDSS 2201+11) with substantial radial changes in . However, for Seyfert narrow-line regions, Bennert et al. (2006b) find a shallower density gradient , which would imply for gas which is optically thin (or has a small covering fraction). These objects have heterogenous behavior; In the ionization cone of NGC 7212, Cracco et al. (2011) find no radial trend of .

Similar conclusions come from the more limited blue-line diagram also considered by Baldwin, Phillips, & Terlevich (1981), which we can apply to the objects for which we have only blue-light spectra from KPNO. Some of objects in this diagram as well as in Fig. 8 show systematic changes in ionization level with radius, manifested as offsets from upper left (higher ionization) to lower right (lower ionization). We show this behavior in Fig. 10.

Figure 8: Baldwin-Phillips-Terlevich (BPT) diagrams for spatial slices in Lick data. Each row shows the three classical BPT diagrams for each object, highlighting radial ionization behaviors. The nucleus is indicated by crosshairs, with colors changing from white to red with increasing distance from the nucleus. The greyscale background and dividing lines are the same as in Figure 4; these show only the region around the AGN loci in each case for discrimination of detail. Nearly all measurements lie firmly in the AGN domain, with possible exceptions in some regions of SDSS 1005 and SDSS 0955. The starburst/AGN ionization boundary from Kewley et al. (2001) is shown as the red full curve, while the boundary from Kauffmann et al. (2003) is the black dashed curve. All these measurements lie firmly in the AGN domain, with the possible exception of two regions in SDSS 1005 and the nucleus of Mkn 883, whose red spectrum shows a broad-line region and strong [O I]. The greyscale show the density of points representing low-redshift galactic nuclei in the SDSS, from Schawinski et al. (2010b), which we also follow in adopting the straight line as the LINER/Seyfert boundary
Figure 9: continued
Figure 10: An alternate Baldwin-Phillips-Terlevich (BPT) diagram, usable when only blue spectra are available (in this case, from the Kitt Peak GoldCam). As in Fig. 8, points are colour-coded with projected distance from the galaxy nuclei. Since Mkn 739 has a significant broad-line region, its H components were deblended using a broad Gaussian and narrow Lorentzian, with narrow-line ratios plotted here.

We use far-IR data to estimate (or limit, for nondetections) the amount of AGN radiation absorbed (and reradiated) by nearby dense material, whether in an AGN “torus” or in the inner parts of the host interstellar medium. The FIR luminosity is conservatively high as an estimate of the potential obscured AGN luminosity, since there may be a nontrivial contribution from star formation in the host galaxy as well as the AGN, and in some cases companion galaxies might blend with the target in the FIR beam. In a simple picture where a fraction of the AGN radiation is absorbed by nearby dust and reradiated, the FIR luminosity will be of order , with an additional scaling factor of a few to account for non-ionizing radiation heating the grains (which we omit at this point for the sake of a conservative calculation). For convenience, we approximate the total far-IR output by the FIR parameter introduced for Infrared Astronomical Satellite (IRAS) point-source catalog data (Fullmer & Lonsdale, 1989), a linear combination of flux values in the 60 and 100 bands which gives a reasonable approximation to the total flux between 42-122. Numerically,

for IRAS fluxes in the 60 and 100 bands given in Jy (multiplied by for a result in ergs cm s). IRAS data were supplemented, where possible, by Akari data (Murakami et al. 2007, Kawada et al. 2007, Yamamura et al. 2010) of higher accuracy. The positions of all these galaxies were covered in the IRAS survey, so we can assign typical upper limits to nondetections depending on ecliptic latitude; Akari added two additional detections not found in the IRAS data, using only quality 1 (confirmed detection) fluxes. For non-ULIRG objects ( erg s), we can reproduce the IRAS FIR parameter from Akari 90 fluxes and mean colours via

with 30% accuracy ( dex), and we use this to fill in FIR luminosities for the objects detected only by Akari. The input values and results of this energy-balance test are shown in Table 8. Within our sample, Mkn 273 and Mkn 266 are classic ultraluminous infrared galaxies (ULIRGs), with erg s. Some of the others remain undetected in both the IRAS and Akari surveys, leading to limits typically erg s. An index of whether the extended clouds can be ionized by an obscured AGN is provided by the ratio of required ionizing luminosity to FIR luminosity, tabulated in Table LABEL:tbl=-luminosity. These values are all lower limits, since the ionizing luminosity is a lower limit This ratio ranges from 0.02 to values (FIg. 11). Low values clearly represent AGN which are strongly obscured along our line of sight but not toward the EELR clouds. Large ratios indicate long-term fading of the AGN, a spectral shape strongly peaked in the ionizing UV, or very specific geometry for obscuring material, and thus indicate objects worthy of close attention.

Figure 11: Distribution of the ratio in the log. Small values are consistent with AGN heavily obscured along our line of sight but not toward the emission-line clouds, while large values indicate fading of the AGN. Shaded regions show that there is no obvious differences in the distributions among type 1 Seyferts (black), intermediate type 1.5 and 19 objets grey), and type 2 nucle (white).

Arguments for long-timescale variations in the central sources here depend on our having estimates for their total luminosity as seen directly, which could in principle fail either if their ionizing radiation were collimated by something other than obscuration, or the spectral shapes in the deep ultraviolet differ from our expectations based on the UV and X-ray behavior of familiar AGN. Collimation by relativistic beaming would not account for the combinations of opening angle and flux ratio required (as found for Hanny’s Voorwerp; Keel et al. in preparation). A spectral solution to the behavior would require an extreme-ultraviolet ’bump” dominating the ionizing flux from 13.6-54 eV by more than an order of magnitude. However, known AGN do not provide evident examples of either solution; the most straightforward interpretation of the data suggests that some of these clouds are ionized by AGN which have faded over the differential light-travel time between our views of the clouds and nuclei.

4 Nuclear and extended radio emission

To further characterize the AGN in these galaxies, we collected radio fluxes at 1.4 GHz from the National Radio Astronomy Observatory Very Large Array Sky Survey (NVSS) source catalog (Condon et al., 1998). All but two objects (SDSS 1510+07 and 1005+28) were detected above the 2.5 mJy survey limit; the source luminosity L(1.4 GHz) ranges from W Hz to , the latter for the double source associated with NGC 5972 and comprising 94% of the galaxy’s total flux. Eight of the galaxies qualify as radio-loud if one uses a simple, representative division at W Hz, and only one lies above 10. This one - NGC 5972 - represents an interesting departure from the usual alignment of emission-line and radio structure (section 7).

5 Host and cloud morphology

The examples of Hanny’s Voorwerp (Josza et al., 2009) and NGC 5252 (Prieto & Freudlng, 1996) suggest that a common source of extraplanar gas at large radii is tidal debris. The host morphologies of the galaxies where we find extended ionized clouds support this notion. Table 10 lists morphological information on these galaxies, including warps, close companions, asymmetries, or ongoing mergers. The actual incidence of tidal structures will be higher - for example, the inclined ring of gas with distinct kinematics in NGC 5252 has no optical counterpart. Of the 19 confirmed large-scale clouds, 14 are in systems classified from SDSS data alone as interacting, merging, or postmerger (still showing tidal tails). This remarkably high incidence of disturbed systems (at least 73%, even without including NGC 5252) supports the idea that most very extended emission-line clouds around local AGN represent illuminated tidal debris. We illustrate this in Fig. 12, showing the SDSS colour images with the -band [O III] contribution enhanced to show the clouds’ locations. In this section, we include IC 2497/Hanny’s Voorwerp in the tabulations for comparison. A striking instance of a QSO ionizing gas in a companion and tidal tail, on similar scales kpc, has been reported by da Silva et al. (2011).

Figure 12: Host morphologies and ionized-gas structures in the Galaxy Zoo sample of AGN-ionized clouds. These are based SDSS colour image products, with filters mapped to BGR for display. However, to enhance visibility of the ionized clouds, the contrast of the filter has been increased in this figure, across the intensity ranges needed to show the gas effectively. Each image cutout spans kpc with north at the top. As in Table 10, IC 2497 is included for comparison.

