Gas-rich mergers and feedback are ubiqitous amongst starbursting radio galaxies, as revealed by JVLA, IRAM PdBI and Herschel
We report new, sensitive observations of two 3–3.5 far-infrared-luminous radio galaxies, 6C 1909+72 and B3 J2330+3927, in the CO transition with the Karl Jansky Very Large Array and at 100–500 m using Herschel, alongside new and archival CO observations from the Plateau de Bure Interferometer. We introduce a new colour-colour diagnostic plot to constrain the redshifts of several distant, dusty galaxies in our target fields. A bright SMG near 6C 1909+72 likely shares the same node or filament as the signpost active galactic nuclei (AGN), but it is not detected in CO despite 20,000 km s of velocity coverage. Also in the 6C 1909+72 field, a large, red dust feature spanning 500 kpc is aligned with the radio jet. We suggest several processes by which metal-rich material may have been transported, favouring a collimated outflow reminiscent of the jet-oriented metal enrichment seen in local cluster environments. Our interferometric imaging reveals a gas-rich companion to B3 J2330+3927; indeed, all bar one of the eight radio galaxies (or companions) detected in CO provide some evidence that starburst activity in radio-loud AGN at high redshift is driven by the interaction of two or more gas-rich systems in which a significant mass of stars has already formed, rather than via steady accretion of cold gas from the cosmic web. We find that the CO brightness temperature ratios in radio-loud AGN host galaxies are significantly higher than those seen in similarly intense starbursts where AGN activity is less pronounced. Our most extreme example, where , provides evidence that significant energy is being deposited rapidly into the molecular gas via X-rays and/or mechanical (‘quasar-mode’) feedback from the AGN, leading to a high degree of turbulence globally and a low optical depth in CO – feedback that may lead to the cessation of star formation on a timescale commensurate with that of the jet activity, 10 Myr.
keywords:galaxies: active — galaxies: high-redshift — galaxies: starburst — submillimetre — infrared: galaxies — radio lines: galaxies
High-redshift radio galaxies (HzRGs) are typically identified via their ultra-steep-spectrum radio emission ( where ) in flux-limited surveys (e.g. Tielens, Miley & Willis, 1979; Röttgering et al., 1994). Despite the inevitable and extreme youth of the radio jets that draw our attention to these distant galaxies, they are associated with the most massive stellar populations of any known galaxy class – and presumably the most massive black holes and host galaxies – out to the highest redshifts (Best, Longair & Röttgering, 1998; Blundell & Rawlings, 1999; Seymour et al., 2007).
In the submillimetre (submm; rest-frame far-infrared, FIR) regime, HzRGs were first explored using relatively primitive submm detectors such as UKT14 on the James Clerk Maxwell Telescope (e.g. Dunlop et al., 1994). Later, with the advent of sensitive submm cameras, their submm luminosities (and hence dust masses) were found to be a strongly increasing function of redshift (Archibald et al., 2001; Reuland et al., 2004), even beyond the peak epoch of activity for submm galaxies (SMGs, –3 – Chapman et al., 2005).
As rare, massive systems, HzRGs are often employed as signposts to what are expected – based on our understanding of how cosmic structures form and evolve (e.g. Davis et al., 1985) – to be over-dense regions of the early Universe. Surveys found excesses of various galaxy types around HzRGs, including Lyman-break galaxies, Ly emitters and SMGs (Ivison et al., 2000; Miley et al., 2004; Greve et al., 2007; Venemans et al., 2007; Overzier et al., 2008; Hatch et al., 2011). Stevens et al. (2003) presented submm imaging of seven HzRGs, several of which appear to contain extended (5–20 arcsec, 35–150 kpc) dust emission, co-spatial with similarly extended UV emission in several cases (Hippelein & Meisenheimer, 1993; Hatch et al., 2008). This suggests that obscured starbursts in these over-dense regions at may differ from the compact (0.5 arcsec, or 4 kpc) events seen in local ultraluminous IR galaxies (ULIRGs) and, indeed, as seen by high-resolution CO and radio continuum imaging of the general SMG field population (Tacconi et al., 2008; Biggs & Ivison, 2008; Younger et al., 2008). From this we might conclude that the mechanism for the formation of the very massive galaxies in these over-dense regions may be fundamentally different to that of the L galaxies forming from SMGs (Smail et al., 2004), although the earliest CO imaging of SMGs (e.g. Carilli et al., 2011; Ivison et al., 2011; Riechers et al., 2011a, b; Hodge et al., 2011) using the Karl Jansky Very Large Array (JVLA) (Perley et al., 2011) suggests their cold gas may be more extended than the high- CO tracers employed previously.
