Study of galaxies in the Lynx-Cancer void. I. Sample description
The evolution of galaxies is influenced by the environment in which they reside. This effect should be strongest for the least-mass and -luminosity galaxies. To study dwarf galaxies in extremely low density environments we have compiled a deep catalogue of dwarf galaxies in the nearby Lynx-Cancer void. This void hosts some of the most metal-poor dwarfs known to date. It borders the Local Volume at the negative supergalactic coordinates and has the size of more than 16 Mpc. With a distance to its centre of only 18 Mpc it is close enough to allow the search for the faintest dwarfs. Within the void 75 dwarf (–11.9 –18.0) and 4 subluminous (–18.0 –18.4) galaxies have been identified. We present the parameters of the void galaxies and give a detailed analysis of the completeness of the catalogue as a function of magnitude and surface brightness. The catalogue appears almost complete to –14 mag, but misses part of the fainter low surface brightness (LSB) face-on galaxies. This sample of void galaxies builds the basis of forthcoming observational studies that will give insight into the main stellar population, Hi-mass-to-light ratio, metallicity and age for comparison with dwarfs in higher density regions. We briefly summarize the information on the unusual objects in the void and conclude that their concentration hints that voids are environments that are favourable for finding and studying unevolved dwarf galaxies.
keywords:galaxies: dwarf – galaxies: evolution – galaxies: distances and redshifts – galaxies: luminosity function – large-scale structure of Universe
The modern models of large-scale structure and galaxy formation, including the state-of-art N-body simulations, predict that galaxy properties and evolution significantly depend on global environment (e.g. Peebles, 2001; Mathis & White, 2002; Tully et al., 2002; Gottlöber et al., 2003; Hoeft et al., 2006; Arkhipova et al., 2007; Hahn et al., 2007, 2009, and references therein). While the effect of a denser environment on galaxy properties and evolution has been known for a rather long time (e.g. Haynes, Giovanelli & Chincarini, 1984; Boselli & Gavazzi, 2006), the role of the most rarefied environment such as voids on galaxy formation and evolution is less studied, either theoretically and observationally.
The latter is due to observational selection effects. Most galaxies with known radial velocities are found in spectral surveys of magnitude-limited samples. Wide-field spectral surveys have typical apparent magnitude limits corresponding roughly to 18 mag. This apparent magnitude limit implies that for distances well beyond the Local Supercluster ( km s), where the great majority of large voids (with sizes of 20–40 Mpc) were found, the faintest selected galaxies will have absolute magnitudes of –16 mag or brighter. This implies that the study of distant voids is limited to galaxies 3–4 mag fainter than galaxies (–19.5 to –20.0 mag). galaxies are typically the ones that mark the borders of voids. Therefore, even the most advanced studies of the void galaxy population, based on very large samples with redshifts from the Sloan Digital Sky Survey (SDSS) (e.g. Sorrentino, Antonuccio-Delogu & Rifatto, 2006; Patiri et al., 2006) of 0.03–0.05 (), were limited to galaxies with luminosities of only 2 magnitudes fainter than . Only studies of photometric and spectroscopic properties for SDSS samples with 0.025, (Rojas et al., 2004, 2005) were able to probe less luminous galaxies. A new detailed study of the properties of galaxies in voids, described by Stanonik et al. (2010) and van de Weygaert et al. (2009) also mainly deals with more luminous dwarf galaxies, namely with –16.0 mag.
The smaller a galaxy, the more fragile it is in respect to external disturbance. Therefore, the possible difference of galaxy properties in various types of environments is expected to depend on galaxy mass. Thus, the relatively ‘shallow’ probes of ‘distant’ void galaxy population based on the SDSS samples leave significant room for deeper insight into the question.
|km s||hour||°||km s||°||°||km s||km s||km s|
|Inner Local Void||2000||18.5||–01||900||30||+02||–500||–200||700|
Earlier studies found galaxies in voids to typically be low-mass actively star-forming galaxies, like BCGs and Hii galaxies (in particular, Salzer, 1989; Pustilnik et al., 1995; Popescu, Hopp & Elsässer, 1997). Lindner et al. (1996) have shown that BCGs are not the only objects in voids. More typical dwarfs with lower star formation (SF) activity also populate voids. Statistical studies of optical properties of void galaxies in the SDSS data did not provide further clues to the evolutionary state of these galaxies, but confirmed the increased fraction of blue galaxies and Star Formation Rate (SFR). The only proxy of evolutionary parameters for void galaxies studied up to now, is the ratio , but only for a limited sample of BCGs (Pustilnik et al., 2002). The recent study of void galaxies via Hi imaging by Stanonik et al. (2010) is less selective, but still is rather limited with regard to luminosity.
The amount of evidence that BCGs in underdense regions may represent a less evolved population was growing during the last decade (e.g. Peebles, 2001; Pustilnik et al., 2003, 2004, 2006). However, this might be due to selection effects which favour actively star-forming galaxies. Therefore, there is a need to address this issue by defining samples that also contain the more typical late-type galaxies, particularly because void environments are thought to be favourable for sufficiently quiet galaxy evolution. Hence, one can hope to use the statistics of void galaxy ensembles as a good instrument for comparison with predictions of cosmological galaxy formation scenarios. Such large and deep samples of void galaxies would also allow a more detailed comparison with predictions from cosmological N-body simulations (e.g. Tikhonov & Klypin, 2009).
To construct void galaxy samples with absolute magnitudes down to –12, from samples with the typical apparent magnitude limit of 18–19, one needs to limit the distant boundary by 10–16 Mpc. The nearest voids (e.g. Fairall, 1998), adjacent to the Local Volume (distances of 10 Mpc, as defined, e.g. by Karachentsev et al., 2004) are suitable for this task. Due to their relative proximity (10–15 Mpc), the low-luminosity galaxies can be relatively easily identified. Moreover, spectroscopic studies of their element abundances in the relatively faint Hii regions of these fairly nearby galaxies is feasible with modern large telescopes.
