A Deep K-Band Photometric Survey of Merger Remnants

A Deep K-band Photometric Survey of Merger Remnants

B. Rothberg and R. D. Joseph

Institute for Astronomy, University of Hawaii, Honolulu, HI 96822

ABSTRACT

K-band photometry for 51 candidate merger remnants is presented here to assess the viability of whether spiral-spiral mergers can produce bonafide elliptical galaxies. Using both the de Vaucouleurs r1/4 and Sersic r1/n fitting laws it is found that the stellar component in a majority of the galaxies in the sample have undergone violent relaxation. However, the sample shows evidence for incomplete phase-mixing. The analysis also indicates the presence of “excess light” in the surface brightness profiles of nearly one-third of the merger remnants. Circumstantial evidence suggests this is due to the effects of a starburst induced by the dissipative collapse of gas. The integrated light of the galaxies also shows that mergers can make L∗ elliptical galaxies, in contrast to earlier infrared studies. The isophotal shapes and related structural parameters are also discussed, including the fact that 70% of the sample show evidence for disky isophotes. The data and results presented are part of a larger photometric and spectroscopic campaign to thoroughly investigate a large sample of mergers in the local universe.

Subject headings: galaxies: evolution — galaxies: formation — galaxies: interactions — galaxies: peculiar — galaxies: starburst — galaxies: structure

1. Introduction

The study of galaxy mergers is not new. As early as 1940, Holmberg studied galaxy interactions using analog models. In 1960, Zwicky & Humason suggested gravitational tidal forces were responsible for producing ﬁlaments connecting neighboring galaxies. These novel theories languished until Toomre & Toomre (1972) postulated that observed tidal tails and bridges found in the Atlas and Catalogue of Interacting Galaxies (Voronstov-Velyaminov 1959) and the Atlas of Peculiar Galaxies (Arp 1966) were the relics of previous encounters between galaxies. Toomre (1977) took this a step further by suggesting that disk-disk mergers could create a new elliptical galaxy. He presented 11 objects which were in various stages of interaction. The earliest of Toomre’s objects still had two distinct nuclei, and the latest had a single coalesced nucleus. If these objects are in the process of forming elliptical galaxies, then the most advanced candidates in the series should exhibit some of the same properties commonly found in ellipticals. – 2 –

One important photometric test of the merger scenario is to determine whether the stellar component in the merger is spatially distributed in the same way as an elliptical galaxy. de Vau- couleurs (1953) characterized the stellar distribution of elliptical galaxies by an r1/4 fit to their surface brightness profiles. The r1/4 stellar distribution is produced as a result of the dissipationless collapse of the system during its formation (van Albada 1982). One way to initiate this is the process of violent relaxation (Lynden-Bell 1967, Hjorth & Madsen 1991), in which the stars are scattered by the net gravitational field of the system. This has the effect of altering the kinetic energy of the stars depending on their initial positions and velocity and thus altering their orbits in the galaxy. This leads to another important process, phase-mixing. Violent relaxation alters the kinetic energy of the stars, which has the effect of smoothing out the once-correlated orbits of the stars in the galaxy over time. The effect on a merger is that structures are slowly smoothed out as the stars, now with different periods, drift out of phase with each other. Generally speaking, spiral and elliptical galaxies are structurally different; they have dis-similar stellar distributions, and elliptical galaxies are smoother in appearance than spirals. In order for mergers to make ellipticals, the merging event must trigger violent relaxation and subsequently induce phase-mixing to “erase” structures in the merger. Early numerical modeling of mergers (i.e. Barnes 1988, 1992) showed that the merger of two spiral galaxies can produce an object in which the cold stellar component undergoes violent relaxation rather quickly and produces an object with an r1/4 stellar distribution. The remnants produced in these simulations all had effective radii, shapes, and velocity dispersions consistent with elliptical galaxies. The process of phase-mixing takes much longer to complete than violent relaxation. While the central body may become devoid of distinct structure, models show that plumes and shell are often present, and they are composed of stars with a wide range of orbital periods. Over time, these features are smoothed out. However, the slow return of tidal material can lengthen the completion of the phase-mixing process, extending it over ≃ 109 years. One way to test for violent relaxation is to use photometric observations to construct surface brightness profiles for mergers. If the profile can be characterized by an r1/4 de Vaucouleurs law, it suggests that violent relaxation has redistributed the stellar component. An important aspect of this is the spatial extent to which the profile can be fit by a de Vaucouleurs law. Merger models, such as those used by Barnes (1988, 1992) incorporate bulge components into the progenitors. Since bulges have luminosity profiles that are approximately similar to r1/4, it is not surprising to find a similar profile in a merger, at least within the central few kiloparsecs. However, violent relaxation does smooth the transition between the bulge and disk profile and replaces the outer exponential profile with an r1/4 profile. A broad measure of the completeness of the violent relaxation can be made by investigating how well and the radial extent over which the luminosity profile is fit by an r1/4 law. The question of whether phase-mixing has occurred and to what extent can be addressed by using the images to look for well-resolved structures. While plumes, ripples, shells and tidal tails are present in the outer regions of the merger, the greater degree to which the central body is uniform can be taken as a measure of how far the phase-mixing has progressed. – 3 –