Valiant, early attempts to detect CO towards HzRGs with single-dish telescopes ended in failure (e.g. Evans et al., 1996; van Ojik et al., 1997), but the stable spectral baselines afforded by interferometers eventually allowed the secure detection of CO towards 53W002 at (Scoville et al., 1997; Alloin, Barvainis & Guilloteau, 2000) and CO towards 4C 60.07 at and 6C 1909+72 at (Papadopoulos et al., 2000, hereafter P00), B3 2330+3927 at (De Breuck et al., 2003, hereafter DB03), TN J0121+1320 at (De Breuck, Neri & Omont, 2003) and 4C 41.17 at (De Breuck et al., 2005). Detections of the CO line – that most sensitive to cold gas (Papadopoulos & Ivison, 2002) – are particularly difficult towards radio galaxies: synchrotron emission often dominates at the frequency of the line, 115 GHz, which is expected to be roughly an order of magnitude fainter than CO . The robust detection of CO by Emonts et al. (2011) towards MRC 0152209 is the exception that proves the rule, albeit at a modest redshift (). More often, has proved elusive (Ivison et al., 1996; Papadopoulos et al., 2005) or detections have been of a tentative nature (e.g. Greve, Ivison & Papadopoulos, 2004; Klamer et al., 2005).
Here, we present new observations of two HzRGs, 6C 1909+72 (also known as 4C 72.26 and TXS J1908+7220) and B3 J2330+3927 (Pentericci et al. 2000, DB03) using JVLA, the Institut de Radioastronomie Millimétrique’s Plateau de Bure Interferometer (IRAM PdBI) and Herschel111Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. (Pilbratt et al., 2010). The PACS and SPIRE instruments (Griffin et al., 2010; Poglitsch et al., 2010) aboard Herschel offer unprecedented sensitivity, with resolution well-matched to ground-based predecessors such as SCUBA (Holland et al., 1999). They cover the decade of wavelengths from 70 to 500 m with , which allows us to probe the peak of the spectral energy distribution (SED) of a dusty galaxy out to , thereby determining its star-formation rate (SFR, via its FIR luminosity, , across rest-frame 8–1,000 m, e.g. Kennicutt, 1998) and its characteristic dust temperature, . Beyond the local Universe, these quantities have only rarely been well constrained until now. Alternatively, we can adopt a reasonable value for and then estimate the redshifts of the SMGs seen in the vicinity of HzRG signposts (e.g. Stevens et al., 2003) using their rest-frame FIR colours (e.g. Eales et al., 2003; Greve et al., 2008; Penner et al., 2011), i.e. without resorting to conventional spectroscopy, thereby determining the likelihood that they inhabit a massive structure alongside the HzRG.
In the next section we describe an extensive set of observations, then present the reduced images, spectra and associated analysis in §3. §4 contains our interpretation of those data and discussion of their implications. We finish with our conclusions in §5. Throughout the paper we use a cosmology with km s Mpc, and .
|(J2000)||(J2000)||(mJy)||(km s)||(Jy km s)|
|B3 J2330+3927 (AGN host)||–||–||–||–|
|JVLA J233024.69+392708.6 (c)||23:30:24.690.02||+39:27:08.580.16|
|B3 J2330+3927 (AGN host)||23:30:24.850.02||+39:27:12.040.15|
|JVLA J233024.69+392708.6 (c)||23:30:24.620.02||+39:27:08.460.18|
The observations described hereafter targeted the HzRGs, 6C 1909+72 and B3 J2330+3927. The former presents a classic double-lobed (plus core) radio morphology, subtending 15 arcsec, and has a total 1.4-GHz flux density, mJy; the latter is brighter, mJy, with what appears at first sight to be a similar morphology, if more compact (2 arcsec), but which high-resolution radio imaging revealed to be an unusually one-sided jet driven by a compact, flat-spectrum core (Pérez-Torres & De Breuck, 2005). Alongside PKS 1138262, 6C 1909+72 and B3 J2330+3927 are the most luminous of the 69 HzRGs explored by Seymour et al. (2007) in the rest-frame near-IR using Spitzer.
2.1 JVLA observations
Whilst the JVLA was in its most compact configuration during 2011 September–November, we acquired amongst the first data taken in a new mode offering almost an order of magnitude more bandwidth than was previously possible, tuning to the CO transition at 115.27120256 GHz (Morton & Noreau, 1994) for our target HzRGs.
Short slots, usually 2–3 hr long, were scheduled dynamically to ensure excellent phase stability and transparency in the K and Ka atmospheric windows for 6C 1909+72 and B3 J2330+392, respectively. During these slots, data were recorded222A bug in the Correlator Back End computer during this period resulted in only 1 sec of data being saved per integration, meaning that a significant fraction of the useful data were lost. every 3 sec in each of contiguous baseband (dual-polarisation) pairs, each baseband comprising -MHz channels for a total dual-polarisation bandwidth of 2,048 MHz, well over 20,000 km s at the redshift of 6C 1909+72. Bright, compact calibration sources – flat-spectrum blazars lying within a few degrees of our target galaxies – were observed every few minutes to determine accurate complex gain solutions and bandpass corrections. 3C 48 and 3C 286 were also observed to set the absolute flux scale, and the pointing accuracy was checked locally every hour.