Several of these galaxies show discs seen nearly edge-on. From these, it is clear that the ionizing radiation can emerge well away from the disc poles. The projected angles from stellar disc to the axis of the ionized clouds, when it is well defined, range from 30–54. This fits with the statistics reported by Schmitt et al. (1997), in which obscured (type 2) objects show a wider range of angle than type 1 objects between the host-galaxy axes and the AGN axes as traced by radio jets.

We see both one- and two-sided emission regions. The two-sided regions are generally highly symmetric in angular location, although not necessarily in radial extent or surface brightness, fitting with biconical illumination patterns. As listed in Table 10, 9 of 19 of our confirmed objects have emission detected on both sides of the nucleus. Particularly in very disturbed systems, a strong asymmetry may reflect the location of cold gas rather than the pattern of escaping ionizing radiation, so that we cannot necessarily conclude that the one-sided clouds are in galaxies that do not have two-sided radiation patterns

The angular width of regions of escaping radiation may constrain the geometry of obscuring regions, if the ionization is bounded by the availability of radiation rather than gas. We list, in Table 10, a cone angle, which is the projected angular width of each half of a notional bicone encompassing all the high-ionization regions seen in our images, outside of a usual nuclear emission region (Fig. 13). Projection effects make the observed angle an upper limit to the three-dimensional opening angle of a cone. The sample of large emission clouds exhibits a wide range, from 23–112. The narrower ones are challenging to understand from obscuration by a circumnuclear torus, suggesting absorbers that are geometrically quite thick compared to the opening angle for escape of ionizing radiation, to an extent which might better be described as an obscuring shell with small polar holes. However, some of these objects have dual clouds in very symmetric locations, which would be most naturally explained by such a scheme.

Figure 13: Illustration of angles listed in Table 10. The disc angle, defined only for highly inclined and reasonably symmetric host disks, is measured from the projected major axis of the disc to the midpoint of the cloud (bi)cones. The cone angle, as shown, measures the width of the paired triangles about the nucleus that encompass the extranuclear emission detected in available images. The base image is the SDSS observation of SDSS 2201+11, with the [O III] clouds shown schematically.

Several of the two-sided clouds show near-symmetry in radial extent on opposite sides of the galaxy. This could reflect episodic activity on the light-travel time scale, although front-back geometric effects would generally break an exact symmetry.

6 Kinematics of ionized gas

Extended ionized regions around AGN may commonly be separated into kinematically quiescent components, such as would be seen for disk gas ionized from the nucleus, and outflow, with additional superimposed radial motion which might be manifested in a well-sampled velocity field as misalignment of the velocity field with the galaxy morphology if the superimposed velocity components are not spectrally resoved (Barbosa et al., 2009). In addition, for disturbed systems, tidal features may show motions decoupled from the disk itself. We consider here the information on gas kinematics provided by our spectra, noting that in most cases we sample only a single position angle through each galaxy.

Redshifts were measured for each pixel along the slit using Gaussian fitting in IRAF. We show results for [O III], H, and when observed, H and [N II]. Velocity errors are based on propagation of photon statistics (Keel, 1996).

Fig. 14 shows a selection of these velocity slices, relative to the nucleus in each case. Despite the angular offset from the edge-on disk, the gas velocities in SDSS 2201+11 are continuous with the pattern in the inner rotating section, and closely symmetric. Similarly, the emission clouds in NGC 5972 fall along an extrapolation of the inner-disk rotation curve (as traced by [O III]). Despite its very disturbed morphology, UGC 11185 shows near-symmetry in kinematics, with a very strong velocity gradient crossing the nucleus.

Other systems in our sample show less ordered velocity slices. The gas in UGC 7342 at all radii has a single sense of motion on each side of the nucleus, but local departures have amplitudes up to 120 km s. A central gradient in the Teacup (SDSS 1430+13) may be reversed where the slit crosses its loop of emission. The kinematics in Mkn 883 and Mkn 739 are very disordered, as expected for a merging system. In Mkn 78, multiple components are seen in the inner few arcseconds, even in [Ne V] (Fischer et al., 2011).

The northern filament in Mkn 266 presents interesting kinematic behavior, with a large and consistent velocity offset between [O III] and H, H. This difference is seen in spectra from both spectrographs. A likely explanation is superposition of structures with quite different ionization states as well as velocity, so that the weighting of lines in our spectra, even though they are not separately resolved, gives different velocity centroids. The offset is close to 50 km s along the entire filament. Localized instances of similar mismatches between [O III] and H velocities on one side of SDSS 2201+11 and possibly in NCG 5972. Outflows are typically inferred from blue wings on [O III], but far from the nuclei where disk extinction is unlikely to be a major effect; outflows could produce relative redshifts or blueshifts. One corollary of this distinction is that there exists a gas component of much higher excitation than implied by the ionization parameter we derived from ratios of total line flux at these locations, suggesting higher ionizing luminosities in these galaxies.

Figure 14: Radial-velocity slices (shown relative to the nuclei) along our slit locations for selected hosts and AGN clouds. Nuclei are indicated by dark red circles. Lines used in calculating radial velocites from GCam and Lick data sets are indicated in the plots for NGC 5972, one of several objects observed at both locations though typically at slightly different position angles. In some cases, the extended emission follows the inner rotation closely even for extraplanar gas, as in SDSS 2201+11 and NGC 5972. Others are more chaotic, as expected for mergers. Particularly for Mkn 266, some structures show significant differences between the lines, indicating that we are seeing blends of multiple components with very different line ratios. Intensity slices in the continuum and lines are plotted across the bottom to show correlations between location and velocity structure, scaled in flux to match [O III] . Velocity errors are calculated as from photon statistics following Keel (1996); in some cases, larger errors may be appropriate from such factors as blending of multiple components.
Figure 15: continued

The relatively quiescent kinematics of most of these features may indicate a contrast in origin of the extended gas when compared with the radio-loud QSOs (Fu & Stockton 2009a, Fu & Stockton 2009b). In their sample, modest star-formation rates led them to suggest that much of the ionized gas was expelled from the system by the launch of powerful radio jets. The galaxies in our EELR sample are mostly radio-quiet (or at least radio-weak), as noted above. Also unlike their QSO sample, we see significant metallicity differences between the nuclei and EELRs, most strongly shown in the [N II]/H ratio, again consistent with the EELR gas having an external tidal origin.

7 Noteworthy systems

From our results or previously reported data, several of these galaxies have interesting individual features.

The inner parts of the EELR in Mkn 78 have long been known (Pedlar et al. 1989, who detected much of the [O III] extent we observe), and observed with HST in both imaging and spectroscopic modes (Capetti et al. 1994, Whittle & Wilson 2004). A detailed fit to the HST radial velocities explains the double line profiles near the core without requiring a second AGN (Fischer et al., 2011); optical and near-IR line spectra suggest that the gas is photoionized from the nucleus with at most a very localized role for excitation by interaction with small-scale radio jets (Whittle et al. 2005, Ramos Almeida et al. 2006). Our data also show the complex spatial and velocity structure in the inner few arcseconds. The outer emission is spatially smooth, and is measured to much larger radii in our spectra than in the initial SDSS imaging detection.

In Mkn 883, the blue line ratios indicate that it lies near the Seyfert/starburst boundary. Only in the red do weak broad H and [O I] definitely indicate an AGN. We do not detect a broad component at H.

For NGC 4388, the SDSS images detect only a few inner knots of the extensive emission region revealed by, for example, Subaru imaging Yoshida et al. (2002). Our spectra detect more of this structure. Our cone angle is estimated from the Subaru image. Detailed spectroscopy by Yoshida et al. (2004) confirms that this distant gas is photoionized by the AGN.

