Polar-ring galaxies: the SDSS view on the symbiotic galaxies
aa r X i v : . [ a s t r o - ph . GA ] D ec Mon. Not. R. Astron. Soc. , 1–8 (2014) Printed 21 September 2018 (MN L A TEX style file v2.2)
Polar-ring galaxies: the SDSS view on the symbioticgalaxies
V. Reshetnikov , F. Combes St.Petersburg State University, Universitetskij pr. 28, 198504 St.Petersburg, Stary Peterhof, Russia LERMA, Observatoire de Paris, CNRS (UMR 8112), 61, Av. de l’Observatoire, F-75014 Paris, France
Accepted 2014 December 5. Received 2014 November 23; in original form 2014 October 1
ABSTRACT
Polar-ring galaxies are multi-spin systems, showing star formation in a blue late-typecomponent, perpendicular to a red early-type one, revealing how galaxy formationcan sometimes occur in successive steps. We perform two-dimensional decompositionin the g , r , i bandpasses of 50 polar-ring galaxies (PRGs) from the Sloan DigitalSky Survey. Each object was fit with a S´ersic host galaxy and a S´ersic ring. Ourgeneral results are: (i) The central (host) galaxies of the PRGs are non-dwarf sub- L ∗ galaxies with colors typical for early-type galaxies. (ii) Polar structures in our sampleare, on average, fainter and bluer than their host galaxies. (iii) In most galaxies, thestellar mass M ∗ of the polar component is not negligible in comparison with thatof the host. (iv) The distributions of the host galaxies on the size – luminosity andKormendy diagrams are shifted by ∼ m to fainter magnitudes in comparison withE/S0 galaxies. It means that the PRGs hosts are more similar to quenched disksthan to ordinary early-type galaxies. (v) All the PRGs in our sample are detected inmid-infrared by WISE, and we derive from the 22 µ m luminosity their star formationrate (SFR). Their SFR/M ∗ ratio is larger than for the early-type galaxy sample ofAtlas , showing that the star forming disk brings a significant contribution to thenew stars. Globally, PRGs appear frequently on the green valley in the mass-colordiagram, revealing the symbiotic character between a red-sequence host and a bluecloud ring. Key words: galaxies: statistics – galaxies: structure.
Polar-ring galaxies (PRGs) are peculiar galaxies, consistingof a ring or disc of gas, stars and dust orbiting in a planenearly perpendicular to the major axis of a central galaxy(Whitmore et al. 1990 = PRC). The central object is usuallyan early-type galaxy (ETG), poor in gas. The polar ring isgenerally younger and looks similar to late-type galaxies,with large amount of HI gas and young stars. PRGs arevery rare objects – the observed fraction of PRGs has beenestimated to ∼ c (cid:13) V. Reshetnikov, F. Combes
The main purpose of this paper is to study the generalphotometric characteristics of PRGs from the Sloan Survey.Throughout this article, we adopt a standard flat ΛCDMcosmology with Ω m =0.3, Ω Λ =0.7, H =70 km s − Mpc − . Our sample uncludes 46 objects from the “best candidates”of the SPRC. This group consists of 70 galaxies which aremorphologically similar to well-studied “classic” PRGs fromWhitmore et al. catalogue. These galaxies tend to havesymmetrical central body, resembling early-type galaxies(E/S0), and extended, mostly visible “edge-on” structurescrossing the central galaxy at large angles. At the inter-sections of polar structures and host galaxies are some-times visible absorption band. Initial analysis of the sampleshowed that at least two galaxies (SPRC-22 and SPRC-43)are line-of-sight projections of two galaxies, and these ob-jects were excluded from further consideration. From thesample were excluded also too weak and small by angularsize objects and also galaxies, showing strongly asymmetricpolar structures. The final sample was supplemented with 4kinematically-confirmed PRGs from Whitmore et al. (1990)catalogue. The list of the sample galaxies is presented in theTable 1, where we provide the basic information