For 6C 1909+72 the two sets of eight contiguous baseband pairs were themselves placed contiguously. We shifted the CO line (expected at 25.434 GHz for ) by 180 MHz from the centre of the available bandwidth to avoid the edge of a baseband, or one of the end basebands.
For B3 J2330+3927, the CO line lies below 32 GHz in the Ka band. As such, only one set of eight contiguous baseband pairs could be deployed on the line (expected at 28.156 GHz for , but purposefully offset by 64MHz to avoid the edge of a baseband); the other set of eight basebands were tuned to 32.5 GHz.
The data were reduced and imaged using following the recipes described by Ivison et al. (2011), though with a number of significant changes: data were loaded using bdf2aips, avoiding any compression, and fring was used to optimise the delays, in software, based on 1 min of data for 3C 48 or 3C 286. The basebands were knitted together using the noifs task, yielding datasets with either 1,024 or 512 2-MHz channels. Continuum images were made, using all of these data to obtain very deep maps of the synchrotron emission at 25.4 and 28.2 GHz (for 6C 1909+72 and B3 J2330+3927, respectively), which was then cleaned and subtracted from the data (using uvsub). The channels were then imaged in groups of four (yielding 90 km s velocity resolution) over 3-arcmin-diameter fields, with natural weighting (robust = 5), to form large cubes centred on the radio galaxies. The resulting spatial resolutions were arcsec at position angle (PA), 171, for 6C 1909+72 and arcsec at PA, 72, for B3 J2330+3927.
2.2 IRAM PdBI imaging
Observations of 6C 1909+72 were made in CO using PdBI’s C and D configurations in the 3-mm atmospheric window during 2007 July (with five antennas) and December (with six antennas) in a series of 6–11-hr tracks with good phase stability (0.6–1.0 arcsec seeing) and transparency (1.5–2.5 mm of water vapour). The local calibrator, 1928+738, was used to track amplitude and phase, with 1642+690 used for an independent check. Absolute fluxes were calibrated using MWC 349. Data cubes were made via using a natural weighting scheme, yielding a arcsec beam with the major axis at a PA of 47.
The original PdBI data acquired by DB03 for B3 J2330+3927 were re-imaged using using a natural weighting scheme, discarding those channels observed only in the compact configuration, yielding a arcsec beam with the major axis at a PA of 72.
2.3 Herschel imaging
The Herschel data presented in this paper comprise a small part of the Herschel Radio Galaxy Evolution Project (HeRGE) OT1 programme (Seymour et al., 2012). With PACS, we made near-orthogonal scans of our HzRG targets at nominal speed (i.e. with the telescope tracking at 20 arcsec s) using mini-map mode, spending a total of 288 sec on source, recording data simultaneously at 100 and 160 m, each scan comprising -arcmin scan legs separated by 4 arcsec. With SPIRE we approached the confusion limit (Nguyen et al., 2010) with just three repetitions (111 sec on source) of its ‘small map’ mode, obtaining data simultaneously at 250, 350 and 500 m. The SPIRE images were produced using the standard pipeline; the PACS data were tackled with a variant of the pipeline developed by Ibar et al. (2010).
Flux densities for SPIRE were measured using sussextractor (Smith et al., 2012), as implemented in the Herschel Data Processing System. For PACS, we used apertures of radius 9 and 13.8 arcsec at 100 and 160 m, respectively, with appropriate aperture corrections, with errors determined by placing many random apertures in regions of the image with integration times similar to those of our targets.
3.1 6c 1909+72
Uncertainty does not include the contribution from confusion noise,
nor any possible systematic offset in flux calibration.
Astrometry is consistent with a significant fraction of the PACS emission originating from JVLA 233024.69+392708.6 (Fig. 3).
The synchrotron continuum emission from 6C 1909+72 dominates the observed flux density at 25 GHz, being almost an order of magnitude brighter than the CO line peak (Fig. 1, left). To assess the CO line properties we subtracted an accurate model of the continuum emission, exploiting the impressive bandwidth now available at JVLA.
In the resulting continuum-free JVLA datacube, CO line emission is seen clearly, centred at (Fig. 1, right), closely matching that seen in our CO spectrum from IRAM PdBI, . The basic characteristics of the CO line emission – peak and total flux densities, line widths, etc. – are listed in Table 1, for both transitions. It is notable that we measure Jy km s, cf. the Jy km s measured by P00, who may also have under-estimated the line width due to a lack of available bandwidth.
Both the CO and emission are partially resolved, even with a natural weighting scheme. Some velocity structure can be discerned in Fig. 1, common to both transitions, but there does not appear to be a strong, coherent gradient.
From the measured CO properties, we find k km s pc and the molecular gas mass, (H + He), is therefore likely around M, naively adopting M (k km s pc) (Downes & Solomon, 1998).