NGC 5252 has been considered the archetypal Seyfert galaxy with ionization cones. The implied energetics of the nucleus depend critically on the density in the ionized filaments. Our implied limits from photoionization balance via surface brightness in H are significantly greater than the values suggested from pressure balance with the galaxy’s hot ISM (Dadina et al., 2010), while we concur with the X-ray results that the ionization parameter remains roughly constant with radius among the ionized features. In turn, the interpretation of this behavior depends on fine structure (much still unresolved) in the emission-line filaments, as seen in Hanny’s Voorwerp (Keel et al., 2011).

Two objects in this sample appear to violate the usual pattern of ionization cones encompassing radio-source axes. NGC 5972 is the most radio-luminous of our galaxies, and shows a typical double-lobed structure Condon & Broderick (1988). The lobes are separated by 9.4’ (330 kpc) in projection, and are oriented near PA , quite different from the optical emission at PA . In this source, the most radio-powerful in our sample, the very different geometries of the line and radio emission make ionization from interaction with the radio plasma unlikely, and their near-perpendicular orientation is unlike the typical case for Seyfert galaxies (Wilson & Tsvetanov, 1994). This could be explained if the ionization cones have extremely broad opening angles, or if the radio structure makes a dramatic and yet-unobserved twist on small scales. Similarly, Mkn 1498 is associated with a giant low-frequency double radio source Röttgering et al. (1996), with projected separation 1.1 Mpc. In this case as well, the orientations of the emission-line structures and the large radio source differ strongly, by about .

SDSS 1430+13, the “Teacup” AGN, is distinguished by a 5-kpc loop of ionized gas. The Faint Images of the Radio Sky at Twenty-one centimeters (FIRST) Very Large Array data at 20 cm show extended structure roughly coextensive with this feature, possibly indicating a related origin.

In both SDSS 2201+11 and SDSS 1111-00 (the latter observed spectroscopically but with emission smaller than our 10-kpc limit), the extranuclear clouds outshine the AGN itself in the [O III] lines.

UGC 11185 shows a second, weaker set of emission-line components near the nucleus, peaking about 1.8” to the east along our slit, roughly 600 km s to the red of the main peaks, and including about 1/4 of the nuclear [O III] flux within a ” aperture.

In both Mkn 463 and Mkn 739, Chandra imaging has furnished evidence for double AGN (Koss et al. 2011, Bianchi et al. 2008). In both cases, the emission regions are much larger than the separation between AGN components, so we do not know whether the ionization is associated mostly with one or the other. More detailed [O III] images could resolve this. However the flux sources are apportioned between components, one AGN in each system must have an ionizing/FIR ratio at least as high as our tabulated limit. Several earlier studies have noted the extended [O III] emission around Mkn 463 (Mazzarella et al. 1991, Uomoto et al. 1993, Chatzichristou & Vanderriest 1995).

Wu et al. (2011) summarize polarimetric detections of “hidden” broad-line regions in nearby AGN. Their list includes four of the nuclei in our sample: NGC 4388, NGC 5252, Mkn 78, and Mkn 463. Broad wings to the Balmer lines are seen in Mkn 266 (southwestern nucleus) and Mkn 739 (eastern nucleus), making them clear Sy 1 nuclei with “non-hidden” broad-line regions. Weak wings are seen at H in Mkn 1498, which would then be classified as a type 1.8 object (Osterbrock, 1977).

8 Conclusions

Volunteers in the Galaxy Zoo project have carried out a search for AGN-ionized gas clouds on large scales (10-40 kpc). This paper has documented the search, and spectroscopic observations of candidates yielding 19 such features. These clouds were classified as AGN-photoionized based on their locations in the Baldwin-Phillips-Terlevich (BPT) line-ratio diagrams, strength of [Ne V] and He II emission, and (when measurements are sufficient) modest electron temperatures K, consistent with photoionization but not with shock heating. Most of the host galaxies show signs of interaction, suggesting that the extended ionized gas in many cases rises from tidal tails.

We consider upper and lower bounds to the luminosity of the AGN as it reached the clouds - lower limits from recombination and upper limits from density and ionization parameter. We compare these with the obscured luminosity estimated from far-infrared measurements; an excess in ionizing luminosity (or deficit in the far-IR) could signal long-term variability of the AGN. The ratio of ionizing to obscured luminosity spans a wide range, from 0.02 to . Over a third of them (7/19) exceed unity, making this kind of energy deficit a common issue. Small values fit with an origin in obscured AGN, requiring only a small fraction of the extreme ultraviolet to escape. In contrast, large values may require a long-term fading of the AGN. An extreme case of this is represented by Hanny’s Voorwerp near IC 2497 (Lintott et al., 2009). In this object, the required ionizing flux indicates that the AGN has faded by a factor within the last years, sampling a timescale on which we otherwise have no information. More detailed observations of this new sample, including pending X-ray measurements, could give statistics adequate to show how common such variations are.

An important use of this sample is in addressing the history of AGN luminosity - on what timescales do episodes of high luminosity persist and fade? Broad arguments suggest that AGN episodes extend over spans comparable to the duration of a galaxy merger (several 10 years), if statistics associating excess AGN with strong interactions and mergers are representative. We note that establishing a link between galaxy interactions and AGN episodes has proven remarkably elusive, with the results depending on details of comparison sample selection and what kind of AGN is studied; as recent examples, Maia et al. (2003), Alonso et al. (2007), and Li et al. (2008) reached different conclusions - a null result, enhancement limited to certain kinds of AGN, or a weak overall enhancement of AGN - from similar analysis of nearby galaxy samples. Therefore, even within such long timespans, we have little information on how episodic the accretion and associated luminous output might be.

The relevant equation for time delay between radiation reaching us directly from the nucleus and that reprocessed in a cloud follow usage for light echos in ordinary reflection, except that here we are constrained by the location of gas so we deal with a constant observed radius and unknown angle between the illumination direction and the plane of the sky; and the long recombination times at low density impose a convolution with a nontrivial time span for response. With the geometry defined in Fig. 16, keeping fixed by the observations means that the geometrical time delay for observing reprocessed nuclear radiation depends on the the viewing angle (from observer to nucleus to the cloud, with a cloud along the line of sight at zero and increasing away from the observer) as given by


derived in the approximation of infinite distance from the observer. Two-sided symmetric sets of clouds have progressively much longer differential delays when seen with their axis near the line of sight, so that a faded source in this regime should eventually be seen ionizing only the farside cloud. Our ability to reconstruct the actual distribution of is hampered by an inner cutoff in (10 kpc, so that the cloud detection is not hampered by galaxy starlight) and lack of knowledge of the distribution of cloud extent from the nuclei. To be conservative, our calculations of ionizing luminosity (above) assume , the minimum possible distance for the nucleus and thus minimal ionizing luminosity,

Figure 16: Behavior of the differential time delay between radiation seen directly from the nucleus and reprocessed through a cloud at distance from the nucleus, as a function of angle from the line of sight, for fixed value of projected radius . The inset diagram shows the geometric quantities. Units of are

We might expect our sample to be complete at least for the lowest redshifts and highest surface brightnesses, but there are a few objects with selection priority as high as some of our cloud hosts for which we do not yet have confirming optical spectra. Of our 19 confirmations, 14 were found in both the targeted and serendipitous searches. Two were found only in the targeted search, and 3 in the serendipitous search. Of these 3, one (SDSS J095559.88+395446.9) had no previous optical spectrum and could not have been included in the targeted sample.

