aa r X i v : . [ a s t r o - ph ] J un How Dry Are Red Mergers?
Jennifer L. Donovan , J. E. Hibbard , J. H. van Gorkom ABSTRACT
The focus of current research in galaxy evolution has increasingly turned tounderstanding the effect that mergers have on the evolution of systems on the redsequence. For those interactions purported to occur dissipationlessly (so called“dry mergers”), it would appear that the role of gas is minimal. However, if thesemergers are not completely dry, then even low levels of gas may be detectable.The purpose of our study is to test whether early type galaxies with HI in oraround them, or “wet” ellipticals, would have been selected as dry mergers bythe criteria in van Dokkum (2005, AJ, 130, 2647). To that end, we examine asample of 20 early types from the HI Rogues Gallery with neutral hydrogen intheir immediate environs. Of these, the 15 brightest and reddest galaxies matchthe optical dry merger criteria, but in each case, the presence of HI (for themajority, at levels > M ⊙ ) – as well as significant star formation in some cases– means that they are not truly dry. Subject headings: galaxies: evolution, galaxies: interaction
1. Introduction
The ways in which early type galaxies form and evolve continue to be matters of muchdebate. Proposed mechanisms of formation have gone full circle, from monolithic collapse(e.g. Eggen, Lynden-Bell, & Sandage 1962) through late mergers of fully formed disk galax-ies (e.g. Toomre & Toomre 1972, Kennicutt, Schweizer, & Barnes 1998), back to very earlyformation of at least the stars (though perhaps not the assembly of the galaxies themselves,e.g. Bower, Lucey, & Ellis 1992, Ellis et al. 1997, van Dokkum et al. 1998.) Certainly obser-vations suggest that at least some ellipticals were formed via mergers of gas-rich disks (e.g.Toomre & Toomre 1972, Schweizer 1982, Hibbard & van Gorkom 1996), though it is not ob-vious how common this formation history is. More recently, the debate has focussed on the Department of Astronomy, Columbia University, 550 West 120th St., Mail Code 5246, New York, NY10027; [email protected], [email protected] NRAO; [email protected] . < z ≤ . >
2. Recentstudies, however, have also produced observations consistent with a dusting of more recentstar formation in some ellipticals with otherwise ancient stars (e.g. Trager et al. 2000).Studying the B-band luminosity density evolution with redshift of the above early-typegalaxies over the range of 0 . < z ≤ . M/M = 0.09 ± − . From this rate, it can be inferred that between 0 < z <
1, the masses ofbright red galaxies have undergone an increase in their stellar mass density by a factor of ≥ ∼
2; however, they contend that decliningstar formation rates in blue galaxies are sufficient to produce L* and smaller red galaxies.On the high mass end of the red sequence, they find that ≈
80% of the stellar populationswithin the largest ( 4 L *) red galaxies were in place by z=0.7, and more recent mergers ofred galaxies are not the sole cause of the subsequent increase in stellar mass. Scarlata et al.(2007) argue against dry mergers as well, indicating that blue, irregular, and disk galaxiescan account for the mass increase since z=0.7 in < . L * early type galaxies via their evo-lution (and possibly via dissipational merging) into low- and intermediate-mass red earlytypes.At lower redshift, Schweizer et al. (1990) and Malin & Carter (1983) find that a signif-icant fraction of nearby field ellipticals exhibit signs of fine structure such as shells, indicat-ing the presence of a cold component. Among these samples, van Gorkom & Schiminovich(1997) find that 50% of ellipticals in gas-rich environments have associated HI at the ∼ M ⊙ level. So how “dry” exactly are the mergers in the more complete sample of VD05?What role do mergers of any type play in the evolution of red galaxies on the red sequence?We explore these questions in this paper. Specifically, we select red normal and peculiargalaxies which are morphologically similar to the sample in VD05 and whose HI content weknow in order to examine the likelihood that the VD05 sample can be truly “dry”. In § §