In this paper we present the sample of galaxies residing in one of the nearest, Lynx-Cancer, voids mentioned in Pustilnik et al. (2003). The choice of the void is motivated by a good coverage of this sky region by the SDSS spectral and image databases (Fukugita et al., 1996; Gunn et al., 2003; York et al., 2000; Smith et al., 2002; Pier et al., 2003), which leads to a significant increase in the number of known void galaxies and provides the photometric properties of the void galaxies. The study of this sample is also motivated by the prominent concentration of atypical dwarf galaxies in this volume. Half a dozen objects in this relatively modest void are very metal-poor galaxies and/or reveal no visible old stellar population (Pustilnik et al., 2003; Pustilnik, Kniazev & Pramskij, 2005a; Pustilnik et al., 2010, Pustilnik et al. 2011, MNRAS, submitted).
The lay-out of the paper is as follows. In Section 2 we summarize information on the nearby voids, and describe the Lynx-Cancer void. Section 3 presents the void galaxy sample. In Section 4 we discuss the completeness of the void galaxy sample, summarize the properties of the most unusual void galaxies and the prospects to increase this galaxy sample with LSB dwarf galaxies that are missed in the SDSS spectral database. Section 5 presents the summary of the main results. We adopt the value of Hubble constant as =73 km s Mpc.
2 Nearby voids. The Lynx-Cancer void description
2.1 Nearby voids
The issue of the nearest voids was briefly addressed by Fairall (1998). In Table 1 we summarise (ranked on the void size, apart from the Lynx-Cancer subvoid) and update on the main parameters of the nearby voids. In comparison to the original Fairall (1998) list, the Inner Local Void from Tully et al. (2008) is added. The misprint is corrected for RA of the Monoceros void centre (A. Fairall, private communication) and for all the related parameters as well. Finally, the Lynx-Cancer void and its subvoid are added with their parameters. The void maximal extent in units of km s is shown in column 2. In columns 3 and 4 we present the approximate equatorial coordinates of the void centres. Column 5 gives the distances to the void centres ( relative to the restframe of the Local Group in km s). In columns 6 and 7 the Galactic longitudes and latitudes of the void centres are given, respectively, while columns 8, 9 and 10 present their approximate supergalactic and coordinates, respectively.
The giant Local Void described by Tully et al. (2008) begins close to the Local Group and its neighbouring groups at the positive supergalactic . The Local Void appears pretty empty. Its expected population of dwarf galaxies remains almost elusive. Whether this is the effect of obscuration by the Galaxy disc or the intrinsic property of the Local Void, or the combination of both, remains unclear. While other nearby voids are also interesting for detailed studies of their galaxy population, we concentrate here on the Lynx-Cancer void.
The Lynx-Cancer void was discovered as a result of a study of the very low-metallicity BCG HS 0822+3542 (one of many such galaxies found in the Hamburg–SAO Survey; Ugryumov et al., 1999; Pustilnik et al., 2005b, and references therein) and its companion, the LSB dwarf galaxy SAO 0822+3545. Pustilnik et al. (2003) noticed that they are situated in a very rarefied environment. This was the motivation for a more careful analysis of the galaxy content in this volume which led to the identification of another nearby void, similar to those described by Fairall (1998). This void is adjacent to the Monoceros void and they probably represent the parts of a larger ’empty’ volume at the negative . In comparison to other nearby voids, the Lynx-Cancer void has two advantages: (a) the major part of the respective sky region is covered by the SDSS imaging and spectroscopy database (York et al., 2000; Abazajian et al., 2009, and references therein), and (b) a large part of this sky region will be covered by the blind high-sensitivity HI surveys, including the Arecibo survey ALFALFA (e.g. Haynes, 2008), the Australian survey ASKAP (Johnston et al., 2008), the Westerbork survey Apertif (e.g. Oosterloo, Verheijen, van Cappelen, 2008) and the Effelsberg survey EBHIS (e.g. Winkel et al., 2010).
Surprisingly, several other very metal-poor dwarf galaxies, including two objects with 1/30 [or 12+7.16], were found in this volume (Pustilnik, Kniazev & Pramskij, 2005a; Izotov & Thuan, 2007; Pustilnik et al., 2010, see details in Section 4). These and other findings hint at the probable effect of the void environment on dwarf galaxy formation and evolution. However, to study the evolutionary status of void dwarf galaxies statistically, one needs to describe the void geometry and boundaries more carefully, to better define the dwarf galaxy sample falling within the void. The statistical approach also requires a well-defined control sample in regions of higher galaxy densities. Fortunately, the detailed studies of a large number of dwarf galaxies have appeared recently, which can be used for comparison; such as the FIGGS sample of 65 faint ( –15.7) late-type dwarfs (Begum et al., 2008), of which 3/4 belong to the Local Volume groups.
2.2 Luminous galaxies delineating the Lynx-Cancer void
We show the boundaries of the Lynx-Cancer void, as delineated by individual ‘luminous’ galaxies (here, with 19.0), or the individual galaxies of pairs and groups (large dots) in Fig. 1. A sample of luminous galaxies, delineating the void, is presented and discussed below. Their lists are given in Tables 3 and 4 in Appendix A.
Constructing this bordering luminous/massive object sample, we first picked up a greater number of luminous objects in the surrounding volume, defined by the sky region with R.A. between 6 h and 11 h and Declinations of 0° and radial velocities 1800 km s, or distances 30 Mpc. Then, analysing the intermediate results in 3D pictures, we reduced the sample of luminous galaxies in order to leave only those objects which really define the boundaries of this void. For galaxies with known non-redshift distances, either from the photometric methods - cepheid, Tip of Red Giant Branch (TRGB), or surface-brightness fluctuations (SBF) or from the Tully-Fisher luminosity-rotation velocity relation (see ‘The Extragalactic Distance Database’, Tully et al., 2009), we computed the ‘distance’ velocity as follows: = 73 (Mpc).
Tully et al. (2008) noted that galaxies in the region considered here have a large peculiar radial velocity component. This is induced by the giant Local Void being situated approximately on the opposite side of the Local Sheet. The geometry is illustrated in Fig. 8 in the appendix. It shows on a larger scale the position of the Lynx-Cancer void in supergalactic coordinates in two slices, together with both the Local Sheet and the Local Void. The Local Sheet galaxies (including the Local Group), being at the Local Void border, move with =323 km s along the vector n, directed to the sky position with coordinates of =220°, =32° (Tully et al., 2008). To account for this large peculiar velocity, the galaxies for which only redshifts are available require a correction , where = 323() km s with being the angle between the vector n and direction to a galaxy.