A first hint as to characteristic timescales comes for the relative numbers of galaxies with and without deficits in ionizing luminosity, since the ones with deficits in the energy budget would be seen during the appropriate delay time after fading of the nucleus. There is no obvious reason for this ratio to be biased in our sample, since the serendipitous survey was independent of the presence of an AGN, and even in the targeted search there are many AGN which are too weak to ionize the extended gas; in essence, given a luminous AGN, our selection is for objects with outlying gas available to be ionized. In a toy model where all objects’ delay times are equal, the timescale for the AGN to be at high luminosity before fading would be of order . From Table 8, our estimate is when we divide the bright and faded groups at an ionizing/FIR ratio of 1.5. The projected extent of the clouds from our [O III] data is listed in table 10; for the 19 galaxies in our sample, the mean value is 19 kpc with a median of 17. For a typical projected extent 20 kpc, this becomes a range 25,000–175,000 years, taking the sample to populate values of at this small sample size. For the luminosity range of Seyfert galaxies we have probed, the fading may be an order of magnitude in ionizing luminosity, but this sample includes no cases in which we see AGN-ionized clouds around a galaxy with no optical trace of an AGN. IC 2497 (Lintott et al., 2009) must be extreme in this respect, having faded from a QSO to a borderline LINER./Sy 2 nucleus. As noted by Schawinski et al. (2010a), these timescales are rapid compared to expectations from scaling up the behavior of accretion disks around stellar-mass black holes, perhaps indicating that disk self-gravity enhances the growth of accretion instabilities.

There are several directions in which we can expand this study. In a “Dead Quasar Survey”, we are conducting [O III] imaging of samples of luminous AGN hosts and galaxies without AGN signatures, to seek fainter (and possibly older) clouds than can be detected from the SDSS images. H I selection should help pick out objects with tidal tails in suitable positions to be ionized at tens of kpc from the core; we are beginning with the Kuo et al. (2008) sample of Seyfert galaxies mapped in H I. For the “faded” galaxies in thes sample, we are pursuing XMM-Newton and HST observations to clarify the obscuration toward the nucleus, seek any signs of outflow-induced star formation as seen in Hanny’s Voorwerp, and refine estimates of the ionizing luminosity through the highest recombination-line surface brightness in the clouds.


This work would not have been possible without the contributions of citizen scientists as part of the Galaxy Zoo project. We particularly thank Hannah Hutchins, Elizabeth Baeten, Massimo Mezzoprete, Elizabeth Siegel, Aida Berges, and users voyager1682002 and Caro, who each examined all of the galaxies in the targeted AGN sample, and in addition Christian Manteuffel, for assistance in compiling the list of SDSS AGN candidates. We are grateful to the following additional Galaxy Zoo participants who contributed to the targeted AGN search: Michael Aarons, Mark Ackland, AdrianusV, Aerial, alexob6, Daniela Alice, Norvan Allen, Anderstp, AndrewM, angst, Anjinsan, ARCHEV, artemiit, aryamwojn, astrobrainiac, astronomicom1, Markku Autio, Michelle Ayers, Elisabeth Baeten, R. Balick, Michael Balzer, Michael Derek Barnett, Kirsten Barr, Barbara Ann Barrett, David Bartlett, Coral Benham, Aida Berges, Mark Bernaldo, Chiheb Boussema, Gwen Brogmus, Dave Browne, buddyjesus, David Burt, cadou, caliz83, Capella, Alice Carlsen, Caro, Jiri Cejka, Theodore J. Celaya Sr., chairstar, Bruno Chiaranti, citisue3, Nick Clarke, Ana Claudia, cloud9, clua, David N. Cook, coral, Gemma Coughlin, Rob Cowhey, Penny Cox, Laurence Cuffe, cyprien, DancesWithWords, Darren, DarthKeribo, Lloyd Daub, daveb, dave3, david_mbe, david_nw, Michael C. Davis Jr., distel, Dobador, Shane Dobkins, drawm, Juliette Dowle, Elizabeth Duff, Graham Dungworth, dxjerlubb, dzd, Michael Easterly, echo, Alan Eggleston, Thomas Erickson, ErroneousBee, Falconet, firejuggler, frisken, Gino, glyphon, GNB080, gordhaddow, Michael Gronceski, grrower1, Michael Hand, Thomas Hardy, Hans Heilman, Steph Hill, Thomas Hobbs, Rick Holtz, Rob Hounsell, hrutter, Mikko Huovinen, IC1101, ixzrtxp, Nina Jansen, Alain Jaureguiberry, jayton, jczoehdo, jhyatt, David E. Johnson, Steve Johnson, David James Jones, John Kelly, khwdfnwit, Pat Kieran, KillerSkaarj, kiske1, knuid, kokdeblade, Anuradha Koratkar, Michi Kovacs, kzhndepnd Marc Laidlaw, laihro01, landersonzych, Lily Lau WW, lawless, Bill Lawrence, Kathleen Littlefield, Liz, Marc Lluell, Michael Lopez, lpspieler, luigimx, Lzsp, Michael MacIsaac, Christine Macmillan, Katie Malik, Steve Malone, mardo, Lelah Marie, Mark, Michael Marling, Stephanie R. Marsala, Mauro Marussi, marxpmp, Mark McCormack, Rob Mellor, Massimo Mezzoprete, mgn, Michaelr1415, MichaelRoberts, MichaelSangerTx, milkncookies, miraculix250, Elspeth Mitchell, Graham Mitchell, mlvgofjedxv, mothic, Mukund, mykyij, NGC3372, Julian Nicol, Rick Nowell, nrbeuw, Richard Oram, orion, oswego9050, pbungaro, Alice Peachey, Thomas Perraudin, Amanda Peters, Erica Pinto, plummerj, Jim Porter, Steven Porter, Richard Proctor, ptkypxdh, randa, RandyC, Kim Reece, Jessica Reeder, RelativisticDog2, Thomas Rickenbach, ripw, rjwarmv, rnjrchd, Michael Roberts, RobinMiller, Jim Robinson, roborali, Rona, Geoff Roynon, Paul Rutten, Rynnfox, S4CCG, Michael Salmon, salteV, Jeroen Sassen, second_try, Matt Sellick, sheba, Alice Sheppard, SianElderxyz, Nanne Sierkstra, Michael Simmons, SJPorter, skepticdetective, Stephen Sliva, Mark Smith, Sophie378, spat, Maria Steinrueck, stella13, stellar390, John H. Stewart, Doug Stork, sumoworm, superhouse, tadaemdg, Auralee Tamison, Chet Thomas, thom_2, Michael Thorpe, timchem, torres, Trixie64, Ramon van der Hilst, Marcel Veillette, Rob H.B. Velthuis, John Venables, Michael Viguet, vkhtmhfigou, Aileen Waite, David Walland, wbybjbpv, weezerd, Mark Westover, Julia Wilkinson, Nat T. Winston III, Windsmurf, wpubphx, xuhtjhc, xzxupfqjd, and Mairi Yates. We also thank the referee, who caught a mistake in calculating light-delay times and helped make the discussion more comprehensive. Jean Tate helped to untangle some issues of participant discovery order.

W.C. Keel acknowledges support from a Dean’s Leadership Board faculty fellowship. C. J. Lintott acknowledges funding from The Leverhulme Trust and the STFC Science in Society Program. Galaxy Zoo was made possible by funding from a Jim Gray Research Fund from Microsoft and The Leverhulme Trust. S. D. Chojnowski participated through the SARA Research Experiences for Undergraduates program funded by the US National Science Foundation. This research is based on observations with AKARI, a JAXA project with the participation of ESA. We thank the Lick Observatory staff for their assistance in obtaining the data. Support for the work of KS was provided by NASA through Einstein Postdoctoral Fellowship grant number PF9-00069 issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060.

Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS Web site is The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Princeton University, the United States Naval Observatory, and the University of Washington.

This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration.