about them.Currently, 12 of 50 galaxies in the sample are kinematically-confirmed PRGs (8 from the SPRC – see Moiseev et al. 2014and 4 from the PRC).The 2D decomposition of the galaxy images from thesample of 50 PRGs was performed using the GALFIT code(v3.0.4; Peng et al. 2010). GALFIT allows for various para-metric functions (S´ersic, Gaussian, etc.) to be modelled si-multaneously as either multiple subcomponents of a singleobject, multiple objects in a frame or combination thereof.Galaxy images in the g , r and i bands were downloadedfrom the SDSS DR9 (Ahn et al. 2012). All stars, backgroundand underground objects were masked. Nearby unsaturatedstars were used as estimates of the PSF at the objects posi-tions. We fit a two-component model (host galaxy + ring) toall galaxies in the sample. Both components were fitted bya single S´ersic function with seven free parameters (objectcentres, total magnitude, effective radius r e , S´ersic index n ,ellipticity, position angle). In several cases (e.g. SPRC-7 –galaxy with non edge-on ring – see Fig. 1) we approximatedthe ring by the inner-truncated model in order to describeclearly visible hole in the ring component. Fig. 1 shows theresults of decomposition for two galaxies with different mor-phology (edge-on ring, non edge-on ring).The comparison of our derived apparent magnitudes ofPRGs with the SDSS modelMag values shows general agree-ment. Mean differences of the SDSS and our magnitudesare +0 . m ± . m
31 ( g ), +0 . m ± . m
25 ( r ) and +0 . m ± . m i ). After excluding 6 most deviated galaxies with magni-tudes difference in the g band ∆ g > +0 . m . m ± . m
16 ( g ),+0 . m ± . m
14 ( r ), and +0 . m ± . m
16 ( i ).For two galaxies with ∆ g > +0 . m Table 1.
Basic data for the PRGs sampleGalaxy Redshift r (mag) r e ( ′′ ) n ring/host(1) (2) (3) (4) (5) (6)SPRC-2 – 14.37 6.9 2.5 0.31SPRC-3 ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ r filter (present work),(5) S´ersic index in the r band (present work),(6) ring-to-central galaxy luminosity ratio in the r band(present work). c (cid:13)000
Basic data for the PRGs sampleGalaxy Redshift r (mag) r e ( ′′ ) n ring/host(1) (2) (3) (4) (5) (6)SPRC-2 – 14.37 6.9 2.5 0.31SPRC-3 ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ r filter (present work),(5) S´ersic index in the r band (present work),(6) ring-to-central galaxy luminosity ratio in the r band(present work). c (cid:13)000 , 1–8 olar-ring galaxies Figure 1.
Examples of the PRGs images modeling: SPRC-58(top), SPRC-7 (bottom). From left to right: original SDSS r-bandimage, 2D model, residual image. g , r , i passbands) are in good agreement withBrosch et al. results (16.80, 16.41, 16.12). The parame-ters of the central galaxy and the ring are in accord also.For the central galaxy Brosch et al. (2010) give such ap-parent magnitudes: 18.22, 17.35, 16.92 ( g , r , i ), the samevalues according to our analysis are 18.28, 17.34, 16.90.The apparent magnitudes of the ring are 17.14, 17.00, 17.01(Brosch et al. 2010) and 17.21, 17.03, 16.78 (present work).Finkelman et al. (2012) have published photometric data for16 candidate PRGs, including SPRC-69 from our sample. Inthe r passband Finkelman et al. give such estimates of ap-parent magnitudes for SPRC-69: 15.40 (host galaxy), 17.14(ring), 15.20 (total). Our values are 15.51, 16.90 and 15.24correspondingly.In the rest of the paper we will use the results of ourphotometry for all the sample PRGs (see Table 1 for totalmagnitudes in the r passband and some other characteris-tics). Fig. 2a-c show distributions of central galaxies by abso-lute luminosity (corrected for Milky Way absorption ac-cording to Schlafly & Finkbeiner 2011 and k -correction fromChilingarian et al. 2010), S´ersic index and physical effectiveradius. As one can see, they are non-dwarf sub- L ∗ galaxieswith average absolute magnitude < M r > = − . ± . n < .