Early optical observations of a handful of merger candidates (Schweizer 1982, 1996, Lutz 1991, Hibbard & van Gorkom 1996) have shown that they are sometimes well described by an r1/4 profile. Unfortunately, optical studies suffer from two serious drawbacks. The first is extinction due to dust within the galaxies, which can block out stars and alter the surface brightness profile from its true shape. The second, is that optical observations are heavily contaminated by the light of younger, hotter stars. The blackbody emission from these stars peaks at UV and optical wavelengths. The problem is amplified if mergers undergo star-formation. Population synthesis models from Bruzual & Charlot (1993) show that for a single population burst model, young, hot stars dominate the contribution to the total light of a galaxy at optical wavelengths. These stars do not represent the majority of the stellar mass in the merger. The stellar mass in both elliptical and spiral galaxies are dominated by older, late-type stars (Aaronson 1981). Since mergers are formed from the collision of two spiral galaxies, most of the stellar mass lies in older, late-type stars. Therefore, an optical r1/4 surface brightness profile is not a true measure of the distribution of the majority of the stellar mass. These drawbacks can be overcome by measuring the surface brightness profiles in the infrared. There are two advantages of using infrared wavelengths. First, extinction is much less than in the optical. Second, the blackbody emission of older, late-type stars, which are excellent tracers of the stellar mass of both elliptical and spiral galaxies, peaks in the near-infrared beyond 1 µm (Aaronson 1977). Silva & Bothun (1998) measured the global infrared colors of the well-known merger NGC 3921, which is included in the sample presented in this paper, and found that (J-H)◦ = 0.63 and (H-K)◦ = 0.21. This is consistent with the near-IR light dominated by older, late-type stars formed in the disk progenitors and later contributed to the merger. One possible form of contamination to deal with in the infrared is the presence of AGB stars if a starburst has recently occurred. The blackbody emission from these stars also peaks in the infrared. Mouhcine & Lan¸con (2002) modeled the contribution of AGB stars to the total K-band luminosity for a single burst population. They found the contribution evolves rapidly from a few percent at ≃ 0.1 Gyr, to ≃ 60% of the light at 0.6-0.7 Gyr, and then quickly declines soon after to ≤ 30%. The contribution to the integrated light is directly proportional to metallicity, and therefore decreases as metallicity decreases. These stars are more likely to contribute to the light from the nucleus where a central starburst may occur, rather than globally to the integrated light of the entire galaxy. Another possible source of contamination of infrared light comes from the presence of hot dust heated from young stars formed during a starburst. However, the emission from hot dust peaks beyond the near infrared and contributes to less than 10% of the light at K-band (Aaronson 1977). Thus, taking the above factors into consideration, the infrared, in particular the K-band at 2.2 µm, is an optimal observational window for testing whether the stellar mass in mergers has undergone violent relaxation. There have been several earlier attempts to measure the surface brightness profiles of a few mergers at K-band (Wright et al. 1990, Stanford & Bushouse 1991, James et al. 1999). These observations did show that a few mergers were well fit by an r1/4 profile and a few were not. However, these early studies were hampered by arrays with very small fields of view, low sensitivity, and – 4 – large plate scales that resulted in very coarse resolution. These factors limited the spatial extent of the observations, typically to less than 2-3 kpc, well within the range of possible bulge contamination. The dynamic range of the measured surface brightness was typically only 4-5 magnitudes. In comparison, simulations of mergers have shown good r1/4 fits to the luminosity profiles over much larger dynamic ranges. Follow-up studies using arrays with larger fields of view and coupled with much deeper integration times are necessary to probe both spatial scales beyond where a bulge may contaminate the profile and to test how well the luminosity profile can be fit with a de Vaucouleurs law over larger dynamic ranges. However, one strong argument against the mergers-make-ellipticals picture is the difference in the stellar phase-space densities between elliptical and spiral galaxies. Carlberg (1986) used surface photometry of galaxy cores to compare the phase-space densities of elliptical and spiral galaxies. The results showed that all but the brightest ellipticals have significantly higher phase-space densities than spirals. According to the collisionless Boltzmann equation, the phase-space density of a stellar system remains constant or decreases. It cannot increase. Therefore, most elliptical galaxies cannot be formed by purely dissipationless mergers. One solution is if mergers undergo dissipative collapse. Only dissipation can increase the central phase-space density. In essence, more stars are needed in the central regions of the merger than can be contributed solely by the progenitor spirals. A starburst triggered by dissipation is one potential solution to this problem. This has been shown by Larson & Tinsley (1978), who suggested the UBV colors of peculiar galaxies were a reflection of strong star-formation, and later by Joseph & Wright (1985) using JHKL colors. These studies, among others, demonstrated that mergers can trigger starbursts in their centers. It can increase the central stellar density of the merger, putting it on par with the levels expected in a typical elliptical galaxy (Kormendy & Sanders 1992). Mihos & Hernquist (1994) (hereafter MH94) were the first to include the effects of dissipative collapse in their merger models. Their models predict that when the gas falls to the center of the merger, a strong starburst is triggered. The resulting central starburst should affect the shape of the surface brightness profile, producing a sharp increase in luminosity near the nucleus. This increase in luminosity should be detectable in the surface brightness profile as a rise above an r1/4 profile near the center. As the starburst fades and the young, hot stellar population evolves into red supergiants, the luminosity fades and the rise in the surface brightness profile decreases to some degree, but should still remain present. Their models are not complete in so far as predicting the extent to which the luminosity increases or how much excess light is present in the surface brightness profile. This is likely due to an incomplete understanding of the behavior of gas on scales of a kiloparsec or less, as well as assumptions made about the parameters of the starburst, including star-formation rates, initial mass function, gas to stellar conversion rates, etc. Moreover, as detailed in Mihos & Hernquist (1996), the presence or absence of a bulge in one or both progenitors, as well as the encounter geometry, can affect the timescale for the onset of the starburst and the intensity of the star-formation that occurs. To date, only one galaxy has been reported as showing signs of excess central luminosity in its surface brightness profile (van der Marel & Zurek 2000). Another related question is whether mergers can produce L∗ elliptical galaxies. If spiral-spiral – 5 – mergers are responsible for creating a significant number of elliptical galaxies in the universe then some of them should be capable of forming an L∗ elliptical galaxy. A number of infrared studies have suggested that mergers do not produce L∗ galaxies (Shier & Fischer 1998 and Colina et al. 2001). While others, most notably, Genzel et al. (2001) find that some mergers can produce objects with L ≃ 1 L∗ elliptical. These studies targeted morphologically disturbed Luminous and Ultra- 11 Luminous infrared galaxies (LIRGs & ULIRGs). These are objects with LF IR ≥ 10 L⊙ (where LF IR is measured from 8-1000 µm; see Sanders & Mirabel 1996 for a review). Earlier studies of mergers, such as those by Schweizer (1982, 1996) of NGC 7252 and NGC 3921 respectively, showed that MB and MV were comparable to luminous elliptical galaxies. Neither of these two mergers are LIRGs/ULIRGs. However, young, hot stars may contribute significantly to the B-band and V-band luminosities of these galaxies. Thus it is useful to investigate a large sample of mergers at K-band, which includes objects in common to the above studies, and determine the number which are consistent with forming an L∗ or brighter elliptical galaxy. In addition to the surface brightness profiles, the K-band photometry can be used to extract structural parameters that provide important insight into both mergers and elliptical galaxies. It is well-known that elliptical galaxies can be divided into two classes, those with boxy isophotes which are anisotropic, slow rotators, with high luminosity, and those with disky isophotes which are isotropic, rotationally supported and low luminosity (Illingworth 1977 and Davies et al. 1983). Bender, Döbereiner, & Möllenhoff (1989) showed a correlation between boxy ellipticals and both radio and X-ray luminosities. The fact that different isophotal shapes show characteristically different physical properties unrelated to their photometry has led some to conclude that disky and boxy ellipticals have separate formation mechanisms (Kormendy & Bender 1996). It has been suggested by Kormendy & Bender, among others, that mergers may be the means by which one type of elliptical is formed. Numerical models of mergers, on the other hand, have shown that isophotal shapes and ellipticity may be a consequence of the mass ratios of the progenitors. If spiral-spiral mergers can make ellipticals, they can make ellipticals with both boxy and disky shapes. Naab, Burkert, & Hernquist (1999) modeled 1:1 and 3:1 mass mergers to determine if either type preferentially formed disky or boxy type objects. They found that, regardless of projection effects, 1:1 mass mergers produced boxy ellipticals and 3:1 mass mergers produced disky elliptical galaxies. Using more sophisticated models, Naab & Burkert (2003) found that 1:1 mergers produce boxy isophotes depending on the alignment of the progenitors during the initial encounter. In many cases, their simulations of 1:1 mass mergers produced ellipticals with disky isophotes. The presence of a disk in ellipticals could be the result of a couple of possibilities. First, in the case of unequal mass ratios, the disk of the larger progenitor can partially survive the merging event (Bendo & Barnes 2000). The second possibility, discussed by Naab & Burkert (2001) and modeled in greater detail by Barnes (2002), is that a stellar disk forms from a gaseous disk after the merger is complete. One problem, until now, is that theorists have only had structural data on elliptical galaxies with which to compare their models. One does not know a priori whether these ellipticals were actually formed in merging events or not. Few, if any, details of the observed isophotal shapes of – 6 – mergers exist in the literature. The structural parameters extracted from the K-band data presented here provide a unique opportunity to compare models with real mergers. Presented here are the results of a K-band imaging survey of 51 merging systems. The survey takes advantage of a relatively newer infrared array, with a larger field of view and better sensitivity than previous devices. In conjunction with the excellent seeing of Mauna Kea, the quality of this data is useful for probing not only out to large spatial scales, in some cases beyond 20 kiloparsecs, but to investigate areas close to the nucleus as well. The results of the survey include surface brightness profiles, integrated K-band magnitudes and structural parameters derived from the photometry. This survey is the first part of a larger campaign to fully investigate the photometric and spectroscopic properties of mergers. For all measurements and calculations in this paper a value of −1 −1 H◦ = 75 km s Mpc is used.