3, followed by our conclusions in §
4. Throughout the paper,we assume H o =75 km s − Mpc − .
2. The Samples2.1. NDWFS and MUSYC
In VD05, the 126 reddest and brightest field galaxies in the NOAO Deep Wide FieldSurvey (NDWFS) and the Multiwavelength Survey by Yale-Chile (MUSYC) around z=0.1were selected in order to uniformly study the morphologies of red galaxies between L* and3L*. The VD05 color selection criteria corresponds to 1.6 ≤ (B-R) ≤ > >
17 (c.f. Figure 2 in VD05). The galaxies wereclassified according to their optical morphology; approximately half of the sample exhibitlow surface brightness features indicative of interactions, such as plumes and tails (VD05).More dramatically, 70% of the bulge-dominated early-type galaxies show such features; thesebright, red, and morphologically selected systems are called red or “dry mergers” by VD05.The presence of tidal features was revealed by the deep imaging survey data, and each objectwas further classified as a “weak,” “strong,” or “ongoing” interaction. The color-magnitudediagram (CMD) of the VD05 sample is shown in Figure 1 with dotted lines highlighting thecolor and magnitude selection criteria.To make the sample of VD05 consistent with ours for the purpose of comparison, wemust adjust it to z=0 and apply a k-correction to both the R magnitudes and B-R colors toaccount for this change in redshift. The 126 galaxies of VD05 have a median redshift of 0.1,and we know the precise redshifts for ∼
80% of this sample; for the other ∼ c magnitudes in Poggianti (1997); for thesesystems we used the values published for Johnson R magnitudes. The difference betweenthe R and R c k-corrections is less than 0.03 mag out to z=0.24, which is accurate enoughfor our purposes.Taking these distances and k-corrections into account, the corresponding z=0 CMDof the VD05 sample is shown beside its z=0.1 counterpart in Figure 1, again with the(z=0.1) color and magnitude cutoffs displayed as dotted lines. The galaxies are no longerwell constrained within an apparent magnitude-dependent parameter space; nine systemsare actually fainter than the magnitude cutoff. For this reason, we define a new magnitudelimit of M R =-20.5, and we also extend the magnitude-color criterion, (B-R) > The purpose of our sample is to test whether early types with HI in or around them, or“wet” ellipticals, would have been selected as dry mergers by the criteria used in VD05. Forthis purpose, it is useful to study systems listed in the HI Rogues Gallery (Hibbard et al.2001), a collection of HI images of peculiar galaxies as well as otherwise normal galaxies with 5 –peculiar HI morphology. Specifically, we consider those listed as being early types (Roguesclasses pecEo, pecEi, EpecH – peculiar ellipticals with HI outside/inside the optical bodyand normal ellipticals with peculiar HI), and we include two well known merger remnants forcomparison (NGC 3921 and NGC 7252). Our sample consists of galaxies from these classeswith B and R magnitudes available in the literature that were measured inside appropriateaperture sizes. The sample and each galaxy’s Rogues classification are listed in Table 1. Thetotal number of galaxies in our sample is 20.