5) to bulge-dominated ( n > n is not a direct measure of the galaxytype and it does not translate one to one to the bulge-to-total ratio. But the single S´ersic index is a reasonable sta-tistical characteristic to separate late-type and early-typegalaxies (e.g. Bruce et al. 2012).The relative fractions of disc-dominated and bulge-dominated brightness distributions are 16%, 28%, respec-tively, in the r band and 26%, 40% ( i filter) in our sampleof PRGs hosts. These fractions are consistent with previ-ous findings that late-type galaxies are less frequent among Figure 2.
Characteristics of PRGs hosts in the r passband: (a)absolute magnitude, (b) S´ersic index, (c) effective radius. Figure 3.
Distribution of the sample galaxies over g − r colorof the host galaxies (red solid line) and of the rings (blue dashedline). PRGs in comparison with ETGs (e.g. Whitmore et al. 1990;Whitmore 1991; Reshetnikov et al. 2011).Host galaxies of the PRGs show wide distribution ofrest-frame optical colors (see the g − r distribution in Fig. 3).In general, the g − r color distribution looks similar to thatpresented by Finkelman et al. (2012) (see their fig. 6) butthe peak in our distribution is shifted to bluer color. (Mostprobably, this difference can be explained by the absence of k -correction in the Finkelman et al. 2012 data.) c (cid:13) , 1–8 V. Reshetnikov, F. Combes
The average colors of the PRGs hosts in our sample are < g − r > = +0 . ± . < r − i > = +0 . ± .
09. Such col-ors correspond to the colors of S0 galaxies (Fukugita et al.2007). Finkelman et al. (2012) have compared the g − r colordistribution of PRGs with that of in several control sam-ples of early-type galaxies (normal ETGs, blue ETGs, dustyETGs). According to fig. 6 in their work, the average colorof the PRGs hosts in our sample matches the color of blueearly-type galaxies better than typical colors of other typesof ETGs. It is important to note that optical colors of PRGscan be distorted by the inner absorption in the hosts andin the rings. Near-infrared photometry of several PRGs waspresented earlier by Iodice et al. (2002a,b). They concludedthat near-infrared colors of host galaxies are bluer on aver-age than those for standard early-type galaxies.Fig. 4 shows standard scaling relations for the PRGshosts. The size – luminosity relation for the hosts is shownin Fig. 4a. PRGs are shifted to lower luminosities in com-parison with relation for early type galaxies (at a fixed ef-fective radius) and are located in the same region as spiralgalaxies. The characteristics of disc-dominated galaxies inthe figure are taken from Simard et al. (2011). Simard et al.(2011) presented results of decomposition for more than 1million galaxies in the SDSS. We took their decompositionresults with pure S´ersic model and selected large ( r e > . ′′ . < z < . n < .
5) galaxiesfor our Fig. 4.Fig. 4b demonstrates the Kormendy relation (the meansurface brightness within effective radius r e vs. r e in kpc) forPRGs and normal E/S0 galaxies. PRGs follow the standardrelation for E/S0 galaxies but, as in Fig. 4a, with notableshift (by ∼ m ) to lower surface brightnesses (at a fixed r e ).This shift is evident for the galaxies with various types ofsurface brightness distribution ( n < n > Polar structures in our sample of PRGs are, on average,fainter ( < M r > = − . ± .
28) and bluer ( < g − r > =+0 . ± . < r − i > = +0 . ± .
22) than their host galax-ies (Fig. 3; see also Reshetnikov et al. 1994; Iodice et al.2002b; Finkelman et al. 2012). Taking into account inter-nal extinction in the polar rings (most of them are seenalmost edge-on), the difference in colors must be larger. Ob-served colors of rings are usual for normal spiral galaxies(e.g. Fukugita et al. 2007).As it was shown earlier by Finkelman et al. (2012),observed luminosities of the hosts and the rings are cor-related – more luminous host galaxies possess more lumi-nous rings, on average (Fig. 5). There is also a weak mutual(ring vs. host) correlation of colors but much weaker thanin Finkelman et al. (2012). The other possible trend is be-tween the relative size of ring ( D ring /D host – diameter ofring normalized by diameter of host galaxy) and absolutemagnitude of host galaxy – see Fig. 6. (The rings sizes weretaken from Smirnova & Moiseev 2013). Among PRGs withbright central galaxies we see relatively compact rings, whilefainter central objects are surrounded by relatively small asfar as more extended rings: for bright hosts with M r − Figure 4.