To verify the choice of such a correction, we compared the resulting for 21 galaxies in the considered region, for which the independent reliable distances are known from TRGB, cepheid or SBF methods. The derived weighted mean difference of –932 km s is consistent with no systematic difference between the real value and that derived through .
2.3 The Lynx-Cancer void description
Real voids are not perfect spheres. But as the first approximation of a void, we use the largest empty spheres which can be inscribed in the distribution of dots, representing ‘luminous’ galaxies. To find such spheres, we accounted for the borders of the studied volume mentioned in Sec. 2.2. The largest sphere appeared significantly larger and more distant (‘Lynx-Cancer main’ void in Table 1) than that previously suggested in Pustilnik et al. (2003). However, we also identified a smaller sphere, significantly intersecting with the former one (called the Lynx-Cancer subvoid). Its parameters are similar to those of the originally described Lynx-Cancer void from Pustilnik et al. (2003). Both of these spheres are shown in contours in the right-hand panel of Fig. 1. The centre of the main void with radius 8.2 Mpc is at distance 18.0 Mpc. The centre of the subvoid with 6.0 Mpc is at 14.6 Mpc.
As described above, in many cases the form of real voids is nonspherical. Also, the modelling of voids (e.g. Lavaux & Wandelt, 2010, and references therein) indicates that smaller voids are expected to be more elongated. Therefore, for the subsequent analysis we not only assign to the Lynx-Cancer void region the interior of the two maximal spheres described above, which includes 45 galaxies with 2 Mpc, but also examine galaxies in the adjacent empty regions between the bordering luminous objects and the surfaces of these empty spheres. If these galaxies fall in the regions situated far from luminous galaxies ( 2 Mpc), these regions are treated also as parts of the void and the related galaxies are also qualified as void objects. About half of the dwarf galaxies classified as void objects in Section 3 belong to these empty regions outside the well-defined maximal void spheres.
3 The dwarf galaxy sample within the Lynx-Cancer void
A sample of 75 dwarf ( –11.9 –17.9) and 4 subluminous [–(18.0–18.3)] galaxies, falling within the Lynx-Cancer void, is presented in Table 2. To separate this sample, we proceeded as follows. First, we selected in two steps all isolated galaxies within the void region. At the first step, all galaxies were considered isolated if they had a projected distance to a luminous nearest neighbour 1 Mpc. At the second step, for the void galaxies with luminous neighbours closer than 1 Mpc in projection, we used a finer criterion, corresponding to the results on satellites of massive galaxies, presented by Prada et al. (2003).
In the analysis of a very large sample of SDSS galaxies, Prada et al. (2003) have shown that for a galaxy with luminosity , the relative r.m.s. line-of-sight velocities of satellites, , change from 120 km s at 20 kpc to 80 km s at 200 kpc, and to 60 km s at 350 kpc. The satellite velocities at large distances scale as of the host galaxy. To qualify the status of a small galaxy, we took these results into account. Namely, based on the and the projected distance of the nearest luminous galaxy, we estimated the respective value of 2 following the results of Prada et al. (2003). If V for the small galaxy in question is larger than the estimated 2, this dwarf was treated as unrelated to the luminous neighbour, that is as an isolated object. Following Karachentsev (2005), we adopted orbital masses in the range (2.61.3)10 for groups. According to Prada et al. (2003), this implies values of of 65-100 km s at 350 kpc, and a factor of 1.5 larger at 100 kpc. Once the isolated galaxies are selected, the distance to the nearest luminous neighbour was then calculated as the length of the 3D radius-vector between the two objects.
Finally, we assign an isolated galaxy to the void galaxy sample if it falls inside the maximal sphere of the main Lynx-Cancer void or its smaller subvoid, as described above, and have 2.0 Mpc. As noted in Section 2.3, we also included in the void sample all isolated galaxies which, due to non-sphericity of the void, appear in the adjacent regions somewhat outside the respective spheres and have 2.0 Mpc.
For two late-type spiral galaxies IC 2233 and NGC 2537, situated on the sky close to each other, we used distances from the detailed study by Matthews & Uson (2008), which finds no evidence or traces of interaction between these two galaxies. A good TRGB distance estimate is known only for the almost edge-on LSB spiral IC 2233. We adopt for this galaxy the mean distance module of three discussed values by Matthews & Uson (2008): =30.15 mag, respectively 10.70.5 Mpc. The less accurate NGC 2537 distance estimators are consistent with the galaxies to be unrelated (see Matthews & Uson, 2008, for references and detailed discussion). We adopted for the latter galaxy the distance determined from its radial velocity and general peculiar velocity correction, which appears consistent with the estimates in literature within their accuracies.
In Table 2 we present the following information on galaxies
falling into the Lynx-Cancer void.
Column 1. Common name or SDSS prefix.
Columns 2 and 3. Epoch J2000 R.A. and Declination.
Columns 4 and 5. and its error (almost all are either from NED or SDSS). For several objects either without SDSS/NED velocities, or for which this is significantly improved or corrected, we give the latter values. They include PGC2807187 (Karachentsev et al., 2008); pair HS 0822+3542 and SAO 0822+3545, KISSB 23 and SDSS J0926+3343 (Chengalur et al., 2006; Pustilnik & Martin, 2007; Pustilnik et al., 2010); UGC 3912 (Springob et al., 2005), SDSS J0723+3621, SDSS J0723+3622, SDSS J0737+4724, SDSS J0852+1351 (Pustilnik et al., MNRAS, submitted), NGC 2537 and IC 2233 (Matthews & Uson, 2008).
Column 6. The respective .
Column 7. The velocity (in km s) corresponds to (Mpc)73 when the photometric (TRGB, cepheids) distance estimate (Mpc) is available (only for 4 galaxies, marked by , typical accuracy 10%). Otherwise, this is + , where 300 km s is a correction for the peculiar velocity described in Section 2.2. In this case the distance accuracy comes from the quadratic sum of ) and ). The first term is 10–15 km s for about 3/4 of all sample galaxies, rising on average to 30–40 km s for the rest objects. The second term is 25 km s (Tully et al., 2008). Hence, for about 3/4 of the sample galaxies the typical uncertainty ) is 30 km s, or 0.4 Mpc. For the remaining galaxies, the typical ) is 40–45 km s, or 0.6 Mpc.