For the 126 galaxies in the VD05 sample, apparent R magnitudes and B-R colors weremeasured inside an aperture of 5 ′′ diameter. The central 5 ′′ diameter area of a galaxy atz=0.1 corresponds to its central 9 kpc diameter area.In order to directly compare the HI Rogues selected above to VD05, absolute R magni-tudes (M R ) and B-R colors are determined for each system from existing photometric datain the literature (see Table 1), using the HyperLeda database compilation of aperture pho-tometry (Paturel et al. 2003). Taking into account the largest quoted uncertainty of theseindividual studies, the photometry of our sample is accurate to better than 0.1 mag.The aperture sizes necessitated by each particular system in our sample differ sincethe galaxies are all located at different distances; the aperture size is chosen to match thecentral 9 kpc of each galaxy as closely as possible in accordance with VD05. In Table 1,we list the adopted distances for each system; distances are taken from the Nearby GalaxyCatalog (Tully 1988) unless otherwise noted. For the galaxies not listed in Tully (1988), wecalculate Hubble flow distances using Virgo-corrected velocities listed in NED. We also list aparameter R ap , defined as the ratio of the equivalent linear diameter of the adopted aperture(D ap ) to 9 kpc, or R ap = D ap , (1)for each galaxy. When R ap > ap is within 10% of 1.00; the four significant deviations arefor NGC 7626 (R ap =0.87), IC 2006 and NGC 2865 (R ap =0.84), and NGC 3921 (R ap =1.26).For the two most distant systems (d >
80 Mpc), NGC 3921 and Mrk 315, we also applymagnitude and color k-corrections as described above.We use the magnitude and color criteria specified in §
3. Results and Discussion3.1. “Wet” Red Rogues
As is evident in Figure 2, 15 (75%) of the HI Rogues sample would have been selected bythe VD05 color criteria for dry mergers. These 15 “wet” Red Rogues (with neutral hydrogenin or around them) are displayed in Figure 3 with HI contours superimposed; the three nor-mal ellipticals with peculiar HI are shown first, followed by the 12 peculiar ellipticals withHI. The presence of HI in these systems varies from lying in a well-defined disk and shells inCen A (NGC 5128) to being confined in a gas-rich companion for NGC 4382.The optical morphologies of these Red Rogues appear very similar to the sample studiedin VD05. 12 of the 15 Red Rogues are classified in the Rogues Gallery as pecEo and pecEi, asthey exhibit signs of tidal interaction at low surface brightness levels in the form of tails andplumes of stars; the other three Red Rogues are EpecH, or normal ellipticals from the opticalstandpoint. This matches the behavior of van Dokkum’s bulge-dominated population, 71%of which – the “dry mergers” – show tidal features, with the remainder exhibiting no clearsign of an interaction history down to very low surface brightness levels, making our “wet”peculiar and “wet” normal populations a counterpart to the “red” peculiar and “red” normalpopulations of VD05. A comparison of the deep co-added BVR optical imaging of cdfs-1100from the MUSYC survey, utilized in VD05, to a DSS image of NGC 7135 (one of the RedRogues) is shown in Figure 4; the galaxies in these two samples are clearly morphologicallysimilar.That 15 out of 20 Rogues would have been selected by the VD05 dry merger color cri-teria is in itself an interesting result, considering that the Rogues are a sample selected tohave gas. However, perhaps even more interesting is the ability to look for trends withinthe sample. In Figure 5, normal ellipticals and peculiar ellipticals are plotted using differentsymbols; no clear trend is apparent. This is possibly due either to our small sample size orto the fact that if we were to obtain deeper optical imaging (such as that used by VD05),it is possible that we would detect tidal features in our “normal” early type systems. The 7 –two merger remnants (MRs), NGC 3921 and NGC 7252, are indicated by asterisks; they areclearly too blue to be selected by the VD05 criteria. In VD05, 18% of the red objects arecurrently undergoing interactions, but our sample was chosen on the basis of looking almostexclusively at galaxies classified as ellipticals or peculiar ellipticals; we added two well knownmerger remnants for comparison, but clearly ongoing mergers are listed under other Roguesclasses. We therefore selected against “ongoing merger” systems in favor of systems whichhave likely already finished merging (i.e. the pecEo and pecEi galaxies, as well as the MRs).VD05 uses a subsample of merger pairs and merger remnants in order to estimate theeffects that dry mergers would have on the evolution of the luminosity function of red galax-ies. He finds that, with some caveats, dry mergers can explain the constant luminositydensity out to z=1, which is incompatible with only passive evolution of red galaxies. We,however, find that if the “wet” peculiar Red Rogues presented in this paper are actually thelocal analogues of these higher redshift merger remnants, then the VD05 systems are notnecessarily dissipationless.