Coordinate name SDSS ObjID Nucleus Name/note Search Posted by
SDSS J005607.66+254804.7 587740589487030353 0.1530 Sy 1 purple haze S ElisabethB
SDSS J003507.44+004502.1 587731187281494175 0.1205 LINER blue arc S scott L
SDSS J004527.06+004237.6 587731187282608299 0.1096 Sy 1.5 S davidjamesjones
SDSS J013037.75+131251.9 587724197207212176 0.0721 Sy 2 CGCG436-065 T
SDSS J014238.47+000514.7 588015509280587804 0.1459 Sy 1 S Tsering
SDSS J014644.82-004043.1 588015508207304746 0.0827 Sy 1 S davidjamesjones
SDSS J030526.96+005144.9 588015510363373793 0.1181 Sy 1 S Mukund Vedapudi
SDSS J030639.58+000343.2 588015509289762862 0.1074 Sy 1 S mitch
SDSS J033013.26-053235.9 587724242842026028 0.0131 Sy 1 NGC 1346 S Half65
SDSS J040548.78-061925.7 587727178476093634 0.0556 Sy 1 S echo-lily-mai
SDSS J074241.70+651037.8 758878270293868614 0.0371 Sy 2 Mkn 78 T
SDSS J075910.44+115156.7 588023046395527377 0.0503 Sy 1 S silverhaze
SDSS J080452.73+212050.2 588016878287650850 0.1242 Sy 1 purple haze S davidjamesjones
SDSS J082034.78+153111.3 587741532229337219 0.1435 Sy 1 S Half65
SDSS J082342.37+482754.4 587725470667833620 0.0935 Sy 2 S spiralmania
SDSS J082642.63+111555.5 587745244691628370 0.0884 trans T
SDSS J083525.51+104925.7 587744873714679862 0.1172 Sy 1 S spiralmania
SDSS J083818.43+333441.3 587732470387703859 0.0621 LINER KUG T
SDSS J084002.36+294902.6 587735240637284507 0.0648 Sy 2 4C 29.30 T
SDSS J084344.98+354942.0 587732484342415393 0.0539 Sy 2 S T laihro
SDSS J084518.51+142034.1 587742062124269645 0.0606 Sy 1 purple haze S mitch
SDSS J084810.10+351534.0 587732470388818038 0.0573 Sy 2 KUG0845+354 T
SDSS J084809.59+351530.3 587732470388818042 0.0567 Sy 2 KUG0845+354 T
SDSS J084917.31+531755.7 587725470670585957 0.1112 Sy 1 purple haze S Tsering
SDSS J085625.93+021310.5 587727944563884115 0.1251 RG S Bruno
SDSS J085729.84+064210.6 587734691423453446 S lovethetropics
SDSS J085837.52+182221.5 587741708866289722 0.0588 Sy 2 T
SDSS J085813.75+385631.8 587732053243855000 0.0884 Sy 2 T
SDSS J090547.33+374738.2 587732152565432366 0.0475 trans T
SDSS J090958.07+621450.4 587737826756198431 0.0261 Sy 2 NGC 2742A S Ioannab
SDSS J091011.34+230717.9 587741421104136355 0.0362 S Citizen_Kirk
SDSS J091708.26+292215.6 587738946130018402 0.0353 Sy 2 KUG 0914+295 S Half65
SDSS J093033.05+034443.6 587728880335257753 0.0911 Sy 1 S ElisabethB
SDSS J094529.64-002154.7 587725074458345485 0.0515 LINER T
SDSS J095559.88+395446.9 588016528244670522 0.0483 violet plume S StephanieC
SDSS J100507.88+283038.6 587741392112451744 0.0517 Sy 2 S T laihro
SDSS J100529.60+275844.2 587741391575580675 0.0555 Sy 2 S mitch
SDSS J101128.26+260655.4 587741490365792357 0.1164 Sy 2 S mitch
SDSS J101645.11+421025.5 588297863112294442 0.0553 NLSy1 S mitch
SDSS J102016.20+524756.9 587731499185602761 0.0689 Sy 2 S
SDSS J102108.59+024058.5 587726033311498259 S mitch
SDSS J103734.22+140120.5 587735349101199404 0.2061 Sy 2 S davidjamesjones
SDSS J104232.05+050241.9 587728880879992930 0.0271 Sy 2 NGC 3341 S mitch
SDSS J104326.47+110524.2 587734948595499096 0.0475 Sy 1 purple haze S lovethetropics
SDSS J104515.28+421331.7 588017626147782828 0.0990 LINER bubble? S RandyC
SDSS J110157.90+101739.2 587732772658806856 0.0340 Sy 1 S davidjamesjones
SDSS J110335.42+032014.7 587726033853022296 0.0531 Sy 2 S skwalker
SDSS J110445.46+041755.2 588010358543351857 0.0252 Sy 2 S ElisabethB
SDSS J110756.53+474434.8 588295840708755475 0.0727 Sy 1 S davidjamesjones
SDSS J111100.60-005334.9 588848898833580220 0.0904 Sy 2 S T ElisabethB
SDSS J111113.00+284242.7 587741532784361479 0.0294 SB NGC 3561A T
SDSS J111113.18+284147.0 587741532784361477 0.0295 Sy 2 NGC 3561 T
SDSS J111349.74+093510.7 587734892748144649 0.0292 Sy 1.5 IC 2637 S T stellar190
SDSS J111653.96+593146.8 587729387686461462 0.0815 trans VII Zw 384 T
SDSS J112534.58+523247.0 587732136456487055 0.0270 SB S errattan
SDSS J112753.87+302138.6 587741491447070913 0.0736 nonAGN S paulrogers
SDSS J112942.51+235014.1 587742189363331247 0.1277 LINER S ElisabethB
SDSS J113323.97+550415.8 587733081347063838 0.0085 Sy 1 Mkn 177 compn S stellar190
SDSS J113629.36+213551.7 587742013279502427 0.0297 Sy 1 Mkn 739 S T Budgieye
SDSS J113849.61+574243.4 587735696978215000 0.1162 Sy 1 S ElisabethB
SDSS J114155.61+010516.7 588848901521277093 0.1365 Sy 2 S lovethetropics
Table 1: Candidate AGN with extended emission-line clouds
Coordinate name SDSS ObjID Nucleus Name/note Search Posted by
SDSS J114454.85+194635.3 588023669168537695 0.0274 Sy 2 S T stellar190
SDSS J114517.10+200121.8 588023669705474229 0.04953 (possible) S Half65
SDSS J115140.70+675041.9 587725552285122567 0.0629 Sy 2 S StephanieC
SDSS J115739.07-023908.3 587724649256779921 0.1308 Sy 1 purple haze S c_cld
SDSS J115906.89+101001.7 587732771591225359 0.1165 Sy 1.8 purple haze S c_cld
SDSS J120114.35-034041.0 587725039018311737 0.0196 Sy 1 Mkn 1310 S Milk_n_cookies
SDSS J120150.80+143323.9 587735348036370587 0.0677 nonAGN S DuffBeer
SDSS J120719.81+241155.8 587742189367066665 0.0505 NLSy1 purple haze Mkn 648 S davidjamesjones
SDSS J120939.43+643107.6 587729154134966352 0.1042 Sy 2 S mitch
SDSS J121418.25+293146.7 587741532253519916 0.0632 Sy 2 Was 49ab S T stellar190
SDSS J121431.32+402902.6 588017979429486656 0.1211 Sy 2 S mitch
SDSS J121452.41+591953.2 587729386079059975 0.0607 Sy 2 VII Zw 444 S mitch
SDSS J121553.08+051447.8 588010359624827047 0.0803 LINER S codexluminati
SDSS J121819.30+291513.0 587739719750058064 0.0477 Sy 2 UGC 7342 S T stellar190
SDSS J122402.57+435814.0 588017603610935505 0.1040 nonAGN S fluffyporcupine
SDSS J122546.72+123942.7 588017566564155399 0.0086 Sy 2 NGC 4388 S T RandyC
SDSS J122802.10+094347.9 587732771057434918 0.1534 LINER S Bruno
SDSS J123034.25+033800.7 587726016682917948 0.01285 LINER S StephanieC
SDSS J123038.98+401614.4 587738947758456849 0.1322 Sy 1.5 purple haze S Tsering
SDSS J123046.11+103317.3 587732772131504164 0.01540 SB VPC 0764 S lovethetropics
SDSS J123113.12+120307.2 588017702933823557 0.1161 Sy 1 S lovethetropics
SDSS J124036.73+365004.3 587739096991334439 0.0404 Sy 2 S stellar190
SDSS J124046.40+273353.5 587741602034090027 0.0565 Sy 1.5 S Tsering
SDSS J124103.66+273526.0 587741602034155555 0.2007 Sy 1 S Tsering
SDSS J124325.65+365525.3 587739096991596570 0.0839 Sy 2 S Bruno
SDSS J124450.84-042604.5 587745544806727722 0.0147 LINER IC 0812 S Milk_n_Cookies
SDSS J124505.56+102433.2 587732772133011652 0.0976 Sy 2 S davidjamesjones
SDSS J124511.84+230210.0 587742014897127434 0.02326 IC 0813 S elizabeth
SDSS J125741.04+202347.7 588023670249750583 0.0807 Sy 2 IC 3929 S T c_cld
SDSS J130007.06+183914.3 587742575372730410 0.1130 Sy 2 S mitch
SDSS J130234.89+184122.3 588023668102856809 0.0656 Sy 2 S Mukund Vedapudi
SDSS J130422.19+361543.1 587738950954385445 0.0443 Sy 2 WR 470 S mitch
SDSS J130258.82+162427.7 587742773491531836 0.0673 Sy 1 Mkn 783 S stellar190
SDSS J130509.98-033209.2 587725039025258588 0.0835 Sy 1 purple haze S lovethetropics
SDSS J131555.15+212521.5 587742013289660465 0.0884 Sy 1 S T davidjamesjones
SDSS J131639.74+445235.0 588017605762482225 0.0909 Sy 1.9 S c_cld
SDSS J131913.93+132030.8 587736802936684556 0.0960 Sy 2 S mitch
SDSS J132340.31-012749.1 587725041711644785 0.0767 Sy 1 S IC 1101
SDSS J132540.23+275146.1 587739719219675227 0.0377 S ElisabethB
SDSS J133227.20+112910.4 588017570316615795 0.0778 Sy 2 S veggy2
SDSS J133416.49+311709.1 587739609171230755 0.0570 SB Was 75 T
SDSS J133718.72+242303.3 587742190986657795 0.1076 NLSy1 S c_cld
SDSS J133815.86+043233.3 587729158970736727 0.0228 Sy 1.5 NGC 5252 S T laihro
SDSS J133817.11+481636.1 587732483292266549 0.02786 Sy 2 NGC 5256, Mkn 266 S T Gumbosea
SDSS J134442.16+555313.5 587735666377949228 0.0373 Sy 2 Mkn 273 S T stellar190
SDSS J134608.10+293810.4 587739504478060626 0.0776 Sy 1 S lovethetropics
SDSS J134630.29+283646.3 587739707943092392 0.0518 Sy 2 T
SDSS J135255.67+252859.6 587739810484650051 0.06387 Sy 1 KUG1350+257 T
SDSS J135602.62+182217.7 587742550676275314 0.0506 Sy 2 Mkn 463 T
SDSS J135635.73+232135.9 587739845379883044 0.0668 LINER T
SDSS J135712.06-070433.0 587746236298231865 S stellar190
SDSS J140037.11+622132.7 587728918446407773 0.0752 Sy 1 S mitch
SDSS J141051.82+410412.5 588017604156457121 0.0812 LINER S Song
SDSS J140506.26+024618.2 587726033335943373 0.0766 Sy 2 S mitch
SDSS J141405.01+263336.8 587739720298201117 0.0357 Sy 1 S davidamesjones
SDSS J142522.28+141126.5 587742609727684701 0.0601 Sy 2 T
SDSS J142925.07+451831.8 587735490282848380 0.0748 Sy 1.5 purple haze S T Aroel
SDSS J143029.88+133912.0 587736809916399664 0.0852 Sy 2 Teacup S T Half65
SDSS J143239.83+361808.0 587736583892238376 0.0132 Sy 2/SB NGC 5675 T
SDSS J144038.10+533015.8 587733427086426161 0.0376 Sy 2 Mkn 477 T
Table 4: Candidate AGN with extended emission-line clouds