Columns 8 and 9. (from NED or from the literature) and the Galaxy extinction (from NED, following Schlegel et al., 1998). The sources of -mag are given by a letter in the superscript for the respective values in the following order. , Karachentsev et al. (2008); , Paturel et al. (2000, 2003); , Barazza et al. (2001); , de Vaucouleurs et al. (1991); , mean of photographic magnitudes from UGC and MCG catalogs, transformed to -band; , Karachentsev et al. (2001); , Karachentsev et al. (2004); , van Zee (2000); , Garnier et al. (1996); , Matthews & Uson (2008); , Pustilnik et al. (2003); , Pustilnik et al. (2010); , Salzer et al. (2002); , Pustilnik, Kniazev & Pramskij (2005a), , Pustilnik et al., MNRAS, submitted. When was unavailable in the literature, we used the brightest (model) values of the SDSS and -filter magnitudes of those presented in NED (if present, or directly from SDSS DR7) and transformed them to -band magnitudes according to the Lupton et al. (2005) formulae [; = 0.0107]. This case is marked by superscript after the value of . The resulting rms accuracy for the great majority of the sample galaxies is 0.1–0.2 mag. However, for several galaxies outside the SDSS zone, for which the CCD photometry appeared unavailable, the can be as large as 0.5 mag.
Column 10. It presents , corrected for , calculated from values in Columns 8 and 9 and the distance (Mpc), corresponding to , that is =73.
Column 11. (Tentative) morphological class.
Column 12. The distance in Mpc to the nearest luminous galaxy or group, .
Column 13. Either an alternative name or some important comments, like the presence of companion, etc.
4 Properties of the void galaxy sample and related issues
4.1 Global properties
The main goal of selecting a galaxy sample falling inside the Lynx-Cancer void is to study evolutionary parameters and to perform a comparison with similar parameters of dwarfs in a more typical and denser environment. The evolutionary parameters can also depend on galaxy global parameters, such as mass or luminosity. Therefore, we present and briefly summarize the distributions of void galaxies with respect to , and morphological types. As one can see in the left-hand panel of Fig. 6, about half of the void galaxies have in the range of 2.0–3.5 Mpc, which is close to the adopted threshold value of 2.0 Mpc. This corresponds to the known fact that a large fraction of void galaxies reside close to the void boundaries. Very isolated galaxies, with from 5 to 13 Mpc comprise 16% of the sample. The blue luminosities of the void galaxies, , spread over a range of more than six magnitudes (Fig. 6, right-hand panel), from –12.0 to –18.3, with the median and peak value of the histogram near –14.5. The latter indicates the substantial incompleteness of this void sample for –14.0.
A preliminary visual analysis of the void galaxy images in Figs 2–5 shows that about half of them appear Low Surface Brightness (LSB) objects. Two objects are classified as blue compact galaxies (BCGs). These are HS 0822+3542 and HS 1013+3809, both having very low O/H. They both are the components of pairs with more massive galaxies (Kniazev, Pustilnik & Ugryumov, 1998; Kniazev et al., 2000; Pustilnik et al., 2003; Kniazev et al., 2003; Pustilnik & Martin, 2007). One more galaxy - NGC 2537=Mkn 86 is also classified as a BCG in some works. The remaining objects are either more or less typical late-type dwarf spirals or irregular galaxies with the ‘normal’ level of current SF. More accurate data on the LSB galaxy fraction will be given in a forthcoming paper where the fitting surface brightness (SB) radial profiles is performed and the value of central SB of the underlying discs is estimated.
4.2 Completeness of the void galaxy sample
The study of the luminosity function of void galaxies for absolute magnitudes –14 is very important for comparison with the results of CDM cosmological simulations. In particular, the question of how faint and slow-rotating galaxies are found within voids was recently addressed by Tikhonov & Klypin (2009). The latter issue requires a careful analysis of selection effects, which limit the number of known faint galaxies in the void sample. A better understanding of the spatial structures (filaments) in the void galaxy distribution can also be used as a test of structure formation in low-density regions (Park & Lee, 2009).
It is well established that the luminosity and central SB of galaxies are correlated albeit with rather large scatter. This is shown, in particular, by Cross & Driver (2002), based on bivariate distributions of , . The ridge of this bivariate distribution along which systematically changes with is read as
_B,0-μ_B,0^* = β_μ ×(M_B-M_B^*)
where =0.2810.007, =22.65 mag arcsec and =–20.20. The r.m.s scatter in around this ridge is estimated as =0.5170.006 mag arcsec. All the values above were transformed to -band from the original magnitudes of the 2dFGRS (Two-degree Field Galaxy Redshift Survey) in Cross & Driver (2002) using the transform of + 0.20 mag. The latter is based on the relation from Hewett et al. (1995) and the typical late-type galaxy colour 0.7. Using these results, we can address the possible incompleteness of the current Lynx-Cancer void galaxy sample.
Blanton et al. (2005) and Geha et al. (2006) indicated that the SDSS spectral survey of galaxies is significantly incomplete for the average SB within -filter half-light radius dimmer than 23.5 mag arcsec. We use the effective surface brightness to derive an estimate of the central SB for comparison with the above study. For this we adopt the colour 0.5 as a value typical of late-type dwarfs. We also use the Lupton et al. (2005) formula to convert the SDSS and magnitudes to Johnson-Cousins . For the adopted colour, we get a typical value of 0.35 and respectively, 0.85. From this, =23.5 mag arcsec to first approximation corresponds to =24.35 mag arcsec. For purely exponential disks with the scalelength , the radius of the disk including its half-light is equal to = 1.678 (e.g. Impey, Bothum & Malin, 1988). Using the definition of , one can show that for purely exponential disks this parameter is related to the central surface brightness: =+1.12. Thus, the above assertion of Blanton et al. (2005) and Geha et al. (2006) can be formulated as a significant incompleteness of the SDSS spectral targets for late-type dwarfs having the visible central SB (uncorrected for the Galaxy extinction and inclination) 23.23 mag arcsec.