It is interesting to note that one of the systems which falls unambiguously within theVD05 sample in Figure 2 is Markarian 315, which has a star formation rate of 30-40 M ⊙ yr − (Ciroi et al. 2005). The presence of HI coupled with such a high rate of star formationmakes this galaxy seem hardly dry. At least one other Red Rogue is also forming stars.Centaurus A (NGC 5128) has 1.5 × M ⊙ of HI associated with its shells and 4.5 × M ⊙ in the disk (Schiminovich et al. 1994), where vigorous star formation is occurring(Ebneter & Balick 1983). CO has also been observed in both the disk (at the level of 2 × M ⊙ ; Eckart et al. 1990), as well as in the shells, where it corresponds to 4.3 × M ⊙ ofH (Charmandaris, Combes, & van der Hulst 2000). Though Cen A has the appearance ofsimply an elliptical galaxy with a dust lane, it actually possesses a disk which is energeticallyforming stars despite its “red” classification. The presence of gas does not necessarily implystar formation in the other Red Rogues, but in the cases of Mrk 315 and Cen A it does.In light of our findings, it is worth considering whether the evolutionary histories derivedby VD05 are strongly constrained by the broadband filters available to him (BVRI). Theinclusion of shorter and/or longer wavelength data (UV, U -band, NIR) or spectroscopy wouldbetter constrain these histories, especially by breaking the degeneracy between extinction andstellar ages (e.g. Gil de Paz & Madore 2002, Anders et al. 2004) or by detecting “frostings”of younger stars (Trager, Faber, & Dressler 2001). If they revealed a component of youngstars, these better constrained star formation histories would then also constrain how “dry” 8 –a previous merger could have been.
4. Conclusion
In this paper, we have examined the selection criteria for picking out dry mergers asutilized in VD05 by comparing his sample to one consisting of early types with known HIproperties. For our small sample of 20 systems, we find that 15 galaxies with HI (or 75%)would have been selected by the VD05 dry merger criteria. VD05 invoked dry merging toexplain the evolution of the luminosity function of bright red galaxies since z=1. However,we show that selecting dry mergers on the basis of their colors is not sufficient to ensure thatthe systems will not have gas. The presence of HI in turn allows for the possibility of signifi-cant star formation, and our sample contains at least two examples for which this is the case.The authors wish to thank the referee, Dr. F. Schweizer, for his suggestions, whichhave greatly improved this manuscript. The authors also wish to thank A. Basu-Zych andB. Johnson for helpful comments and useful discussions. This work was supported in partby an NSF grant to Columbia University. We acknowledge the usage of the HyperLedaDatabase (http://leda.univ-lyon1.fr) and 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. Optical imageswere taken from the Digitized Sky Survey, produced at the Space Telescope Science Instituteunder U.S. Government grant NAG W-2166. The images of these surveys are based onphotographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain andthe UK Schmidt Telescope. 9 –Table 1: The SampleObject M R B-R d (Mpc) R ap Rogues Class. Ref.IC 2006 -19.81 1.44 15.7 0.84 EpecH dMCG -5-7-1 -20.27 1.60 58.3* 0.98 pecEi iMrk 315 -22.11 1.65 157* 1.01 pecEi jNGC 474 -20.92 1.54 32.5 1.05 pecEo c*NGC 680 -21.25 1.68 37.7* 0.95 pecEi *NGC 1052 -21.27 1.62 17.8 1.04 EpecH gNGC 1210 -20.33 1.25 48.7* 1.04 pecEi dNGC 1316 -22.25 1.55 16.9 0.97 pecEo aNGC 2768 -21.69 1.44 23.7 0.96 EpecH gNGC 2865 -21.24 1.43 35.7 0.84 pecEi eNGC 3921 -21.77 1.27 81.7* 1.26 MR bNGC 4125 -21.96 1.58 24.2 1.10 pecEo cNGC 4382 -21.67 1.45 16.8 0.90 pecEo hNGC 4406 -21.60 1.49 16.8 1.09 pecEo cNGC 5018 -22.12 1.53 40.9 0.96 pecEo eNGC 5128 -21.07 1.65 3.8** 1.12 pecEi dNGC 5903 -21.26 1.71 35.9 1.05 EpecH aNGC 7135 -20.75 1.64 34.7 1.11 pecEo cNGC 7252 -21.78 1.31 62.9* 1.01 MR cNGC 7626 -21.42 1.72 46.0* 0.87 pecEo fTable 1:
The sample. Columns display object name, M R in the Cousins system, B-R, dis-tance (Mpc), aperture fraction (defined in the text), HI classification in the Rogues Gallery(Hibbard et al. 2001), and references to the photometry: a, Sandage 1975; b, Huchra 1977; c,Sandage & Visvanathan 1978; d, Lauberts & Valentijn 1989; e, Poulain & Nieto 1994; f, Sandage1973; g, Peletier et al. 1990; h, Schroeder & Visvanathan 1996; i, Lauberts 1984; j, Moles et al.1987. Distances are taken from Tully (1988), except for those marked with an asterisk (*), whichare calculated using Virgo-corrected velocities from NED and assuming H o =75 km s − Mpc − inorder to be consistent with Tully (1988). The distance to NGC 5128 is taken from Rejkuba et al.(2005). Photometry from references a, b, c, f, and k have been converted to Cousins. The photom-etry for NGC 474 is marked as suspect by Sandage & Visvanathan (1978), and the unpublishedphotometry for NGC 680 is available through the HyperLeda aperture photometry database (adescription of this specific database is available in Prugniel & Heraudeau (1998)). A Rogues clas-sification of pecEo refers to peculiar ellipticals with HI outside of the optical body, pecEi refers topeculiar ellipticals with HI inside of the optical body, EpecH refers to normal early type galaxieswith peculiar HI, and MR refers to merger remnants.
10 –Fig. 1.— ( left ) CMD of sample used in VD05 at z=0.1; R magnitudes are apparent. ( right )Same sample adjusted to z=0 and k-corrected; R magnitudes are absolute. Adjusting thez=0.1 sample (at left) to z=0 (at right) makes the specified apparent-magnitude dependentcriterion no longer a valid photometric constraint on the population. All crosses encircledwith diamonds represent galaxies for which the redshift is not known; for these galaxies, wehave assumed z=0.1. 11 –Fig. 2.— CMDs of the HI sample (squares) and the VD05 sample (+) with both data setspresented in M R . The dotted lines display the VD05 selection criteria as well as our extensionof the color and magnitude criteria to fainter magnitudes. 12 –Fig. 3.— The Red Rogues, as published in the Rogues Gallery, with HI contours over-laid on optical DSS images. Row 1: NGC 2768 (Schiminovich et al. 2001), NGC 5903(Appleton, Pedlar, & Wilkinson 1990), NGC 1052 (van Gorkom et al. 1986), NGC 5128(Schiminovich et al. 1994); Row 2: NGC 680 (van Moorsel 1988), NGC 1316 (Horellou et al.2001), NGC 7626 (Hibbard & Sansom 2001); Row 3: NGC 4125 (Rupen, Hibbard, & Bunker2001), NGC 4382 (Hibbard & Sansom 2001), NGC 4406 (Li & van Gorkom 2001),NGC 5018 (Kim et al. 1988); Row 4: Mrk 315 (Simkin & MacKenty 2001), NGC474 (Schiminovich et al. 2001), NGC 2865 (Schiminovich et al. 1995), NGC 7135(Schiminovich et al. 2001). 13 –Fig. 4.— Deep optical imaging of two morphologically similar galaxies: cdfs-1100 from VD05(top) and NGC 7135 from the Red Rogues sample (bottom). 14 –Fig. 5.— HI sample divided into normal (+) and peculiar (diamonds) ellipticals. The colorand magnitude selection criteria used in this paper are also displayed. The two asterisks (*)indicate the merger remnants, which are two of the bluest systems in the HI sample. 15 – REFERENCES
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