Table 3: continued
Coordinate name SDSS ObjID Nucleus Name/note Search Posted by
SDSS J144240.79+262332.5 587739457225097282 0.1071 Sy 1 S spiralmania
SDSS J144331.19+191121.0 587742062161428638 0.0598 Sy 2 S Bruno
SDSS J145724.63+105937.3 587736807771930760 0.1227 Sy 1 S davidjamesjones
SDSS J150408.46+143123.3 587742575922708553 0.1181 Sy 1 Mkn 840 S mitch
SDSS J150756.88+032037.3 587726100952449048 0.1369 Sy 1 S mitch
SDSS J151004.01+074037.1 588017991773520114 0.0458 Sy 2 S T whitefluffydogs
SDSS J151141.26+051809.2 587736546312323142 0.0845 Sy 1 S Half65
SDSS J151915.98+104847.8 587736813131989104 0.0988 Sy 1 S T spiralmania
SDSS J152412.58+083241.2 588017703489372418 0.0371 Sy 2 S T Alice
SDSS J152549.54+052248.7 587730022796755031 0.048 Sy 2 T
SDSS J152907.45+561606.6 587742882456731737 0.0998 Sy 1 S spiralmania
SDSS J153355.15+585756.4 587725818571063416 S lovethetropics
SDSS J153432.52+151133.2 587742013841145937 0.0066 Sy 2 NGC 5953 S Half65
SDSS J153508.93+221452.8 587739814240190581 0.0858 trans purple haze T
SDSS J153703.36+135944.1 587742590401904799 0.0737 LINER S RandyC
SDSS J153854.16+170134.2 587739845390761994 0.02974 Sy 2 NGC 5972 S T NeilGibson
SDSS J155007.62+272814.5 587736941990969374 0.1468 Sy 1 purple haze S ElisabethB
SDSS J160536.79+174807.5 587739720846934175 0.0339 Sy 2 IC 1182 S stellar190
SDSS J162538.08+162718.1 587739814246023211 0.0343 LINER Akn 502 T
SDSS J162804.06+514631.4 587736980102643827 0.0547 Sy 1.9 Mkn 1498 S Budgieye
SDSS J162930.01+420703.2 587729653421441105 0.0717 Sy 1 purple haze S Tsering
SDSS J162952.88+242638.4 587736898503639075 0.0368 Sy 1 Mkn 883 S T Rick Nowell
SDSS J164800.81+295657.4 587733399186898947 0.1059 Sy 1 S mitch
SDSS J172335.75+342133.4 587739849686843709 S Mukund Vedapudi
SDSS J172747.17+265121.4 587729409160183880 0.0291 VV 389 S elizabeth
SDSS J172935.81+542939.9 587725505559855518 0.0820 Sy 2 S Bruno
SDSS J181611.61+423937.3 758879745074397535 0.04120 Sy 2 UGC 11185 S stellar190
SDSS J210918.38-060754.7 587726879412256901 0.0286 Sy 2 S mitch
SDSS J214150.10+002209.4 587731186725683280 0.1068 LINER S echo-lily-mai
SDSS J220141.64+115124.3 587727221400862869 0.0296 Sy 2 S T stellar190
SDSS J233254.46+151305.4 587730774959652922 0.2148 Sy 1 S ElisabethB
SDSS J234413.61+004813.9 587731187275923676 0.0497 LINER S davidjamesjones
Table 4: Candidate AGN with extended emission-line clouds
Table 3: continued
Telescope UT Dates Range, Å Resolution, Å Slit scale ”/pixel Galaxies observed
KPNO 2.1m 2010 June 15-21 3630-5700 3.2 0.78 33
Lick 3m 2010 July 12-15 5450-8260 4.5 0.78 11
3495-5605 1.5 0.43 11
Lick 3m 2010 Dec 1 - 3 4630-7410 4.3 0.78 13
3280-4595 2.7 0.43 13
Lick 3m 2009 Dec 17 5250-9940 13.5 0.78 2
3650-5710 4.8 0.43 2
Table 5: Spectroscopic data
Coordinate name SDSS ObjID Nucleus Name PA Source Region Type
Confirmed AGN-ionized:
SDSS J074241.70+651037.8 758878270293868614 0.0371 Sy 2 Mkn 78 90 Lick AGN
SDSS J095559.88+395446.9 588016528244670522 0.0483 Sy 2 148 Lick AGN
SDSS J100507.88+283038.5 587741392112451744 0.0517 Sy 2 62 Lick AGN
SDSS J111349.74+093510.7 587734892748144649 0.0292 Sy 1.5 IC 2637 47 GCam AGN
SDSS J113629.36+213551.7 587742013279502427 0.0297 Sy 1 Mkn 739 168 GCam AGN
SDSS J121819.30+291513.0 587739719750058064 0.0477 Sy 2 UGC 7342 133 GCam AGN
SDSS J122546.72+123942.7 588017566564155399 0.0086 Sy 2 NGC 4388 26 GCam AGN
SDSS J133815.86+043233.3 587729158970736727 0.0228 Sy 1.5 NGC 5252 175 GCam AGN
SDSS J133817.11+481636.1 587732483292266549 0.0279 Sy 2 Mkn 266 176 GCam Lick AGN
SDSS J134442.16+555313.5 587735666377949228 0.0373 Sy 2 Mkn 273 57 GCam AGN
SDSS J135602.62+182217.8 587742550676275314 0.0504 Sy 2 Mkn 463 8 GCam AGN
SDSS J143029.88+133912.0 587736809916399664 0.0852 Sy 2 Teacup 37 GCam Lick AGN
SDSS J151004.01+074037.1 588017991773520114 0.0458 Sy 2 175 GCam AGN
SDSS J152412.58+083241.2 588017703489372418 0.0371 Sy 2 CGCG 077-117 150 GCam AGN
SDSS J153854.16+170134.2 587739845390761994 0.0297 Sy 2 NGC 5972 167 GCam Lick AGN
SDSS J162804.06+514631.4 587736980102643827 0.0547 Sy 1.9 Mkn 1498 150 GCam Lick AGN
SDSS J162952.88+242638.4 587736898503639075 0.0368 Sy 1 Mkn 883 91 Lick AGN
SDSS J181611.61+423937.3 758879745074397535 0.0412 Sy 2 UGC 11185 90 GCam Lick AGN
SDSS J220141.64+115124.3 587727221400862869 0.0296 Sy 2 19 GCam Lick AGN
Other cloud types:
SDSS J003507.44+004502.1 587731187281494175 0.1205 LINER 35 Lick H II regions
SDSS J005607.66+254804.7 587740589487030353 0.1530 Sy 1 200 Lick no
SDSS J012839.87+144553.8 587724233179660360 0.0452 Sy 2 CGCG436-060 45 Lick no
SDSS J014238.47+000514.7 588015509280587804 0.1459 Sy 1 80 Lick AGN
SDSS J030639.58+000343.2 588015509289762862 0.1074 Sy 1 40 Lick AGN (small)
SDSS J033013.26-053235.9 587724242842026028 0.0131 SB NGC 1346 76 Lick H II
SDSS J040548.78-061925.7 587727178476093634 0.0556 Sy 1 76 Lick H II
SDSS J080452.73+212050.2 588016878287650850 0.1242 Sy 1 150 Lick AGN (small)
SDSS J082642.63+111555.5 587745244691628370 0.0884 trans 72 Lick resolved?
SDSS J083525.51+104925.7 587744873714679862 0.1172 Sy 1 150 Lick small?
SDSS J084344.98+354942.0 587732484342415393 0.0539 Sy 2 141 Lick AGN
SDSS J111100.60-005334.9 588848898833580220 0.0904 Sy 2 101 GCam unresolved cloud at 4”
SDSS J113323.97+550415.8 587733081347063838 0.0085 Sy 1 Mkn 177 136 GCam AGN in small compn
SDSS J114454.85+194635.3 588023669168537695 0.0274 Sy 2 131 GCam unresolved
SDSS J121418.25+293146.7 587741532253519916 0.0632 Sy 2 Was 49ab 63 GCam off-nuc AGN or cloud
SDSS J123046.11+103317.3 587732772131504164 0.01540 SB VPC 0764 30 GCam unresolved
SDSS J124450.84-042604.5 587745544806727722 0.0147 LINER IC 0812 62 GCam
SDSS J125741.04+202347.7 588023670249750583 0.0807 Sy 2 IC 3929 47 GCam H II to 9”
SDSS J134630.29+283646.3 587739707943092392 0.0518 Sy 2 164 GCam 34” dim AGN cloud?
SDSS J135255.67+252859.6 587739810484650051 0.06387 Sy 1 KUG1350+257 163 GCam H II
SDSS J142522.28+141126.5 587742609727684701 0.0601 Sy 2 110 GCam two AGN?
SDSS J150408.46+143123.3 587742575922708553 0.1181 Sy 1 Mkn 840 70 GCam  7”
SDSS J151915.98+104847.8 587736813131989104 0.0099 Sy 1 89 GCam unresolved
SDSS J153508.93+221452.8 587739814240190581 0.0858 trans 70 GCam
SDSS J153703.36+135944.1 587742590401904799 0.0737 LINER 21 GCam unresolved
SDSS J160536.79+174807.5 587739720846934175 0.0339 Sy 2 IC 1182 96 GCam H II
SDSS J210918.38-060754.7 587726879412256901 0.0286 Sy 2 70 GCam unresolved
SDSS J214150.10+002209.4 587731186725683280 0.1068 LINER 70 GCam Lick unresolved
SDSS J233254.46+151305.4 587730774959652922 0.2148 Sy 1 138 Lick unresolved
SDSS J234413.61+004813.9 587731187275923676 0.0497 LINER 71 Lick H II
Table 6: Results of long-slit spectroscopy
Object F(5007) F(H)
IC 2637 nuc 0.23 0.28 0.81 0.11 3.94 2.7e-14
cloud 0.12 2.43 1.34 4.6e-15
Mkn 78 nuc (1) 0.35 0.32 0.10 0.29 13.5 6.8e-14 0.05 1.00 0.23 6.5e-13
cloud 2.00 0.16 0.43 0.37 14.00 7.8e-15 0.094 0.51 0.22 2.0e-15
Mkn 266 nuc 0.69 0.17 0.43 0.19 4.08 3.4e-14 0.052 0.58 0.35 4.0e-14
cloud 0.46 0.40 0.22 0.38 8.20 3.7e-14 0.029 0.25 0.23 1.1e-14
Mkn 273 nuc 0.27 0.11 0.90 0.12 3.93 2.4e-14
cloud 0.06 0.26 0.93 0.27 9.3 1.0e-14
Mkn 463 nuc 0.26 0.29 0.30 0.08 7.7 3.7e-13
cloud 0.41 0.16 0.51 0.17 11.3 1.8e-14
Mkn 739 nuc (2) 1.16 0.49 0.15 0.18 5.88 1.5e-14
cloud 1.31 0.22 0.45 0.24 8.30 6.5e-15
Mkn 883 nuc 0.31 0.12 0.81 0.08 2.82 5.3e-14 0.12 0.47 0.47 8.4e-14
cloud 0.30 0.22 0.30 0.19 7.84 6.6e-15 0.08 0.31 0.49 4.1e-15
Mkn 1498 nuc (2) 0.73 0.82 0.07 0.10 3.72 1.5e-13 0.07 1.00 0.22 4.9e-14
cloud 1.08 0.62 0.18 0.32 12.3 1.0e-14 0.06 0.14 0.20 3.4e-15
NGC 4388 nuc 0.45 0.25 0.27 0.23 11.5 3.6e-13
cloud 0.39 1.15 0.57 4.72 4.3e-15
NGC 5252 nuc 0.31 0.27 0.55 0.19 9.22 6.0e-14
cloud 1.31 0.34 0.31 0.34 10.1 1.1e-14
NGC 5972 nuc 2.25 0.31 0.29 0.40 2.56 5.1e-14 0.07 0.69 0.49 1.9e-14
cloud 0.15 0.20 0.63 0.26 8.48 2.6e-14 0.12 0.79 0.61 9.5e-15
SDSS 0955+39 nuc 0.05 4.60 2.90 6.9e-16 0.32 0.84 1.02 1.6e-15
cloud 2.64 0.47 0.28 0.51 7.00 3.2e-15 0.08 0.26 0.37 9.5e-16
SDSS1005+28 nuc 0.55 0.29 0.30 0.24 7.96 1.1e-14 0.07 0.57 0.48 5.8e-15
cloud 1.52 0.60 0.24 0.58 5.46 4.5e-15 0.25 0.34 3.3e-15
SDSS 1510+07 nuc 0.04 1.15 1.86 2.11 3.2e-15
cloud 1.76 0.22 0.56 0.39 6.64 4.4e-15
SDSS 1524+08 nuc 0.78 0.19 0.91 0.37 8.00 4.8e-15
cloud 1.58 0.15 0.49 0.44 7.84 5.4e-15
SDSS 2201+11 nuc 0.17 0.07 1.65 0.28 5.48 1.7e-15 0.17 1.21 1.02 7.0e-15
cloud 0.53 0.24 0.21 0.18 10.1 7.8e-15 0.07 0.40 0.35 3.0e-15
Teacup nuc 0.31 0.23 0.30 0.09 7.95 1.3e-13 0.15 0.23 0.15 1.1e-14
cloud 0.14 0.17 0.23 0.15 8.07 2.1e-14 0.11 0.34 0.36 7.6e-15
UGC 7342 nuc 0.75 0.22 0.58 0.22 9.14 2.9e-14
cloud 0.77 0.26 0.41 0.55 9.90 8.9e-15
UGC 11185 nuc (1) 0.17 0.19 0.37 0.16 8.89 8.5e-14 0.19 1.29 0.88 3.8e-14
cloud 0.28 0.28 0.25 0.26 8.11 1.4e-14 0.07 1.10 0.72 5.1e-15