2. Sample

It is difficult to select a “complete” sample of mergers in any sense of the word, and no claims of completeness are made for the sample presented here. The source catalogs for disturbed galaxies are all compiled based on their optical features, with little regard for discriminators such as distance, size, luminosity, spectral features, etc. Moreover, many of the galaxies in these catalogs have very little, if any, supplementary data beyond their sky positions and images from the Palo- mar Observatory Sky Survey. The sample presented here is tailored to be reasonably large and to include only objects in the advanced stages of merging. The sample selection was based on optical morphology alone because merger identification is a visual selection criterion. Selections based on other criteria, such as far-infrared luminosities, may introduce a bias into the sample towards objects undergoing star-formation. The 51 objects in the sample presented here were selected primarily from the Atlas of Pecu- liar Galaxies (Arp 1966), A Catalogue of Southern Peculiar Galaxies (Arp & Madore 1987), the Atlas and Catalog of Interacting Galaxies (Vorontsov-Velyaminov 1959), and the Uppsala General Catalogue (Nilson 1973). The selection process varied depending on the source. Objects selected from Arp’s atlas were chosen by visual inspection of the plates because every object listed has an accompanying image. Objects selected from the Arp-Madore, VV Catalog, and UGC sources were first selected by listed morphology. Arp-Madore sources were selected from categories 7, galaxies with linear jets; 15, galaxies with tails, loops, and debris; and 16, disturbed galaxies that are iso- lated. VV and UGC sources were selected from the peculiar, disturbed, eruptive, jet, and irregular sub-categories. Each galaxy making the first cut was then visually inspected using the Digitized Sky Survey and any reference sources with published optical images. The selections were based strictly on optical morphology and according to the following criteria: 1) Tidal tails, loops, and shells, which are induced by strong gravitational interaction. 2) A single nucleus, which, based on numerical studies, marks the completion of the merger. This criteria is important because it marks the point at which the merger should begin to exhibit properties in common with elliptical galaxies. – 7 –

3) The absence of nearby companions which may induce the presence of tidal tails and make the object appear to be in a more advanced stage of merging. 4) The mergers must be observable from Mauna Kea. This limited the survey to objects with declinations ≥ -50◦. One object from this sample, AM 0612-373, has no previously recorded redshift. The redshift used to determine a distance to this galaxy is taken from spectroscopic data currently under analysis. The sample properties are listed in Table 1 and include name, R.A. and Dec., recessional velocity, total integration time, seeing, and the spatial scale of the isophotes (FWHM) in parsecs. The names are broken down based on their catalogs, the first column corresponds to the NGC catalog, the second column corresponds to either the Atlas of Peculiar Galaxies or the Catalogue of Southern Peculiar Galaxies, the third column to the UGC catalog, the fourth column to the Atlas and Catalog of Interacting Galaxies and the fifth column shows the IC number or other well-known designation. Since most of the objects have multiple designations, all subsequent references to sample galaxies within the paper, tables and figures will first use the NGC designation if available, followed by the Arp or Arp-Madore (AM), UGC, VV and lastly the IC designation if no other designation is available. Unless otherwise noted, the galaxies are listed in order of Right Ascension in tables and figures. Within the sample there are two sub-samples. The first are “shell ellipticals” which comprise 6 of the 51 objects in the sample. It is widely suspected that shell ellipticals are formed from some type of interaction, possibly from the accretion or tidal stripping of a less massive companion. How- ever, there is some theoretical evidence that shell ellipticals may be formed from mergers of equal or nearly equal mass galaxies (Hernquist & Spergel 1992). One piece of observational evidence is NGC 3656 (included in the sample), which Balcells (1997) found to have two faint tidal tails (µR ≃ 26.8 mag arcsec−2). A second sub-sample are the LIRGs & ULIRGs. A significant number of these galaxies are morphologically disturbed. This has inspired a number of studies that offer a merger origin for their intense far-infrared luminosities. While none of the objects in our sample were selected based on their LF IR, it is important to note that 3 mergers in the sample are classified 12 11 as ULIRGs (LF IR ≥ 10 L⊙) and 13 are classified as LIRGs (LF IR ≥ 10 L⊙). The shell ellipticals and LIRGs/ULIRGs are noted in Table 1 and marked with different symbols in the figure plots.