Scaling relations for the host galaxies of PRGs in the r band (dots – galaxies with S´ersic index n <
3, red crosses –with n > n > .
5) galaxies, dashedline – for late type with n < .
5, see Shen et al. 2003, yellowdots show the location of ∼
40 000 disc-dominated galaxies fromSimard et al. 2011), (b) Kormendy relation (solid line shows therelation for early type galaxies in Coma cluster (Houghton et al.2012), dashed line represents mean relation for the SDSS early-type galaxies with absolute magnitudes M r between -18 . m . m the mean relative size of rings is 1.4 ± M r > −
20 the mean value is 2.3 ± g − r colors and r -band luminosities. The results are shown inFig. 7. As one can see, stellar mass of a polar componentis not negligible in comparison with mass of a host in mostgalaxies. In some galaxies the masses of two components arecomparable.Fig. 8 presents the observed distribution of angle be-tween the ring and the central galaxy. We see that ringsin most galaxies are within 20 ◦ from polar orientation, soour objects are indeed “polar”-ring galaxies. Relative frac-tion of such galaxies in the combined sample (our work +Whitmore 1991) is 75% (55 of 73 galaxies). Fig. 8 shows the projected, apparent angles between the host galaxy and thering. Recently, Smirnova & Moiseev (2013) performed anal-ysis of spatial angles between two components in the sample c (cid:13)000
20 the mean value is 2.3 ± g − r colors and r -band luminosities. The results are shown inFig. 7. As one can see, stellar mass of a polar componentis not negligible in comparison with mass of a host in mostgalaxies. In some galaxies the masses of two components arecomparable.Fig. 8 presents the observed distribution of angle be-tween the ring and the central galaxy. We see that ringsin most galaxies are within 20 ◦ from polar orientation, soour objects are indeed “polar”-ring galaxies. Relative frac-tion of such galaxies in the combined sample (our work +Whitmore 1991) is 75% (55 of 73 galaxies). Fig. 8 shows the projected, apparent angles between the host galaxy and thering. Recently, Smirnova & Moiseev (2013) performed anal-ysis of spatial angles between two components in the sample c (cid:13)000 , 1–8 olar-ring galaxies Figure 5.
Absolute r -band magnitudes of host galaxies versusrings. Figure 6.
Absolute r -band magnitudes of hosts versus relativesizes of polar rings. of 78 galaxies from the SPRC and PRC. They came to thesame conclusion – in the majority of PRGs the outer struc-tures lie in the plane close to polar within 10 ◦ –20 ◦ . Figure 7.
Distribution of the sample galaxies over the ratio ofstellar mass of a ring to stellar mass of a host galaxy.
Figure 8.
Angular distance away from perpendicularity of thehost galaxy and the ring. Solid line – our measurements ( g pass-band), dotted line – the data from Whitmore (1991), dashed line– combined sample. Figure 9.
Distribution of our PRG sample galaxies (green) com-pared to the Atlas sample (black line), as a function of theratio between 22 µ m and K-band luminosities. The numbers forthe PRGs have been multiplied by 4 to compare with the muchlarger Atlas sample. All galaxies of our sample were detected in the four bands(3.4, 4.6, 12 and 22 µ m) of WISE, and we derived the 22 µ mluminosity in Table 2. From this luminosity, an indicationof the star formation rate (SFR) has been derived, follow-ing the calibration of Calzetti et al. (2007), also displayedin Table 2. The SFR for PRGs are distributed on the highside of what is found for typical early-type galaxy samples.In Fig. 9, we compare the distribution of the ratio between22 µ m and K-band luminosities, with the early-type sam-ple of Atlas (Davis et al. 2014). It is clear that the PRGgalaxies form relatively more stars than normal early-typegalaxies. This shows that the presence of the polar disks,although faint, are significative in this respect.Also, Fig. 10 shows the distribution of polar rings andhosts in the color-mass diagram. Objects with SFR higherthan 2.5 M ⊙ /yr are plotted with large symbols, PRG ringsare clearly in the blue cloud and the hosts are in the yellow c (cid:13) , 1–8 V. Reshetnikov, F. Combes
Figure 10.