Notes: Line fluxes are in units of erg cm s
(1) blend of two velocity components
(2) BLR present; flux is estimated NLR only

Table 7: Emission-line ratios and selected fluxes
SDSS ID Other name IRAS Akari L(FIR) F(H) Ratio
074241.70+651037.8 Mkn 78 1.11 1.13 2.4e44 10 4.0e-16 11 e43 0.25
095559.88+395446.9 5.0e44 7 2.5e-16 16 e43 0.06
100507.88+283038.5 e44 8 4.2e-16 14 e44 0.5
111349.74+093510.7 IC 2637 1.75 3.39 3.0e44 10 3.0e-16 11 e43 0.10
113629.36+213551.7 Mkn 739 1.3 2.4 2.3e44 18 2.1e-16 6.3 e43 0.26
121819.30+291513.0 UGC 7342 44 35 1.8e-16 43.2 e44 3.6
122546.72+123942.7 NGC 4388 10.0 17.1 1.4e44 15 3.8e-16 7.6 e42 0.02
133815.86+043233.3 NGC 5252 4.0e43 32 3.8e-16 3.5 e44 5
133817.11+481636.1 Mkn 266 7.3 10.3 1.0e45 27 6.8e-16 4.2 e44 0.4
134442.16+555313.5 Mkn 273 22.5 22.5 5.0e45 12 9.0e-16 10 e44 0.03
135602.62+182217.8 Mkn 463 2.33 1.94 9.0e44 16 3.8e-16 7 e44 0.3
143029.88+133912.0 Teacup 0.26 e43 5 1.66e-15 22 e44 14
151004.01+074037.1 e44 14 8.2e-16 8 e45 3.4
152412.58+083241.2 0.71 0.73 0.63 1.6e44 5.6e-16 12 10 1.0e44 0.6
153854.16+170134.2 NGC 5972 0.24 e43 35 7.2e-17 3 e43 1.8
162804.06+514631.4 Mkn 1498 0.34 e43 10 9.8e-17 11 e43 0.6
162952.88+242638.4 Mkn 883 1.01 1.13 2.2e44 10 4.0e-16 11 e43 0.26
181611.61+423937.3 UGC 11185 e44 10 1.7e-15 11 e44 1.7
220141.64+115124.3 0.28 e43 23 4.6e-16 5.0 e44 3.4