3. Observations and Data Reduction

Near-infrared images were obtained using the QUIRC 1024x1024 pixel HgCdTe infrared array (Hodapp et al. 1996) at f/10 focus on the University of Hawaii 2.2 meter telescope. The field of view of the QUIRC array is 193′′ x 193′′ with a plate scale of 0.189′′ pixel−1. Exposures from QUIRC were obtained by nodding between the source and a blank field of sky. The on-target observations were dithered so as to remove any array defects in the final processing. The infrared photometric standards were selected based on proximity to the targets in Right Ascension and Dec- lination. Multiple observations of standard stars were taken before and after target observations. Standards were selected from Persson et al. (1998), Carter & Meadows (1995), Hunt et al. (1998), – 8 – and Hawarden et al. (2001), which includes the UKIRT faint standards. The median seeing for the observations was 0′′.8 (corresponding to a median spatial resolution of 377 pc) and all the data were obtained under photometric conditions. The long integration times −2 allowed for 3σ detections out to a surface brightness of µK ≃ 20.7 mag arcsec . This corresponds to a median spatial radius of ≃ 10.1 kpc. The K filter used in this survey for all but two objects conforms to the new Mauna Kea Infrared Filter Set (Tokunaga, Simons, & Vacca 2002). One object, UGC 6 was observed with the K’ filter (Wainscoat & Cowie 1992) and has been converted to K using their conversion equation. Another object, IC 5298, was observed with an older K filter with similar properties to the Mauna Kea K

filter. No conversion was made and it is assumed that Kold ≃ KMaunaKea. The data set was reduced using IRAF. Surface brightness profiles were extracted from elliptical isophotes using the ELLIPSE package from STSDAS. Elliptical isophotes are more sensitive than circular isophotes to the true structure of galaxies. Therefore they are more useful for investigating whether mergers are relaxed or not. The ELLIPSE program failed to properly converge when at- tempting to fit elliptical isophotes to one object, AM 0956-282. As a result no structural parameters were extracted and all photometric work for this object was done using circular apertures. The ELLIPSE fitting was conducted by first using IMEXAMINE to find the maximum brightness at the center of the galaxy. The center position was held constant while the ellipticity and position angle were allowed to vary freely. The choice was made to hold the center position constant, rather than allowing it to vary, in order to guarantee that the fitting algorithm would be able to function out to large radii. Schweizer (1996) noted that the isophotal center of NGC 3921 seemed to move as the isophotes were measured progressively outward. Finally, the algorithm failed to properly measure the isophotes only 20′′ from the nucleus. While it is interesting to note that the center of the merger may appear to “slosh” around, it is more important that the isophotes can be measured out to large radii. The sloshing is merely an artifact of the irregularity of the isophotes, which are better quantified by the a3/a and a4/a parameters. The semi-major axis of each isophote was increased in linear steps of size equal to the seeing. The seeing estimates for the galaxies were obtained by measuring the FWHM of high S/N stars in the field and taking the average value. Stars in the field were masked and those pixels were ignored in the isophote fitting and flux measurement. The following output was taken from ELLIPSE for each isophote: the area in usable pixels for each annular elliptical isophote, the summed flux in the elliptical isophote, the ellipticity, position angle, the B4 and B3 parameters, and the computed errors for ellipticity, position angle and B4 and B3. The two parameters, B3 and B4, are the amplitudes of the cos 3θ and cos 4θ terms which measure the deviations of the isophote in relation to a perfect ellipse. They were converted to the widely used a3/a and a4/a parameters (see Milvang-Jensen & Jörgensen 1999) which are taken relative to the semi-major axis instead of the equivalent radius. The output from ELLIPSE was put into an IDL program which computes the surface brightness, S/N and errors at each isophotal radius. Two separate programs were written to fit a de Vaucouleurs r1/4 and a Sersic r1/n profile to the data (Sersic 1968). The fits were made using a Levenberg-Marquardt non-linear least squares minimization technique to achieve a good fit (Press et al. 1992). All data – 9 – points were used in the fits. It is important to make clear the distinction in terminology between reff and Reff . The for- mer refers to the effective radius in arcseconds, whereas the latter is a metric value in kiloparsecs. Reff will be used throughout the rest of the paper. No corrections to the K-band data have been made for galactic extinction. Since the reddening at K is very small, any such corrections, even for objects at low galactic latitude would be negligible and well within the errors from standard star calibrations.

4. Results

Figure 1 is crafted to provide a visual representation of most of the results of the investigation as it pertains to each galaxy. It shows vertically from top to bottom for each galaxy: the K-band image, the surface brightness profile fit with a de Vaucouleurs law, the surface brightness profile fit with a Sersic law, and plots of the structural parameters for 50/51 of the galaxies in the sample. There are no structural parameters plotted for AM 0956-282, as noted earlier. The images are displayed with a log-stretch in order to both enhance the faint details such as tidal tails, and show details within the central parts of the galaxy. A metric bar is overplotted to provide a scale for the images. A number of galaxies show resolved structure, including possible stellar clusters, central disks and dust lanes. The images are displayed in order to complement the surface brightness profiles and structural information.