The latter value of , according to the above ridge relation (equation 4.2), corresponds to an average galaxy with –18.1. This implies that practically all face-on galaxies (with small contribution of bulge emission) in the void sample are subject to the SDSS spectroscopy SB selection. The most probable observed inclination angles for the random orientation sample fall around =60°. Therefore, one expects a typical increase of the visible surface brightness of ()075. The inclination brightening (and the possible presence of central SF regions) thus should mainly eliminate the SDSS SB selection for more luminous void galaxies. However, it would remain crucial for the fainter void galaxies.
Despite the fact that half of selected Lynx-Cancer void galaxies appear to be LSB galaxies, it is quite evident that LSB dwarfs are under-represented in this sample. This is substantiated by the observation that no SDSS redshifts could be obtained for several of these LSB dwarfs but are found in the literature as mainly measured via dedicated Hi-line observations or through resolved stellar photometry. The examples are LSB dwarfs (LSBDs) UGC 3966 (DDO 46) with Hi velocity from Springob et al. (2005), SAO 0822+3545 with H velocity from Pustilnik et al. (2003), UGC 4426 with photometric distance from Karachentsev et al. (2006), UGC 5209 with Hi velocity from Huchtmeier, Karachentsev & Karachentseva (2003).
The effect of the SDSS spectral database magnitude selection for our void sample can be roughly estimated using the adopted Petrosian magnitude limit =17.77. For the great majority of galaxies with sizes less than 1′–2′, Petrosian magnitudes do not differ more than 0.1 mag from the total galaxy magnitudes. Therefore, using the average colour 0.5 for late-type galaxies, we transform with the Lupton et al. (2005) formula the limiting value =17.77 to the approximate limiting value =18.6. The distances to the near and distant borders of the Lynx-Cancer void and to its centre, 10, 26, and 18 Mpc correspond to distance moduli of 30.0, 32.0 and 31.3 mag. This implies, that for the front void ‘hemisphere’, the SDSS should select [if the additional anti-selection of LSB galaxies (LSBGs) would not work] all galaxies with –12.7, and respectively, for the whole void volume, all galaxies with M–13.4.
In Fig. 7, we show the first approximation of the shape for the void galaxy luminosity function (LF) based on the Table 2 absolute magnitudes. This is fitted by the Schechter function (Schechter, 1976) in the range –14.0 M–18.0. Two values of are adopted, the standard one of –20.2 mag and the ‘reduced’ one of –19.2 mag, suitable for the void conditions. The LF does not look to be affected by incompleteness down to –14.0. The Schechter LF low-luminosity slopes for these two values of are –1.270.10 and –1.220.09, respectively. This slope is close to that of the raw LF for the SDSS low-luminosity galaxy sample from Blanton et al. (2005).
According to the ‘ridge’ relation in equation 4.2, for –14.0 the average value of for disk galaxies corresponds to 24.4 mag arcsec, that is more than 1 mag arcsec dimmer than the derived estimate for the SDSS central surface brightness ‘threshold’. Probably, the significant effect of visible brightening due to galaxy inclination and the presence of central star-forming regions counteracted the SDSS selectivity against LSBGs. However, for galaxies with –12.0 –13.5, our sample could be missing up to 25–30 objects according to the extrapolation of the LF to the lower luminosity range.
There are at least three possible means to find ‘numerous’ missed generic LSB dwarfs within the boundaries of this void. They include: (1) the blind Hi surveys (e.g. those mentioned in Section 2.1); (2a) the optical spectroscopy and H detection of candidate LSB galaxies; and (2b) Hi pointing sensitive observations of candidate LSB galaxies. The latter are separated in this sky region from the SDSS image database. Unfortunately, no reliable criteria exist to identify good candidates in the nearby LSBDs. Therefore, both methods (2a) and (2b) are not expected to result in a ’high’ detection rate for galaxies residing in the nearby voids. However, all new redshifts of LSBDs will advance our understanding of the dwarf galaxy census in the local Universe.
4.3 Other properties of, and the prospects of studying, the void sample galaxies
The whole set of the studied sample galaxies includes 75 dwarfs. Observations and the subsequent analysis of this full dataset and comparison with similar data on the control dwarf galaxy sample in a denser environment will require a significant effort during coming years. However, the data collected so far already suggest differences in the evolutionary status of some Lynx-Cancer void dwarf galaxies and those in denser surroundings. While selection effects might play a role, it is unlikely to influence the resulting parameters of the discovered unevolved void dwarf galaxies discussed below.
To examine possible differences in the evolutionary status of void galaxies, we study the following parameters. The first is the gas metallicity as traced by O/H in Hii regions around the sites of star formation. The second parameter, the gas mass-fraction, defined as the ratio , where =(Hi+He)+. The contribution of molecular gas in the total gas mass of dwarf galaxies is small. While the parameter (Hi+He) is well determined directly from the Hi 21-cm line flux and the adopted ratio , the parameter is a more model-dependent. However, with good surface photometry in several filters, like that extracted from the SDSS images and the adopted stellar metallicity, it can be well estimated within reasonable assumptions using popular models like PEGASE (Fioc & Rocca-Volmerange, 1999). The third parameter related to the evolutionary status of a galaxy is the age of its the oldest resolved or also unresolved visible stars. The former is hard to obtain and would require long observations even with the Hubble Space Telescope or similar telescopes, even for the fairly local Lynx-Cancer void galaxies. The ground-based surface photometry can be a reasonable alternative. However, the age estimate require well-resolved multicolour deep galaxy images as well as accounting for the potential contamination of nebular emission (if present) and corrections for the dust extinction. Again, the SDSS images of well resolved galaxies and the PEGASE package models are quite often suitable for estimates of this parameter. Examples of such an analysis for two very metal-poor Lynx-Cancer void galaxies, DDO 68 and SDSS J0926+3343, are published by Pustilnik et al. (2008, 2010).
It is worth mentioning that a significant fraction of the void galaxies are paired with typical projected distances in pairs of several tens of kpc. These include the following Lynx-Cancer void list entries: (13,14), (16,17), (27,28), (40,41), (46,47), (66,67), (77,78). Other galaxies look like they are forming unbound but probably coherent elongated structures (‘filaments’) with total length of 1–2 Mpc. Examples are the chains of entries: (13,14,15,24), (53,60,63,66,67), (59,70,74). The majority of the void galaxies at the current level of its census appear, however, quite isolated.