3.2 B3 J2330+3927
At the position of the radio galaxy core, as identified by Pérez-Torres & De Breuck (2005) and detected in CO using PdBI by (DB03), we find synchrotron emission ( mJy centred at J2000) but there is no evidence of CO emission in a synchrotron-subtracted data cube. The (3-) limit we can set, assuming the same line width as the CO emission (830 km s fwhm) and following Appendix A of Seaquist, Ivison & Hall (1995) is Jy km s. The basic characteristics of the CO line emission are again listed in Table 1, for both transitions.
Fig. 2 shows the Keck/NIRC -band imaging (from DB03, ) as a greyscale, superimposed with contours to show the emission in 8.4-GHz continuum (synchrotron), CO and CO . Approximately 4 arcsec to the SSW of the AGN (and 260 km s bluewards of its CO redshift) we do see emission in CO . It is coincident with what DB03 labelled component ‘c’ and tentatively suggested might lie at the same redshift as the radio galaxy. We show its spectra in CO and in Fig. 2. Our new CO imaging and our re-analysis of the CO data make it clear that component c is in fact a gas-rich galaxy in the immediate vicinity of the radio-loud AGN, which is presumably interacting with (and thereby triggering activity in) the host galaxy of the AGN. Component c is detected in at least three IRAC bands and so has a significant stellar population. It is not obvious that it contributes significantly to the Lyman halo that surrounds B3 J2330+3927 (Matsuda et al., 2009).
Fig. 3 reveals the dynamical structure of the CO emission, with a blue–red gradient leading from component c to beyond the radio-loud AGN. The continuum emission at , 250 and 850 m (rest-frame 40, 60 and 200 m) seen by PACS, SPIRE and SCUBA are also shown. At m, the emission is predominantly from the AGN; at and 160 m, it is predominantly from component c; at m the emission is centred between the two. We have evidence, then, that dust at a variety of temperatures is distributed throughout the system, yet we lack the spatial resolution to disentangle these components reliably.
The total flux in CO for component c, Jy km s. The excitation situation for component c is comparable to that of SMGs (Harris et al., 2010; Ivison et al., 2011), with a brightness temperature () ratio, (cf. – Bothwell et al., 2012). This differs strikingly from that of the radio galaxy core, where the ratio is entirely inconsistent with thermal emission, (), i.e. the emission is super-thermal333Strictly, super-thermal excitation exists only where local thermodynamic equilibrium has been violated by a population inversion. Here, we use it as short-hand for . (see §4.1).
DB03 assumed to extrapolate their CO measurement to and calculate a gas mass for B3 J2330+3927. We can see now that this will have resulted in an over-estimate of for the AGN host galaxy. Our limit on implies k km s pc and M where M (k km s pc).
For the gas-rich galaxy previously known as component c, which we re-name JVLA 233024.69+392708.6, we find k km s pc and M.
3.3 On the HzRGs and their environments
The over-density of bright SMGs found around HzRGs, their faintness at optical/IR wavebands and the alignment of those SMGs with the jets from the central AGN were taken as tentative evidence that HzRGs are signposting massive structures in which other galaxies are also undergoing intense starbursts (Ivison et al., 2000; Stevens et al., 2003).
Our deep Herschel imaging allows us to assess the likely redshifts of these bright neighbouring SMGs via their FIR/submm colours (e.g. Amblard et al., 2010, though such colours are only sensitive to – see Blain 1999). We report their FIR/submm flux densities in Table 2.
The upper panels of Fig. 4 show two of the colour-colour diagnostic plots employed by Amblard et al. (2010) to assess the redshift and of galaxies detected by H-ATLAS (Eales et al., 2010), probing their colours across the rest-frame 100-m bump. The coloured backgrounds indicate the redshifts of model SEDs. The new, lower panel of Fig. 4 is based on the same model SEDs and is designed to exploit information from our SCUBA 850-m imaging that is particularly relevant to galaxies at .
The red crosses represent B3 J2330+3927 and its brightest SMG neighbour, SMM J233019.1+392703. The position of the latter – particularly in the lower two plots – suggests strongly that it lies at a lower redshift than the radio galaxy, nearer the peak of the radio-detected SMG population at (e.g. Chapman et al., 2005).
The black crosses represent 6C 1909+72 and the very bright, nearby SMG, SMM J190827.5+721928. Their rest-frame FIR colours are remarkably similar in the lower two colour-colour plots. The two galaxies differ in , i.e. the colours on the Wien side of their SEDs. The limit for the bright companion is consistent with the known redshift of the radio galaxy, but the colour of the radio galaxy is bluer than we might expect. On balance, the evidence is consistent with the proposition that these two dusty starbursts share the same node or filament444A typical filament at spans of order Mpc (e.g. Springel et al., 2005). SMM J190827.5+721928 is separated by 45 arcsec (335 kpc) in the plane of the sky from 6C 1909+72. It would need to lie within to inhabit the same sheet or filament. of the cosmic web.
With the km s of velocity coverage available to us via the WIDAR correlator at JVLA, we should be sensitive to CO emission from bright SMGs in our target fields, should they have a similar ratio of and if – as seems likely, at least for 6C 1909+72 and SMM J190827.5+721928 – they lie in the same cosmic structure as the central HzRG.