4.1. Violent Relaxation

4.1.1. The de Vaucouleurs r1/4 Proﬁle

One goal of measuring the surface brightness profiles is to test whether and to what extent the mergers in the sample have undergone violent relaxation. The preferred method for this is to determine if the mergers share the same stellar distribution as elliptical galaxies. The de Vaucouleurs profile has been used for some time as a means of describing the stellar distribution in elliptical galaxies. The de Vaucouleurs profile is defined as:

r 1/4 µ(r)= µeff +b ( ) − 1 (1) reff where b = 8.325 so that reff is the radius of the isophote containing half the total luminosity, and µeff is the surface brightness at the effective radius. The surface brightness profiles for all 51 mergers in the sample are shown fit with a de Vaucouleurs profile in the 2nd plot from the top for each galaxy in Figure 1. Each point plotted represents one seeing radius (see Table 1 for spatial scales for each galaxy) along the major axis. The data are plotted spatially out to a 3σ detection limit over the background. The dashed line is a best-fit model to the data. The vertical lines with – 10 – bars plotted over each point are the 1σ errors in photometry. Points with a “×” symbol overlaid indicate that the data is at least 2σ above or below the r.m.s. of the model fit. The data are plotted against radius1/4, so a straight line is a perfect r1/4 profile. Since the galaxies all lie at different distances and are subject to different seeing limits, the data are plotted in metric units of kpc1/4 along the x-axis. As a reference, a second axis is displayed along the top of each plot that shows the radius in linear units of kpc. In order to assess the extent to which violent relaxation has occurred, if at all, several tests were conducted on the surface brightness profiles. These quality assessments are used to form an overall picture of each galaxy. To begin with, the spatial extent to which the profiles are well-fit by an r1/4 profile was assessed. Column 2 of Table 2 describes whether or not each galaxy is fit by a de Vaucouleurs profile out to the last isophote (within the 1σ photometric errors) and Column 3 of Table 2 lists the radial distance of the last isophote. The spatial extent of the profile is used to determine whether the violent relaxation is complete or incomplete to the limit of the K-band data. In a few cases, the spatial extent of the data is small enough that an r1/4 profile may be the result of the presence of a bulge rather than indicative of the galaxy as a whole. Next, a reduced chi-square (unitless) and r.m.s. (in units of mag arcsec−2) were computed for the de Vaucouleurs 2 profile fit to each galaxy. Equal weighting was assigned to each point in the fit. The χν and r.m.s. for each galaxy is listed in columns 4 and 5 of Table 2. These numbers reflect how well, overall, the surface brightness profile of each galaxy is fit by a de Vaucouleurs profile. Out of the 51 galaxies in the sample, 9 fail to follow an r1/4 fit out to the last isophote. Seven 2 of those ten galaxies have χν values much greater than the median for the sample, with a range of 2.02-4.66, indicating poor fits overall. Of those seven, two galaxies, UGC 2238, NGC 6052 have the largest r.m.s. values. This suggests that these objects may not have undergone violent relaxation at all. The remaining five galaxies can only be classified as questionable. The remaining two galaxies, NGC 2744, and Arp 193, which do not follow an r1/4 profile out to their last isophote, still show 2 reasonably good fits (χν ≃ 1) and may have undergone incomplete violent relaxation. Moving on to the 42 galaxies for which the last isophote still follows an r1/4 profile, it is important to note that three galaxies, Arp 230, AM 0956-282, and NGC 3310 are limited in the spatial extent to which their isophotes are measured. The last isophote for each of these galaxies are 3.2 kpc, 2.6 kpc, and 3.3 kpc respectively. While Arp 230 is classified as a shell elliptical, and therefore likely to have undergone complete violent relaxation, the same cannot be said of the other two galaxies. It is possible that for these objects, a remnant bulge is responsible for, or significantly contributes to, the profile shape. 2 The remaining 39 galaxies show a mix of χν and r.m.s. values, ranging from good to poor. 2 The data show that 15/39 have χν values larger than the median value of 1.08. In particular, 9 objects, UGC 6, NGC 1614, NGC 2623, NGC 2782, NGC 2914, NGC 4004, AM 2038-382, NGC 2 7135 and AM 2246-490 show χν values >> 1. This does not mean that all of these galaxies have not undergone violent relaxation. On closer inspection, many of the these galaxies with apparent poor fits show the strongest deviation from an r1/4 at small scales near the nucleus. Beyond the central regions, the fits appear quite good, save for NGC 4004, which has a rather chaotic profile. – 11 –

The de Vaucouleurs profile seems to work reasonably well on large scales. It appears that the profile shape near the center of the galaxy is most responsible for poor fits. The profile shape near the center may be influenced by the central luminosity of the galaxy. As an experiment, the de Vaucouleurs profile was fit to the galaxies again, but this time ignoring the central 1 or 2 points. The fits tended to improve considerably. The de Vaucouleurs profile suggests a significant number of galaxies have undergone violent relaxation to the limits of the K-band data. However, some 2 of these galaxies have high χν ’s and large r.m.s. values. Additional information is needed to reconcile this problem. Several galaxies show evidence of incomplete violent relaxation as well as 2 poor fits. However, their χν and r.m.s. values are actually smaller than some of the galaxies which show evidence of violent relaxation to the last isophote. It is therefore useful to obtain additional information to confirm whether or not the poor fits indicate that either incomplete or no violent relaxation has occurred.