The g − r color – stellar mass (in units of M ⊙ ) dia-gram (a) of the host galaxies (red symbols) and (b) of the polarrings (dark blue symbols). Galaxies with global star-formationrate > ⊙ /yr are marked by large symbols. Blue and yel-low points show locations of nearby ( z = 0 . − .
03) late-type(with S´ersic index n=0.8-1.2) and early-type (n=3.5-4.5) galaxiesfrom Simard et al. (2011) correspondingly. Galaxies with absolutemagnitudes − > M r > −
23 are shown only. one. In Fig. 11, the global colors are plotted in the samediagram (for polar rings and hosts together). The objects athigh SFR are distributed in the green valley mainly, show-ing that this location is due to the symbiose between twodifferent sub-systems.
We have performed a survey of photometric characteristicsin a sample of 50 PRGs selected from the SDSS. In general,we have confirmed previous results but on the basis of amuch larger and uniform sample.We have found that central objects of PRGs look likeearly-type galaxies by morphology and optical colors. Polarstructures demonstrate optical colors usual for spiral galax-
Figure 11.
The u − r – mass diagram of the PRGs according tothe SDSS total magnitudes corrected for the Milky Way absorp-tion and k -correction. Large symbols show galaxies with star-formation rate > ⊙ /yr, green lines show the green valley(Schawinski et al. 2014). ies. Typical stellar masses of the rings in our sample areabout 10 –10 M ⊙ and they contribute significantly to thetotal stellar mass of PRGs.The most interesting results can be seen in Fig. 4.Fig. 4b shows the Kormendy relation for the PRG hosts.It is evident that the location of PRGs is shifted from therelation for E/S0 galaxies. This feature was first noted byReshetnikov et al. (1994) for bulges of 6 PRGs in the B pass-band. They discussed this point and proposed that the ringsprojection distorts the surface brightness distribution of thecentral regions of galaxies. Projection of an absorbing ringcan reduce the observed luminosity of a galaxy and shiftits characteristics on the Kormendy relation. Iodice et al.(2002b) found a similar behavior in the K passband for thesample of 5 PRGs. But, since Iodice et al. used observationsin the infrared where the dust absorption is minimal, theyconcluded that this shift can be explained by the small sizeof the PRG bulges in comparison with normal early-typegalaxies. From the other side, our Fig. 4a demonstrates thatthe PRG hosts are not compact – they are more extendedin comparison with E/S0 galaxies of the same luminositiesand are located in the locus of spirals. This is true not onlyof late-type galaxies but also of early-type, bulge-dominatedgalaxies with n >
3. Therefore, an alternative explanationfor the shift of the PRGs on the Kormendy relation is thattheir hosts are fainter (in surface brightness and in luminos-ity) in comparison with normal early-type galaxies of thesame size.In general, as one can derive from Fig. 4, PRGhosts may represent a transitional class between early-and late-type galaxies. In this sence, they look similar toblue-sequence E/S0 galaxies described by Kannappan et al.(2009). Blue-sequence E/S0 galaxies are shifted from thestellar mass – radius relation for normal ETGs and arelocated between late-type and early-type galaxies in thisplane (see fig. 9 in Kannappan et al. 2009). At a fixed stellarmass, blue-sequence E/S0 galaxies are somewhat larger insize than E/S0 galaxies. This behavior is similar for PRGs:their hosts are larger at a fixed luminosity and they are c (cid:13) , 1–8 olar-ring galaxies Table 2.