We do not detect CO emission from SMM J233019.1+392703 in the B3 J2330+3927 field. However, this SMG is less than half as bright as the nearby HzRG its emission is attenuated severely (82 per cent) by the JVLA’s primary beam at 28.2 GHz, so this does not rule out the possibility that it shares the same structure at . We set a 3- upper limit of Jy km s where we have adopted a line width of 800 km s, typical for SMGs (e.g. Greve et al., 2005).
SMM J190827.5+721928, in the 6C 1909+72 field, is sufficiently bright that we would expect to detect its CO emission, given our km s  of velocity [redshift] coverage in this field, if it were to have a similar ratio of and be part of the same structure as the nearby HzRG. We searched the CO velocity cube at the position of the SMG, at spectral resolutions ranging from 95–380 km s. Despite being amongst the brightest known SMGs, its position is ill-defined since it lacks a convincing counterpart in the available Spitzer 3.6–24-m imaging. The most convincing peak lies at GHz, which would correspond to Jy km s at (after primary beam correction). The best-fit line width ( km s) would be narrower than the majority of SMG CO lines (Greve et al., 2005; Bothwell et al., 2012) and we do not view this as a robust detection, but it would be an obvious place to begin searching for CO with IRAM’s new WideX correlator. Several other 2–3- peaks are also present, but none with a line width even remotely commensurate with an SMG (e.g. Greve et al., 2005). The 1- level for a line of width 800 km s is Jy km s. We are forced to conclude that the presence of two unusually bright SMGs in this field may be due to the chance alignment of distant starbursts.
The lack of a convincing detection of CO towards the brightest SMG in the vicinity of 6C 1909+72 casts doubt on the idea that it co-habits a proto-cluster environment with the radio galaxy (Stevens et al., 2003). However, our Herschel imaging does hint at the presence of a dust-rich strand of the cosmic web centred on 6C 1909+72. Fig. 5 reveals a large (500 kpc) red structure, the majority of which is co-aligned with a prominent extension (150 kpc, somewhat larger than the 50-kpc scale of the northern radio jet – recall Fig. 1) seen previously at 850 m (Stevens et al., 2003). We might imagine this as a filament of gas that is being accreted slowly onto the AGN host galaxy, but in our view it is no less likely that this structure is due to the outflow of metal-rich material from the radio galaxy host, heated by a network of unresolved LIRGs. A dusty outflow could be driven by radiation pressure from a strong nuclear starburst and/or AGN on dust (e.g. Prochaska & Hennawi, 2009; Lehnert et al., 2011; Faucher-Giguere & Quataert, 2012; Roth et al., 2012; Wagner, Bicknell & Umemura, 2012) with material transported well beyond the AGN host galaxy, as witnessed in the near-IR on smaller scales for radio galaxies at (Nesvadba et al., 2008; Harrison et al., 2012), for M 87 in the Virgo cluster (Simionescu et al., 2008, 2009; Werner et al., 2010), for the brightest cluster galaxy (BCG) in the Hydra A cluster (Kirkpatrick et al., 2009; Gitti et al., 2011) and for BCGs more generally (Kirkpatrick et al., 2011; O’Sullivan et al., 2011). We note that the presence of 3,000-km s absorption features555For a 3,000-km s outflow, with a dynamical timescale of a few yr, a single epoch of starburst and/or AGN activity would be sufficient to generate a structure on the scale seen here. against an unpolarised optical continuum led Dey (1997) to classify 6C 1909+72 as a broad-absorption-line radio galaxy, and is consistent with a powerful outflow. Of course, we must also bear in mind the well-rehearsed argument that radio galaxies are most easily discovered when their jets are encountering a working surface (e.g. Barthel & Arnaud, 1996).
4.1 On the HzRG SEDs and ratios
Fig. 6 shows the observed radio-through-optical SEDs of 6C 1909+72 and B3 J2330+3927. For both galaxies, the photometry between 3 m and 74 MHz can be described adequately with a model comprising only synchrotron and thermal dust emission, following Kovács et al. (2010). Table 3 lists the results of a simultaneous fit to the following free parameters: the dominant cold dust temperature, , and the power-law index, , of a dust temperature distribution, , designed to offer a physically motivated treatment of the Wien side of the thermal emission spectrum (in Kovács et al., , the low-temperature cutoff of the distribution), the dust mass, (for a characteristic photon cross-section to mass ratio, m kg, from Dunne, Eales & Edmunds 2003, where the frequency dependence of the dust emissivity, , was fixed to +1.5) and the synchrotron power-law index, . The resulting measurements of and (as defined by Helou et al., 1985, but where is measured across m, as is ) are also listed.