4.1.2. The Sersic r1/n proﬁle

The Sersic profile was selected as an additional tool to probe the surface brightness profiles of the mergers in the sample and to augment the information obtained from the de Vaucouleurs profile. The profile, r1/n is a generalized form of the r1/4 de Vaucouleurs profile:

r 1/n µ(r)= µeff +bn ( ) − 1 (2) reff where n is a free parameter. The constant b becomes bn = 0.868n - 0.142. This is the numerical solution of the Sersic fitting function taken from Caon, Capaccioli, & D’Onofrio (1993), hereafter CCD93. This solution retains reff as the isophote containing half the total luminosity. The Sersic form is more general than the original de Vaucouleurs law because it can take the form of a de Vaucouleurs profile (n = 4) or an exponential disk profile (n = 1). The profile is more sensitive than the de Vaucouleurs law in fitting the central regions of galaxies. Objects which have values of 2 n > 4.0 may yield poor r.m.s. and χν values if fit with a de Vaucouleurs law. This is because the central luminosity in these galaxies is quite high, which forces a deviation from the r1/4 fit, but this does not mean they haven’t undergone violent relaxation. The Sersic profile is also useful for fitting the profile to more spatially extended areas. A poor fit to a de Vaucouleurs profile at large radii may indicate that violent relaxation has not occurred, but it also may indicate the strong influence of a disk. An n ≃ 1.0 Sersic profile shows a significant luminosity drop on large scales, while Sersic profiles with n > 2.0 are very similar at large radii. Figure 2 (taken from Surace & Sanders 2000) illustrates this quite well. If galaxies which do not follow an r1/4 profile to large radii have values of n ≃ 1.0, it confirms that the galaxy is still dominated by an exponential disk at large radii. Thus the Sersic profile is useful as a means of clarifying information provided by the more limited r1/4 profile. The third panel from the top in Figure 1 for each galaxy shows the surface brightness profile fit with a Sersic fitting function (dashed line). The Sersic number n derived from the fit is displayed – 12 – in each plot beneath the name of the galaxy. The symbols in the plot are otherwise the same as those for the de Vaucouleurs fits. A limitation of 1 ≤ n ≤ 10 was used in the fits. Beyond n = 10, the factor bn becomes less accurate and introduces errors into the solution. It also tends to force the central surface brightness (µ◦) towards unrealistic values. Furthermore, as n exceeds 10, the differences in the shape of the profile become less pronounced (Graham et al. 1996). The Sersic profile was selected over another potential fitting algorithm, the Nuker profile (Lauer et al. 1995), which has been used primarily to analyze high-resolution HST images of the centers of elliptical galaxies. However, a major drawback, pointed out by Lauer et al. is that it is only intended to describe the central regions of galaxies. It is used to analyze images on angular scales smaller than the typical non-AO assisted seeing even at the best ground-based sites (such as Mauna Kea). Graham et al. (2003) showed that the Sersic law can fit an entire profile all the way into the nucleus. It is flexible enough to include not just the outer profile, but areas previously thought to be within the sole purview of the Nuker fit. A second draw-back is that the Nuker law uses a fixed angular range in arcseconds, rather than a metric scale. Graham et al. point out that for a sample of galaxies with a varying range of distances, and/or intrinsically different scale lengths, the Nuker profile will give a range of different inner profile slopes, even if all the galaxies have the same structural shape. Before delving into the results of the Sersic fits to the mergers in the sample, it is useful to put the Sersic parameter n into a physical context. CCD93 demonstrated that the Sersic profile is useful as means of separating the bright family of ellipticals, nominally galaxies with Reff ≥ 3kpc, from ordinary galaxies. They found that n is correlated with both the luminosity and effective radius of elliptical galaxies. Ellipticals with a value of n ≥ 4.0 corresponded to bright ellipticals, while n < 4.0 corresponded to lower luminosity ellipticals, with smaller effective radii. The smallest Sersic number found to fit an elliptical galaxy was n = 1.5. Several S0 galaxies were included in their sample, many of which showed values of n < 1.5 along either or both the major or minor axis. Low values of n appear to have “cores,” while higher values of n could be characterized as “power-law” galaxies. Faber et al. (1997) reached the opposite conclusion. They found brighter ellipticals have cores, while fainter ellipticals have power-law centers. However, the “cores” in Faber et al. are thought to be connected with super-massive black holes which form in high-luminosity, massive galaxies, while the Sersic “cores” are simply an inward extrapolation of the outer galaxy profile. The Nuker cores, in contrast, are not extrapolations of the outer galaxy profile. Graham, Trujillo, & Caon (2001) showed a strong correlation between n and the velocity dispersion. Their work illustrates that n is not simply an extra fitting parameter used to improve profile fits, but has an underlying physical meaning. Finally, Naab (2000), used the Sersic law to fit the luminosity profiles of simulated merger remnants for a variety mass ratios, ranging from 1:1 to 4:1. The fits were made along the intermediate, short, and long axes of the simulated mergers. Since real mergers are most likely measured in some form of projection across the axes, rather than directly along an axis (unless the observer is particularly lucky) it is more useful to show the different ranges of Sersic values that Naab found for the varying mass ratios. 1:1 mergers showed 3.26 ≤ n ≤ 4.67, 2:1 mergers showed – 13 –