As mentioned in Section 2.1, several unusual and/or very metal-poor dwarf galaxies fall in the Lynx-Cancer void volume. These include a very low-metallicity BCG HS 0822+3542 [12+(O/H)=7.44] and its companion, very blue (and presumably relatively young) LSB dwarf SAO 0822+3545 (Pustilnik et al., 2003). The two most metal-poor dwarf galaxies in the Local Volume and adjacent regions, DDO 68 and SDSS J0926+3343, with the parameters 12+(O/H) of 7.14 and 7.12, respectively, are representatives of this void galaxy population. Moreover, they are situated only at 1.6 Mpc from each other. Both of them are also unusual in the blue colours of their outer parts, indicating ages of the oldest visible stellar population of 1–3 Gyr (Pustilnik et al., 2008, 2010), which is in drastic contrast with the colours and ages of the absolute majority of other dwarf galaxies. At least three other Lynx-Cancer void dwarf galaxies have 12+(O/H) 7.3. They include SDSS J0812+4836 (Izotov & Thuan, 2007) and SDSS J0737+4724, SDSS J0852+1350, and one more blue ’young’ dwarf SDSS J0723+3622 (Pustilnik et al., MNRAS, submitted).
The Lynx-Cancer void interior represents a small fraction (0.05) of the whole volume to the distance 26 Mpc. The existence in this void of the concentration of extremely metal-poor galaxies and objects that lack visible old stellar populations is surprising. The former fact is indicative of the physical relationship between the evolutionary status of low-mass late-type galaxies and their global environment. In turn, the latter fact hints at the possibility of retarded galaxy formation in the void environment.
The nearby Lynx-Cancer void is described as a low-density region bordering the Local Volume at negative supergalactic coordinates, with the overall extent of more than 16 Mpc and distance to the void centre of 18 Mpc. A smaller subvoid is identified with the centre position on distance of 14.6 Mpc and radius of 6.0 Mpc (close to that described in Pustilnik et al., 2003).
A sample of 75 late-type dwarf and four subluminous galaxies residing in the void region is presented. Their main observational parameters available in the literature are collected for further study and comparison.
The shape of the Lynx-Cancer void galaxy raw LF indicates a significant drop in the galaxy number at absolute magnitudes –14. This is probably related to the SDSS galaxy spectral database selectivity against LSB galaxies. To remedy this drawback, we mention three possible methods.
Properties of several unusual dwarf galaxies found to date in the Lynx-Cancer void are briefly described, with the emphasis on their very low metallicities and relatively small ages of the oldest visible stellar populations. These data, despite being rather limited, suggest that the void-type environment might effectively slow down the rate of evolution of a fraction of low-mass galaxies and postpone their formation epoch.
We thank the anonymous referee for suggestions and criticism which helped to significantly improve the quality and clarity of the data presentation and discussion. We are thankful to D.I. Makarov for providing the list of groups prior publication and A.Y. Kniazev for useful comments on the manuscript. This work was supported through the RFBR grants No. 06-02-16617 to SAP and ALT and No. 10-02-92650 to SAP. SAP is also grateful for the support through the Russian Federal Agency of Education grant No. 2.1.1/1937. We acknowledge the spectral and photometric data used for this study and the related information available in the SDSS database. The Sloan Digital Sky Survey (SDSS) is a joint project of the University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, the Johns Hopkins University, 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. Apache Point Observatory, site of the SDSS telescopes, is operated by the Astrophysical Research Consortium (ARC). This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We also acknowledge the usage of the HyperLeda database (http://leda.univ-lyon1.fr).
|#||Name or prefix||Coordinates (J2000)||Type||Other names and notes|
|1||PGC2807187||06 21 03.51||+20 10 16.6||1318||7||1263||1513||18.00||3.63||-17.21||Scd||12.76||ADBS J062103+2010|
|2||HIPASSJ0626+24||06 26 20.97||+24 39 20.0||1473||7||1432||1682||17.60||1.79||-16.0||Scd||11.39|
|3||PGC1689759||06 29 58.23||+23 34 28.5||1452||6||1426||1680||17.10||1.18||-15.89||Scd||11.36|
|4||UGC3475||06 30 28.86||+39 30 13.6||487||1||524||752||14.97||0.79||-15.88||Sm||5.11||MCG 7-14-2|
|5||UGC3476||06 30 29.22||+33 18 07.2||469||4||477||718||14.96||1.02||-16.02||Im||4.89||CGCG 175-2|
|6||UGC3503||06 38 01.40||+22 39 06.0||1389||4||1347||1608||15.10||0.66||-17.27||Sd||10.96|
|7||UGC3501||06 38 38.40||+49 15 30.0||449||6||528||735||17.20||0.50||-13.31||Im||4.24||MCG 8-12-031|
|8||UGC3516||06 43 08.51||+22 52 24.9||1287||7||1226||1491||16.97||1.19||-15.79||Sd||10.78|
|9||KKH 38||06 47 54.88||+47 30 50.0||451||2||518||725||17.40||0.40||-12.99||Ir||4.31||LEDA 2807122|
|10||UGC3587||06 53 54.70||+19 17 59.0||1267||1||1194||1470||13.84||0.40||-18.08||S?||10.47|
|11||UGC3600||06 55 40.00||+39 05 42.8||412||4||435||679||16.18||0.29||-13.95||Im||3.85||PGC 019871|