Both radio galaxies have warmer dust temperatures, k, than those seen for similarly luminous, dusty starbursts at ( k when calculated using a power-law temperature distribution, as we have here – Magnelli et al., 2012). Determining the contribution of the AGN to is a long-standing problem that we cannot hope to solve without dramatically improved spatial resolution. The relatively warm dust does imply significant AGN contributions to for these two radio galaxies, though even this cannot be asserted with certainty since the brightest 100–160-m emission in the B3 J2330+3927 system (§3.2) is centred near the gas-rich companion (which may contain its own AGN, if it is anything like the interacting system seen towards 4C 60.07 – Ivison et al. 2008). On the other hand, the detection of dust and molecular gas towards the radio galaxy hosts makes it likely that significant levels of star formation are taking place in both. Given the strong dependence of on (e.g. Eales et al., 2000), we might expect that the fraction of due to the AGN, , where is the fractional AGN contribution to estimated for typical SMGs (e.g. Frayer et al., 1998). This suggests a 90-per-cent AGN-related contribution to for our HzRGs, which still leaves room for vigorous starburst activity in the radio galaxy hosts, with (Kennicutt, 1998).
The super-thermal ratio, , observed towards the AGN host galaxy, B3 J2330+3927, reveals the presence of highly excited molecular gas, given the excitation requirements of CO ( k and cm). Even for 6C 1909+72, the ratio, , is significantly higher than the average value seen for SMGs by Bothwell et al. (2012). IRAM PdBI and VLA observations of 4C 41.17 at revealed a similar story, with likely super-thermal (Papadopoulos et al., 2005). In comparison, the same line ratio averaged over the entire Milky Way is (Fixsen, Bennett & Mather, 1999), indicative of quiescent gas in the interstellar medium (ISM).
|Parameter||6C 1909+72||B3 J2330+3927|
In contrast to B3 J2330+3927, the ratio that we find for its companion galaxy, JVLA 233024.69+392708.6 () is consistent with the average value seen for SMGs and with star-forming regions within nearby starburst galaxies such as M 82 and NGC 253, which have values in the range 0.5–0.8 (Güsten et al., 1993; Mao et al., 2000). The ratio in JVLA 233024.69+392708.6 is thus consistent with gas heated by UV radiation from star formation, i.e. less extreme ISM conditions than those of the radio galaxy.
ratios as high as that observed towards B3 J2330+3927 are expected in environments where the gas energetics are dominated by X-rays. These can penetrate large columns of gas and maintain a high kinetic gas temperature, , much deeper into molecular clouds than far-ultraviolet (FUV) photons (e.g. Meijerink, Spaans & Israel, 2006; Schleicher, Spaans & Klessen, 2010; van der Werf et al., 2010), with this gas thermally decoupled from the cooler dust. Powerful radio-loud AGN are often found to have high X-ray luminosities, capable of powering extended X-ray-dominated regions (XDRs); while B3 J2330+3927 has no X-ray measurements, this is one possible explanation for the observed .
Another possible explanation is shock-excited gas (e.g. Flower & Pineau Des Forêts, 2010) caused by its radio jet ramming into the molecular environment, bearing in mind that the mechanical power of a radio jet can be orders of magnitude larger than that of its synchrotron emission. An example of this is observed in the powerful radio galaxy, 3C 293, which has Milky Way-like levels of star formation and a very low X-ray luminosity (see also Matsushita et al., 2004, regarding the central region of M 51). Its super-thermal CO (as measured in CO and ) has been attributed to turbulent heating from the dissipation of shocks caused by the interaction of its powerful radio jet with its molecular gas (Papadopoulos et al., 2008; Nesvadba et al., 2010; Guillard et al., 2012) – an interaction that seemingly also drives a 1,400-km s outflow in H i (Morganti et al., 2003). Such mechanisms are often proposed to provide the feedback needed to reconcile models of galaxy formation with a variety of observations (e.g. Granato et al., 2004; McNamara & Nulsen, 2012; Fabian, 2012, and references therein).
A large fraction of the radio emission from B3 J2330+3927 comes from its core (50 per cent at 8.4 GHz) and it displays an unusually one-sided jet: ‘one of the most asymmetric radio structures ever reported for a type ii AGN’ (Pérez-Torres & De Breuck, 2005). This may be due to relativistic Doppler beaming (e.g. Riley & Warner, 1990) or because of the launch mechanism (e.g. Chagelishvili, Bodo & Trussoni, 1996) but if a survey of such galaxies were to find relatively high ratios relative to galaxies hosting two-sided jets then it would suggest a more prosaic reason: a blockage that causes an unusually high fraction of the mechanical energy from AGN-driven outflow to be deposited into the molecular gas.