2.85 ≤ n ≤ 4.87, 3:1 mergers showed 2.55 ≤ n ≤ 3.02, and 4:1 mergers showed 2.10 ≤ n ≤ 3.02. Generally speaking, the more unequal the mass ratio, the lower the Sersic n. Returning to the data, the Sersic profile is first applied to the 9 objects which do not follow an r1/4 profile out to their last isophote. Two galaxies have a value of n > 4.0, AM 1300-233 (n = 4.99), and UGC 8058 (n = 10.0). It can be argued that for AM 1300-233, violent relaxation has occurred but is still not complete. UGC 8058, also known as Mrk 231, marks a rather different case. It is both an ULIRG and an AGN (see Sosa-Brito et al. 2001 for a summary of the properties of Mrk 231). The K-band image is dominated by the central point source. The result is that both the de Vaucouleurs and Sersic fits are severely influenced by the central source. This has the effect of making the effective radius unrealistically small, and the central surface brightness unrealistically bright. Further, the bright central source forces the Sersic number n to equal 10.0. The properties of the underlying host galaxy become “lost in the glare” of the central source. However, even though the central source affects the profile, the outer profile clearly shows that incomplete violent relaxation has occurred. Three more of the nine galaxies that do not follow an r1/4 fit to their last isophote have values 1.0 < n < 2.0, UGC 2238, NGC 3597, and NGC 6052. The smallest is NGC 6052 with a value of n = 1.13. UGC 2238 and NGC 3597 have values of n = 1.46 and 1.87 respectively. It would seem that NGC 6052 has not undergone violent relaxation, and is strongly dominated by an exponential disk component. It is also questionable whether UGC 2238 and NGC 3597 are undergoing or have undergone any amount of violent relaxation. However, since elliptical galaxies are known to have values of n as low as ≃ 1.5, UGC 2238 would be considered borderline, while NGC 3597 could still eventually become an elliptical galaxy. The remaining 4 galaxies have Sersic values of 2.17 < n < 2.74. It can be concluded that these objects have undergone incomplete violent relaxation and may have formed from unequal mass mergers. Of the 42 galaxies which do follow a de Vaucouleurs profile to their last isophote, three galaxies, Arp 230, NGC 4004, and AM 1255-430, have low values of n, 1.46, 1.53, and 1.87 respectively. Arp 230 is classified as a shell elliptical, but Iodice et al. (2002) suggest the morphology is more akin to an S0 galaxy. The presence of a disky component is strongly suggested by the low Sersic number. The K-band image shows what appears to be a disk perpendicular to the shell structure. It is also noted above that the spatial extent of the K-band data is not sufficiently beyond the possible 2 contamination of a central bulge. UGC 6950 shows a more confused structure. The Sersic χν = 1.78 and the r.m.s. = 0.25 mag arcsec−2, which is the highest value found for all of the Sersic fits. While the last few isophotes fall within the best r1/4 fit, nearly two-thirds of the isophotes lie quite distant from the best-fit line. Thus, while NGC 4004 appears quite disturbed, the poor quality of the fit, coupled with the low Sersic number suggests that this galaxy is not undergoing violent relaxation. While AM 1255-430 has a low Sersic n value, indicating, perhaps, some disky contribution to its overall shape, it still generally follows the r1/4 profile quite well over most of 2 its radius. The remaining objects appear to be better fit by a Sersic profile. The χν and r.m.s. values are listed in columns 6 and 7 of Table 2. A number of the objects which were well-fit by a de Vaucouleurs profile but showed strong deviations near the center, producing poor fitting – 14 – statistics, show a marked improvement in the quality of their fits with the addition of the Sersic n parameter. This parameter appears sensitive to the deviations in the luminosity near the center of the galaxies. The overall distribution of the Sersic n parameter for the sample is plotted as a histogram in Figure 3. The median and average values for n are 4.08 and 4.97 respectively. Overall the breakdown of n is nearly 50-50 between n ≤ 4.0 and n > 4.0. The distribution is bi-modal, with peaks clustered around n ≃ 3 and n = 10. The second peak is an artifact of the maximum limit imposed on the parameter. These are all objects with a strong concentration of light in their central regions. Furthermore, all of the n = 10 objects are those with at least a 2σ excess in their de Vaucouleurs light profiles. In fact, 15/19 objects with n > 4 are objects which show a rise above an r1/4 profile near the nucleus. Clearly, there is something more going on here. Of those 15 objects, only one, UGC 8058 (Mrk 231), is known to harbor an AGN, which can strongly influence the central luminosity. On first inspection, there appears to be a correlation between Sersic n and central luminosity. This correlation will be discussed subsequently. The objects with at least a 2σ turnover in their de Vaucouleurs light profiles all have Sersic values of n < 4.0. An examination of the first peak in the bi-modal Sersic n distribution shows that median and average n values are 3.43 and 3.60 respectively. Approximately one-third of the objects have a value of n ≥ 4.0. Six objects have a value of n < 2.0. Based on earlier discussions, it is likely that two of these objects, NGC 4004, NGC 6052 have not undergone violent relaxation. Of the remaining four, it is questionable whether UGC 2238 has undergone any violent relaxation, and there is not enough spatial information on Arp 230 to strongly confirm or deny whether the entire galaxy is undergoing violent relaxation. The remaining two galaxies, both with Sersic values of n = 1.87, do show some evidence for violent relaxation. For the sample overall, taking into account both the de Vaucouleurs and Sersic profiles the breakdown of results is as follows: 2 galaxies, NGC 4004 and NGC 6052, show little, if any, evidence of violent relaxation; 1 galaxy, UGC 2238 is questionable based on the profile and Sersic number; 3 galaxies, Arp 230, AM 0956-282, and NGC 3310, do not have enough spatial information beyond the range of possible bulge contamination to confirm or refute evidence of either complete or incomplete violent relaxation, 7 galaxies, NGC 2744, NGC 3256, NGC 3597, AM 1300-233, UGC 8058, Arp 193, and UGC 9829 show evidence of incomplete relaxation; and 38 galaxies show evidence of complete relaxation to the limits of the K-band data. Column 9 of Table 2 provides a synopsis for each galaxy. The first four columns of Table 3 list the properties derived from the Sersic fits.

4.2. Phase-Mixing

The process of phase-mixing, in conjunction with violent relaxation, is a necessary requirement for mergers to make bonafide elliptical galaxies. While violent relaxation can occur rather rapidly, producing elliptical luminosity profiles even in the most disturbed systems, phase-mixing is a much longer, often drawn-out process. It is difficult to quantify the level to which phase-mixing has occurred in mergers. However, in a more qualitative way, it is possible to investigate whether the – 15 – mergers in the sample show evidence for the presence of discrete structures, or if the process of phase-mixing has “smoothed away” any residual structures. Two methods are applied. The first uses an unsharp masking technique to look for resolved central structures in the galaxies which may signal the presence of incomplete phase-mixing. The process removes fainter, lower surface brightness structures such as tidal tails, loops and shells. The unsharp masking technique removes small scale fluctuations from the image by first smoothing the image with a median filter of size ≃ 3× FWHM of the seeing, and then subtracting it from the original image. The images are then categorized based on whether any distinct features remain. Otherwise, if it is a featureless, it is labeled as such. The unsharp masking results are listed in Table 4 and the images for each galaxy are displayed in Figure 4. The second technique uses a residual image created by subtracting a galaxy model from the actual data image. The model is made using the BMODEL task in the STSDAS package. It uses the properties extracted for each galaxy from the ELLIPSE task in conjunction with the original image to create a model galaxy image. The residual image is useful for identifying large scale fine structure, also indicative of incomplete mixing. Ideally, the residual image of a phase-mixed galaxy should show very little, if any, signs of the actual galaxy. Again, these images are described in a very qualitative manner in Table 4. If the residual image is nearly featureless, it is noted as such, otherwise a brief description is given of any structure that is detected. Large-scale tidal tails, shells and loops are not mentioned in the table since their presence is assumed, given the sample selection criteria. However, loops and shells on small spatial scales are noted. The residual images are also displayed in Figure 4 for each galaxy, adjacent to the unsharp mask image. In the residual image, bright patches correspond to negative intensity, while dark patches correspond to positive intensity. A quadrupole feature is noticeable in the centers of several galaxies. A similar feature is noted by Ebneter et al. (1988). They conducted a search for dust lanes, stellar disks, bars, shells and other deviations in elliptical galaxies using a variety of image processing techniques, including both unsharp masking and residual images created by models. They suggest that the quadrupole feature is due to the presence of a disk. After reprocessing images by shifting the centers of model images, they noticed a disk feature seemed to appear. They concluded that galaxies with quadrupole features have disks that cause the isophotes to become non-elliptical and produce a numerical instability in the isophote fitting algorithm. Overall, very few mergers in the sample show evidence for complete or advanced phase-mixing. The unsharp masking technique reveals only 15 objects in the sample that are devoid of significant structure. These objects are listed as “featureless” in Table 4, however, they do include a shape description of the central core. The residual images show only 3 objects that are somewhat featureless, outside tidal tails present at large radii. Two of the three objects are in common with the 15 objects devoid of features in the unsharp masking technique. Several objects show evidence of either central face-on disks or edge-on disks. The residual images reveal that for a number of objects, tidal tails and plumes can be traced all the way from the nucleus to large radii. The residual images of the shell ellipticals all show some degree of fine structure, while 3/6 of the unsharp mask images are rather featureless. While violent relaxation is apparently complete (to the limit of the – 16 –