WISE and 2MASS data for our PRG sampleGalaxy F L SFR L K (1) (2) (3) (4) (5)SPRC-3 14.41 42.792 0.94 10.32SPRC-5 9.13 42.345 0.38 9.36SPRC-6 6.74 41.859 0.14 9.05SPRC-7 7.71 42.961 1.33 10.32SPRC-9 11.58 43.954 10.1 11.28SPRC-10 10.89 42.797 0.95 10.63SPRC-11 10.46 43.176 2.06 11.12SPRC-12 13.51 43.240 2.35 10.27SPRC-13 8.04 42.406 0.43 10.48SPRC-14 10.32 42.518 0.54 10.83SPRC-15 6.42 42.376 0.40 11.02SPRC-16 11.58 43.138 1.91 10.62SPRC-17 10.47 42.357 0.39 10.64SPRC-18 18.05 43.613 5.03 10.59SPRC-19 10.06 43.592 4.81 10.76SPRC-20 13.92 43.411 3.33 10.84SPRC-23 61.94 43.175 2.06 10.64SPRC-24 12.07 42.935 1.26 11.14SPRC-25 6.78 43.081 1.70 10.58SPRC-27 6.99 42.722 0.82 10.53SPRC-28 6.43 43.112 1.81 10.67SPRC-29 5.55 42.602 0.64 11.05SPRC-30 8.23 43.193 2.14 10.54SPRC-31 13.00 43.016 1.49 10.97SPRC-34 11.43 43.408 3.31 10.47SPRC-35 7.39 43.049 1.60 10.38SPRC-37 10.39 43.198 2.16 11.00SPRC-39 13.02 42.545 0.57 10.27SPRC-42 8.25 42.145 0.25 10.39SPRC-44 8.71 43.600 4.90 11.47SPRC-47 10.32 42.500 0.52 11.00SPRC-48 8.08 42.922 1.23 11.13SPRC-49 8.39 43.116 1.83 11.23SPRC-51 4.76 42.959 1.33 10.81SPRC-53 5.79 43.129 1.88 –SPRC-55 4.97 43.097 1.76 10.82SPRC-56 4.91 42.685 0.76 11.38SPRC-57 6.44 43.027 1.52 10.82SPRC-63 6.61 43.084 1.71 10.55SPRC-66 117.91 44.488 29.9 11.29SPRC-67 4.00 41.984 0.18 10.90SPRC-69 14.93 42.451 0.47 10.43SPRC-70 13.82 43.336 2.86 10.37PRCA-1 7.61 41.896 0.15 10.40PRCA-4 10.86 42.266 0.32 10.67PRCA-6 5.83 41.763 0.12 10.40PRCB-17 21.92 41.077 0.03 9.02Columns:(1) name (SPRC or PRC),(2) Flux (22 µ m) in mJy,(3) log Luminosity (22 µ m) in erg/s,(4) SFR in M ⊙ /yr,(5) log L K (L ⊙ ). located between spirals and ellipticals on the Kormendy re-lation (Fig. 4a,b).Kannappan et al. (2009) have noted that blue-sequenceE/S0s are often associated with counter-rotating gas andpolar rings. We confirmed this conclusion from the oppositepoint of view – central galaxies of PRGs look similar to blue-sequence E/S0s.One interesting result is also the distribution of the starformation rate for PRGs. The ratio between 22 µ m and K- band luminosities is significantly larger for our PRGs samplethan for the early-type sample of Atlas . PRG galaxies area symbiotic objects, with two different sub-systems livingtogether, one in the red sequence, the other in the blue cloud,and the global ensemble appears to be in the green valley.Our results show that the structure of the PRGs hostsare more similar to quenched disks than to ordinary early-type galaxies. More detailed observations and sophisticatedmodeling required to solve the puzzles of this mysteriousclass of extragalactic objects. ACKNOWLEDGMENTS
We thank the referee for useful comments. VR express grat-itude for the grant of the Russian Foundation for Basic Re-searches number 13-02-00416. This work was partly sup-ported by St. Petersburg State University research grants6.0.160.2010, 6.0.163.2010, and 6.38.71.2012. VR acknowl-edges the hospitality of Paris Observatory, where a largepart of this work was done. FC acknowledges the Euro-pean Research Council for the Advanced Grant ProgramNumber 267399-Momentum. This publication makes use ofdata products from the Wide-field Infrared Survey Explorer,which is a joint project of the University of California, LosAngeles, and the Jet Propulsion Laboratory/California In-stitute of Technology, funded by the National Aeronauticsand Space Administration, and data products from the TwoMicron All Sky Survey, which is a joint project of the Uni-versity of Massachusetts and the Infrared Processing andAnalysis Center/California Institute of Technology, fundedby the National Aeronautics and Space Administration andthe National Science Foundation.
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