According to Wagner & Bicknell (2011), the kinetic energy and momentum of a jet can be transfered to dense gas with a high efficiency (10–70 per cent), causing turbulent heating and shock excitation of the molecular gas and – in extreme cases – driving gas outflows and inhibiting star formation. The kinetic energy of the radio jet in B3 J2330+3927 can be estimated following Punsly (2005) and Punsly et al. (2008), assuming Jy (from Jy and Douglas et al., 1996; Cohen et al., 2007, though see also Bîrzan et al. 2008 and Cavagnolo et al. 2010). The jet’s kinetic luminosity, , is then erg s, roughly two orders of magnitude larger than the CO luminosity, erg s. This simplistic approach assumes that the 151-MHz emission comprises optically thin radiation from the lobes, whereas a substantial fraction may come from a Doppler-boosted jet. Nevertheless, over the 10-Myr lifetime of the radio source we can expect a few erg to be deposited into the multi-phase ISM (enough to quench a cooling flow for yr, as argued by McNamara et al., 2005). The turbulent kinetic energy of the molecular gas can be calculated through , where fwhm is the velocity dispersion of the gas. From §3.2, M so if km s and is solely due to the velocity dispersion of the gas (i.e. no components due to rotation or bulk outflow), then erg. We can thus conclude, as did (as did Nesvadba et al., 2008, for an analagous situation) that the radio jet carries sufficient energy to explain the observed CO characteristics. The jet can influence the gas properties in precisely the way required to stop star formation, though questions clearly remain about the ultimate fate of the gas and deeper observations are required to search for any high-velocity molecular component (see, e.g., Polletta et al., 2011).
Taking another approach, if we adopt a fiducial jet advance speed, , then the gas is heated to k, where is the reduced mass and is Boltzmann’s constant. For a large fraction of the gas that has been shock-heated by the jet, the density will be low (– cm, Wagner & Bicknell, 2011). This over-pressurised gas will expand (Begelman & Cioffi, 1989), running into the denser, ambient ISM, which will be engulfed and destroyed by the resulting shock (e.g. Klein, McKee & Colella, 1994). The velocity of the shock that is driven into the dense clouds, , where is the density of the hot plasma created by the outward-moving radio jet and is the density of the engulfed ISM. For typical molecular clouds, 10 cm, so we arrive at –40 km s (Begelman & Cioffi, 1989). Such molecular shocks would be significant emitters in high- CO lines (Flower & Pineau Des Forêts, 2010).
At what rate must the gas must be shock-heated to maintain this CO emission? For a C-type shock wave with km s (assuming a lower velocity or a denser ambient ISM will make little difference, and a J-type shock will produce a line luminosity almost two orders of magnitude fainter), we find that to explain requires a mass shock rate of M yr (Flower & Pineau Des Forêts, 2010). Over the age of the jet, only a few M of gas needs to be shock heated, perhaps much less if this is a special, short-lived phase due to the initial passage of the radio jet. The gas may return to a higher density following the passage of the shock (Cooper et al., 2008), perhaps re-forming H (Guillard et al., 2009), so the amount of shock-heated gas required to explain the observed ratios is not restrictive.
Whatever the excitation mechanism, the CO optical depth must be moderate to low () since in the optically thick regime the of a transition is equal to the excitation temperature, i.e. (with the latter being the same for all levels when thermalised – see Papadopoulos et al. 2011). In the optically thin case, and since (at least up to ), we find (true for thermalised gas, but also for sub-thermal gas if the effect due to optical depth is sufficiently large). Optically thin CO emission can be achieved if the medium is highly turbulent, as is the case for the Galactic Centre – a local region where the CO ladder is observed to be super-thermal. The super-thermal ratio observed towards B3 J2330+3927 is thus likely an indication of mechanical energy deposited into the ISM from the radio jet and/or the ongoing starburst, inducing a degree of turbulence sufficient to make the CO lines optically thin, allied with a high density () and a relatively high kinetic temperature; moreover, this must happen globally, rather than in some small fraction of the gas, as seen in the nucleus of M 82, for example (Knapp et al., 1980; Weiß et al., 2001).
4.2 The merger fraction amongst starbursting HzRGs
The degree to which various populations of star-forming galaxies are driven by violent mergers of gas-rich galaxies or by the accretion of cold gas streams – sometimes smooth, sometimes clumpy – via dark matter filaments into spiral disks (Dekel, Sari & Ceverino, 2009) has been a topic of intense debate in recent years. Even for SMGs – the most extreme star-formation events in the Universe, where the evidence for merger-driven activity was thought by many to be overwhelming (Engel et al., 2010) – the seeds of doubt have been sown.
What of radio galaxies? We cannot generalise to the entire population, even though that population is small in number: the time-consuming interferometric observations of molecular gas that are capable of laying bare the processes leading to star formation have generally been obtained towards systems from which rest-frame FIR emission has already been detected, i.e. we are limited to systems pre-selected to be undergoing intense starbursts. Even so, it is interesting to add our findings to those studies of radio galaxies in the literature and explore the overall statistics. Taking the eight systems listed in §1 in order of decreasing redshift:
The interferometric CO study of 4C 41.17, at , by De Breuck et al. (2005) revealed activity driven by a merger between two massive, gas-rich galaxies separated by 2 arcsec (13 kpc) and 400 km s.
High-resolution interferometric imaging of 4C 60.07 at by Ivison et al. (2008) revealed two components of roughly equal integrated flux, separated by 30 kpc – evidence of an early-stage merger between the host galaxy of an actively-fueled black hole (the HzRG), and a gas-rich starburst/AGN, caught prior to its eventual equilibrium state. This separation is typical of SMGs with multiple radio identifications (Ivison et al., 2007) and for the intense burst of star formation near first passage in merger simulations (e.g. Springel, Di Matteo & Hernquist, 2005).