Notes: FIR fluxes are in Jy
Mean IRAS detection limits are used when no specific value is available
Luminosities are in erg s; H fluxes are in erg cm s
Values of ionizing/FIR luminosity ratio are all lower limits

Table 8: Ionizing and far-infrared luminosity comparison
Object Distance: arcsec kpc [S II] ratio (cm) [O II]/[O III] log (erg/s)
Teacup 5-14 8-22 130-240 0.15 -2.16
Mkn 883 12-16 9-12 12- 100 2.62 -3.34
SDSS 2201 8-16 5-10 0.25 -2.42
NGC 5972 26-30 15-18 3-300 0.11 -1.99
Mkn 266 20-29 11-16 12-100 0.22 -2.36
Table 9: [S II] density measures and limits
SDSS designation Sy type Name , kpc Morphology cone angle disc/cloud angle Sides
SDSS J074241.70+651037.8 0.0371 2 Mkn 78 16 E 55 2
SDSS J095559.88+395446.9 0.0483 2 10 Interacting S 88 1
SDSS J100507.88+283038.5 0.0517 2 13 Sb, disturbed companion 92 1
SDSS J111349.74+093510.7 0.0292 1.5 IC 2637 11 Merger remnant 60 1
SDSS J113629.36+213551.7 0.0297 1 Mkn 739 17 Ongoing merger 28 1
SDSS J121819.30+291513.0 0.0477 2 UGC 7342 38 Ongoing merger; tails 86 2
SDSS J122546.72+123942.7 0.0086 2 NGC 4388 13 Edge-on Sc 80 53 1
SDSS J133815.86+043233.3 0.0228 1.5 NGC 5252 21 Edge-on S0, tilted H I ring 59 31 2
SDSS J133817.11+481636.1 0.0279 2 Mkn 266 21 Ongoing merger 112 2
SDSS J134442.16+555313.5 0.0373 2 Mkn 273 19 Ongoing merger 75 2
SDSS J135602.62+182217.8 0.0504 2 Mkn 463E 16 Ongoing merger 55 2
SDSS J143029.88+133912.0 0.0852 2 Teacup 18 Stellar tail and arc 80 1
SDSS J151004.01+074037.1 0.0904 2 10 Symmetric disc; S0 or Sa 85 2
SDSS J152412.58+083241.2 0.0371 2 CGCG 077-117 19 Merger remnant 56 1
SDSS J153854.16+170134.2 0.0297 2 NGC 5972 33 Warped disc and tails 35 18 2
SDSS J162804.06+514631.4 0.0547 1.9 Mkn 1498 21 E 42 1
SDSS J162952.88+242638.4 0.0368 1 Mkn 883 37 Ongoing merger 73 1
SDSS J181611.61+423937.3 0.0412 2 UGC 11185 11 Strong interaction 48 1
SDSS J220141.64+115124.3 0.0296 2 16 Edge-on warped disc, tails 23 30 2
SDSS J094104.11+344358.4 0.0499 LINER IC 2497 40 Warped disk, H I tail 46 65 1
Table 10: Morphologies of AGN hosts with extended clouds


  1. pagerange: The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black-hole accretion eventsReferences
  2. pubyear: 2011
  3. in
  4. IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation.


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