|12||UGC3672||07 06 27.56||+30 19 19.4||994||4||968||1236||15.40||0.32||-16.08||Im||8.54||PGC 20154|
|13||UGC3698||07 09 16.80||+44 22 48.0||422||1||465||701||15.41||0.42||-14.92||Im||3.82||Pair with NGC 2337|
|14||NGC2337||07 10 13.60||+44 27 25.0||436||1||479||715||13.48||0.38||-16.85||IBm||3.91||Pair with UGC 3698|
|15||UGC3817||07 22 44.48||+45 06 30.7||437||1||478||717||15.96||0.44||-14.44||Im||3.71||PGC 20852|
|16||SDSS||07 23 01.42||+36 21 17.1||885||3||882||1050||17.01||0.23||-14.19||Sm?||6.55|
|17||SDSS||07 23 13.46||+36 22 13.0||974||3||971||1050||19.25||0.23||-11.95||dI||6.55||Companion? of J0723+3621|
|18||PGC020981||07 25 38.95||+09 10 59.8||1202||6||1064||1363||16.69||0.27||-14.94||I||8.18||CGCG 057-013|
|19||UGC3853||07 27 39.26||+48 26 45.4||936||4||992||1231||15.96||0.46||-15.65||Sdm||4.20||MCG 8-14-018=RFGC 1217|
|20||UGC3860||07 28 17.20||+40 46 13.0||354||1||371||570||14.96||0.25||-14.75||Im||2.51||DDO 43|
|21||UGC3876||07 29 17.49||+27 54 01.9||854||6||806||1096||13.70||0.19||-17.39||SAd||7.81||MCG 5-18-15=KARA 193|
|22||SDSS||07 30 58.90||+41 09 59.8||874||3||892||1146||16.67||0.27||-14.58||dI||5.25||SDSS J073058.90+410959.8|
|23||UGC3912||07 34 12.63||+04 32 47.1||1240||5||1065||1368||14.72||0.23||-16.87||S?||7.62|
|24||SDSS||07 37 28.47||+47 24 32.8||476||2||524||761||18.06||0.47||-12.50||LSB||3.22||SDSS J073728.49+472432.8|
|25||UGC3966||07 41 26.00||+40 06 44.0||361||5||370||631||14.44||0.22||-15.46||Im||2.37||DDO 46|
|26||SDSS||07 44 43.72||+25 08 26.6||749||4||680||945||18.35||0.18||-12.39||Ir||5.82||SDSS J074443.72+250826.6|
|27||MCG9-13-52||07 46 56.36||+51 17 42.8||445||2||510||737||16.78||0.27||-13.51||Sm||3.02||KKH 40; Comp. of MCG9-13-56|
|28||MCG9-13-56||07 47 32.10||+51 11 29.0||439||3||503||730||15.48||0.30||-14.90||Sm||2.93||CGCG 262-28=KUG 0743+513|
|29||UGC4117||07 57 25.98||+35 56 21.0||773||1||754||1031||15.34||0.20||-15.61||IBm||5.24||MCG 6-18-3=KUG 0754+360|
|30||UGC4148||08 00 23.68||+42 11 37.0||716||3||729||989||15.63||0.18||-15.21||Scd||3.73||MCG 7-17-6=KUG0756+423|
|31||NGC2500||08 01 53.30||+50 44 15.4||504||1||562||794||12.23||0.17||-18.13||SBcd||2.27||UGC4165=KARA 224|
|32||MCG7-17-19||08 09 36.10||+41 35 40.0||704||1||712||976||16.65||0.22||-14.20||Sc||3.76||KUG 0806+417|
|33||SDSS||08 10 30.65||+18 37 04.1||1483||3||1371||1683||18.29||0.16||-13.68||Sm:||4.39|
|34||SDSS||08 12 39.53||+48 36 45.4||521||5||565||807||17.23||0.22||-13.21||dIr||2.50||SDSS J081239.53+483645.4|
|35||NGC2537||08 13 14.73||+45 59 26.3||445||1||475||720||12.27||0.23||-17.93||SBm||2.95||UGC4274, not pair of IC2233|
|36||IC2233||08 13 58.93||+45 44 34.3||553||1||572||781||13.05||0.22||-17.32||Sd||3.04||UGC4278=RFGC 1340|
|37||NGC2541||08 14 40.18||+49 03 42.1||548||1||594||876||12.25||0.22||-18.36||SABc||2.33||UGC4284, pair with NGC2552?|
|38||NGC2552||08 19 20.14||+50 00 25.2||524||1||574||811||12.92||0.20||-17.51||SAm||2.24||UGC4325 pair with NGC2541?|
|39||KUG 0821+321||08 25 04.90||+32 01 05.1||648||16||601||894||16.10||0.20||-14.54||Ir||4.41||SDSS J082504.94+320105.1|
|40||HS 0822+3542||08 25 55.43||+35 32 31.9||720||2||691||985||17.92||0.20||-12.93||BCG||4.46||Pair with SAO 0822+3545|
|41||SAO0822+3545||08 26 05.59||+35 35 25.7||740||1||711||985||17.56||0.20||-13.29||Im||4.45||Pair with HS 0822+3542|
|V calculated from photometric distance.|
Appendix A Lists of galaxies delineating the Lynx-Cancer void
In Tables 3 and 4, we summarize the parameters of objects delineating the Lynx-Cancer void, which include, respectively, 10 ‘isolated’ luminous galaxies and 34 pairs and groups hosting of luminous galaxies. The following information is presented in columns of Table 3. Column 1 - the galaxy name. Columns 2 and 3 - J2000 R.A. and Declination, respectively. Columns 4 and 5 - heliocentric velocity and its error (as extracted from NED). Columns 6 and 7 - the velocity relative to the LG centre and the adopted ‘distance’ velocity (D(Mpc)=V/73). Columns 8 and 9 - the total blue magnitude and the adopted -band the Galaxy extinction correction for the absolute blue magnitude in column 10. In column 11 the ‘NED based’ galaxy morphological type is shown. In column 12, the distance in Mpc to the nearest luminous galaxy is presented. In column 13, some alternative galaxy names are given. Three galaxies with formal M –19.0 are included, since the internal extinction correction of these almost edge-on galaxies, will brighten them above the adopted threshold absolute magnitude.
In columns of Table 4 the following information is presented. Column 1 - the group name, which is either the Tully group (TG) number (for those from Tully et al., 2008), or the name of the brightest member for pairs/triplets/groups from Karachentsev & Makarov (2008); Makarov & Karachentsev (2009) and Makarov & Karachentsev (2011). Columns 2 and 3 - J2000 coordinates of group centre. Column 4 - the group heliocentric velocity; column 5 - V, the group velocity relative to the Local Group centre. Column 6 - the group distance velocity V, which are taken either from the non-redshift measurements for Tully et al. (2008) sample objects or from V with the account for the discussed in Section 2.2 V.