Smith et al. (2010) use rest-frame UV spectroscopy to show that 6C 1909+72, at , comprises two galaxies separated by 1,300 km s along the line of sight, with the radio-loud AGN hosted by the more distant component.
TN J0121+1320 at was detected in CO by De Breuck et al. (2003). There were no obvious signs of a gas-rich companion, but the available sensitivity, velocity coverage and spatial resolution were poor compared to recent studies.
We have shown here that the immediate environment of B3 J2330+3927 at contains at least two gas-rich components, again separated by 30 kpc.
Nesvadba et al. (2009) presented interferometric CO imaging of the radio galaxy, TXS 0828+193, finding two large gas reservoirs separated by 720 km s along a shared line of sight roughly 80 kpc SW of the AGN host galaxy, which was itself detected only in synchrotron at 96.6 GHz.
53W002 at was observed and detected in CO (Yamada et al., 1995; Scoville et al., 1997) without knowledge of its FIR luminosity, though it was known to sit amongst a clump of Ly- emitting galaxies (Pascarelle et al., 1996) through which the radio galaxy is suspected to be assembling a large fraction of its eventual stellar mass through mergers (e.g. Motohara et al., 2001). At least one of these – 330 kpc and 300 km s from 53W002 – is FIR-luminous (Smail et al., 2003). Based on their CO imaging, Scoville et al. (1997) concluded ‘it is clear that the emission is resolved’, but this was not corroborated by Alloin et al. (2000). However, with a 6-arcsec synthesised beam the latter data would be insensitive to close companions.
Finally, Hubble Space Telescope near-IR imaging of MRC 0152209 at revealed a morphology consistent with an advanced-stage merger (Pentericci et al., 2001) where we would expect the gas to have been driven into a compact configuration, well within the 7–10-arcsec fwhm synthesised beam of the available CO observations (Emonts et al., 2011).
Overall, we have six mergers or close galaxy pairs, one ambiguous case, and one case where no merger activity can be discerned (albeit with relatively poor sensitivity and resolution available). The statistics suggest, then, that violent interactions are as ubiquitous amongst starbursting high-redshift radio galaxies as they are amongst SMGs (Engel et al., 2010).
Combining our new interferometric CO data for two distant radio galaxies with those for six more, reported elsewhere, we find that activity in starbursting radio-loud AGN at high redshift is driven by the merger or interaction of two or more systems in which significant masses of molecular gas and stars have already formed, rather than the steady666Often denoted ‘secular’, which in astronomy indicates that a process occurs continuously, as opposed to a discrete or periodic event such as a merger. accretion of cold gas from the cosmic web.
We introduce a new colour-colour diagnostic plot to constrain the redshifts of several distant, dusty galaxies found previously in our target fields. We conclude that the SMG south of 6C 1909+72 likely shares the same node or filament, but we fail to detect this FIR-luminous galaxy in CO, despite 20,000 km s of velocity coverage.
We introduce a new colour-colour diagnostic plot, exploiting the power of 350–850-m photometry to constrain the redshifts of several distant, dusty galaxies found previously in our target fields. We conclude that the bright SMG near 6C 1909+72 likely shares the same node or filament of the cosmic web at as the signpost AGN. However, we fail to detect this SMG in CO , despite our 20,000 km s of velocity coverage.
Also in the 6C 1909+72 field, we find an unusually large, red dust feature, aligned with the radio jet. We suggest that metal-rich material may have been dispersed on 100-kpc scales by a collimated outflow, reminiscent of the jet-oriented metal enrichment seen in X-ray observations of local cluster environments.
We find that CO brightness temperature ratios in the host galaxies of radio-loud AGN are significantly higher than those seen in similarly intense starbursts where AGN activity is less pronounced. In our most extreme example, the super-thermal ratio suggests that significant energy is being deposited rapidly into the molecular gas via X-rays and/or mechanical (‘quasar-mode’) feedback from the AGN, leading to a high degree of turbulence globally and a low optical depth in CO – feedback that will lead to the cessation of star formation on a timescale commensurate with that of the jet activity, 10 Myr.
We are grateful to Arjun Dey, Axel Weiss and Padelis Papadopoulos for sharing their wisdom regarding super-thermal ratios and unpublished optical spectroscopy. We appreciate the remarkable efforts of the NRAO staff that have significantly upgraded what many of us already regarded as the finest telescope ever built. In particular, we thank Frazer Owen and Gustaaf Van Moorsel for their invaluable help with the data used here. IRS acknowledges support from STFC and through a Leverhulme Senior Fellowship. NS is the recipient of an Australian Research Council Future Fellowship. TRG acknowledges support from STFC, as well as IDA and DARK. This work is based on observations carried out with the Karl Janksy Very Large Array. The NRAO is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. It is also based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
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