|1||NGC2543||08 12 58.0||+36 15 17||2471||9||2449||1920||12.86||0.30||-19.54||1102||1125||-1098||4.90|
|2||NGC2654||08 49 11.8||+60 13 14||1349||3||1478||1924||12.67||0.28||-19.70||1439||1226||-356||2.55|
|3||NGC2880||09 29 34.6||+62 29 27||1608||21||1716||1600||12.46||0.14||-19.38||1157||1091||-171||1.59|
|4||NGC3041||09 53 07.2||+16 40 40||1408||2||1271||1829||12.31||0.15||-19.81||208||1476||-1060||2.27|
|5||NGC3198||10 19 54.9||+45 32 59||663||4||682||1008||11.07||0.05||-19.68||482||855||-231||1.33||UGC5572|
|6||NGC3239||10 25 05.7||+17 09 37||753||3||622||912||11.71||0.14||-18.91||52||798||-438||2.04||UGC5637|
|7||NGC3319||10 39 09.5||+41 41 13||739||1||740||971||11.48||0.06||-19.20||387||864||-215||1.33|
|8||NGC3344||10 43 30.2||+24 55 25||586||4||498||788||10.38||0.14||-19.93||113||726||-284||2.40||UGC 5840|
|9||NGC3365||10 46 12.6||+01 48 47||986||1||789||1336||13.17||0.20||-18.34||-294||1116||-674||4.85|
|10||NGC3432||10 52 31.0||+36 37 10||613||4||606||1117||11.64||0.06||-19.33||341||1031||-259||2.45|
|absolute magnitudes are corrected only for Galactic extinction according to NED;|
|Almost edge-on galaxies. The account for internal extiction leads to M –19.0.|
|1||NGC2273||06 50 07.4||+60 50 30||1967||2190||1917||928||-509||7.44||NGC2273 -brightest|
|2||NGC2460||07 57 02.5||+60 22 38||1558||1723||1388||945||-384||3.62||NGC2460 -brightest|
|3||IC2267||08 18 03.4||+24 45 33||1999||2307||977||1315||-1624||6.02||IC2267 -brightest|
|4||TG 179||08 18 58.3||+57 48 44||1082||1173||925||708||549||-231||3.02||NGC2549 -brightest|
|5||TG 579||08 27 07.0||+25 57 27||2068||1988||1876||790||1130||-1272||4.90||NGC2592 -brightest|
|6||TG 282||08 52 41.4||+33 25 19||420||374||581||272||401||-320||2.16||NGC2683 -brightest|
|7||TG 292||08 53 24.0||+51 18 54||707||758||1266||844||857||-395||2.38||NGC2681 -brightest|
|8||NGC2685||08 55 35.5||+58 44 11||992||1196||872||782||-240||2.21||NGC2685 -brightest|
|9||TG 167||09 07 02.5||+60 07 44||1386||1482||1656||1205||1101||-277||1.59||NGC2768 -brightest|
|10||TG 389||09 09 33.5||+33 07 25||2028||1978||2221||963||1630||-1162||5.28||NGC2770 -brightest|
|11||NGC2775||09 10 20.1||+07 02 17||1350||1169||1491||88||964||-1134||4.12||NGC2775 -brightest|
|12||NGC2798||09 17 22.3||+41 59 56||1707||1974||1047||1475||-791||3.20||NGC2798 -brightest|
|13||NGC2820||09 21 36.7||+64 15 23||1697||1898||1415||1253||-174||2.55||NGC2820 -brightest|
|14||NGC2844||09 21 48.7||+40 09 14||1478||1750||877||1329||-727||2.82||NGC2844 -brightest|
|15||TG 293||09 22 01.5||+50 59 30||637||684||1029||642||752||-283||2.29||NGC2841 -brightest|
|16||NGC2859||09 24 03.0||+34 30 51||1679||1636||1854||788||1425||-886||2.82||NGC2859 -brightest|
|17||TG 271||09 32 09.9||+21 30 00||554||441||615||140||474||-365||2.16||NGC2903 -brightest|
|18||TG 168||09 42 35.8||+58 51 08||1362||1451||1090||738||788||-151||2.21||NGC2950 -brightest|
|19||TG 378||09 45 13.5||+32 21 53||1461||1403||1634||585||1333||-742||1.36||NGC2964 -brightest|
|20||TG 401||09 50 15.0||+12 47 43||1437||1281||1717||104||1347||-1060||2.27||NGC3020 -brightest|
|21||TG 379||09 52 08.3||+29 14 11||1499||1427||1628||490||1352||-764||2.13||NGC3032 -brightest|
|22||TG 373||09 55 17.8||+04 16 15||1335||1128||1498||-119||1122||-985||4.46||NGC3044 -brightest, NGC3055|
|23||TG 157||10 01 59.5||+55 40 04||1126||1199||1413||880||1086||-210||3.85||NGC3079 -brightest|
|24||NGC3118||10 07 03.6||+33 00 43||1362||1312||1592||522||1368||-625||1.96||NGC3118 -brightest|
|25||NGC3184||10 17 55.2||+41 24 33||592||892||380||768||-248||1.40||NGC3184 -brightest|
|26||NGC3227||10 23 41.4||+19 54 48||1034||1552||159||1366||-718||1.45||NGC3227 -brightest|
|27||TG 370||10 29 02.5||+28 44 23||1345||1272||1526||346||1374||-567||2.59||NGC3245 -brightest|
|28||NGC3301||10 36 25.3||+21 49 35||1219||1504||163||1364||-613||1.45||NGC3301 -brightest|
|29||NGC3338||10 42 27.1||+13 57 56||1123||1833||-46||1640||-816||3.76||NGC3338 -brightest|
|30||TG 266||10 48 15.7||+12 33 32||819||670||811||-47||729||-353||1.33||M105 group|
|31||TG 364||10 48 29.6||+12 31 07||1285||1136||1565||-93||1407||-679||2.04||NGC3373 -brightest|
|32||TG 268||10 51 23.2||+05 51 00||1023||844||825||-139||717||-383||1.33||NGC3423 -brightest|
|33||TG 361||10 56 18.6||+17 30 53||1104||1526||7||1416||-569||2.04|
|34||TG 269||11 00 36.2||+28 58 08||683||620||1000||177||946||-272||2.55||NGC3486 -brightest|
|group coordinates are from Makarov & Karachentsev (2011) [MK11]; ‘groups’ are from MK11, V are calculated from V and V;|
|MK11 groups, absent in Tully sample; MK11 groups with a member(s), from the Tully list. Their V are adopted as for the respective galaxies.|
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