K-band data) for a majority of the sample, the process of phase-mixing is incomplete in nearly all the objects.

4.3. ”Excess Light” in the proﬁles

As noted earlier, there are a number of galaxies in the sample that show significant deviations from an r1/4 profile near the nucleus. Some of these are objects that appear to be well described by a de Vaucouleurs profile beyond the nucleus, but exhibit a rise in luminosity above an r1/4 profile near the nucleus. There are 17 galaxies in the sample that show this behavior. The main reason that the Sersic law was selected as an additional method for investigating the mergers in the sample is that it is more sensitive than the de Vaucouleurs law to the behavior of the surface brightness profile near the nucleus. The addition of the extra fitting parameter n allows the Sersic profile to better fit the galaxy’s true surface brightness profile at small radii. At large radii, the de Vaucouleurs law is still a useful fitting function. Moreover, for a number of galaxies in this sample, as well as many elliptical galaxies, the r1/4 profile fits the true luminosity profile well into the center of the galaxy. The de Vaucouleurs profile is not merely an empirical fitting formula, but is based on certain underlying physical similarities between ellipticals, such as the overall density distribution, which constrains their formation mechanisms (Burkert 1993), as well as central phase-space density. Models have shown that deviations from a de Vaucouleurs profile are likely due to the effects of dissipation (Hjorth & Madsen 1995). As has been demonstrated by CCD93, and pointed out earlier by Schombert (1986, Figure 14 in particular) in relation to deviations from a de Vaucouleurs profile, there is a correlation between the shape of the profile and luminosity. Thus, the deviation from an r1/4 profile, in this case, the presence of excess luminosity near the nucleus, is physically significant. Moreover, the MH94 models were quantified by deviations from a de Vaucouleurs profile. Because they address the fundamental issue of dissipation and a central starburst, it is useful to compare the excess luminosity detected here with those models. Other possible explanations for the excess luminosity will also be addressed subsequently. The question of “excess light” is approached in three steps. First, galaxies were identified as having “excess light” from their de Vaucouleurs profiles. The detections are quantified by compar- ing the measured surface brightness with the best-fit r1/4 model. Those profiles that show points near the center which are at least 2σ above the r.m.s. of the fit were considered galaxies with “excess light.” As a second check, the total MK measured using both a Sersic and de Vaucouleurs profile were compared. For 16/17 objects the Sersic MK is brighter, indicating that the de Vau- couleurs fit is failing to account for all of the light. One galaxy, NGC 1210, showed a ∆MK = 0. Its Sersic n value is 4.08, which indicates virtually no deviation from a de Vaucouleurs profile. This galaxy was rejected as a merger with “excess light.” All the objects with a minimum 2σ “excess” −2 have at least a maximum ∆µK ≥ 0.3 magnitudes arcsec rise in the surface brightness profile above the best-fit model. The decision to use the r.m.s. of the fit as a discriminator is justified in order to take into account objects with a significant number of “bumps” and “wiggles.” These objects may appear to have an excess near the center, but also have significant deviations along – 17 – the entire profile and would therefore have a very high r.m.s. as well. The galaxies with “excess light” are noted with a “\” symbol in column 10 of Table 2. The next step is to quantify the “excess light” in terms of luminosity. The program written to find a best-fit r1/4 model also calculates the model surface brightness at each radial point. This was then converted to an absolute magnitude and compared with the real measured absolute magnitude at that radius. The absolute magnitudes were then converted into a luminosity relative to L⊙. Table 5 lists the 16 galaxies with a minimum 2σ excess. Table 5 also includes the radius of the “excess light” (in cases of multiple points, the furthest point out is used), the maximum ∆µK between the actual surface brightness and the model, and the excess luminosity in L⊙. The range 8 10 11 in “excess luminosity” extends from ≃ 6×10 -3×10 L⊙. This range increases to 7×10 L⊙ when UGC 8058 is included. The median radial extent of the “excess light” is ≃ 360 pc, the average is 394 pc, with no object showing “excess light” beyond 1 kpc. The final step is to address the “excess luminosity” using the Sersic profiles, which are sensitive to profile deviations near the center of the galaxy. As noted above, all of the objects determined to have “excess light” have a Sersic n > 4.0. There are 19 objects with Sersic n > 4.0, of those, 15/19 show “excess light.” This seems to indicate a correlation between the Sersic number and “excess luminosity.” CCD93 showed that a correlation exists between the total luminosity (measured in −1 −1 terms of MB) of an elliptical galaxy, and its Sersic number n. Converting to H◦ = 75 km s Mpc and from MB to MK assuming (B-K) = 4.0 (Hibbard & Yun 1999), gives a correlation coefficient of p = -0.755 for the CCD93 data. Recently, Khosroshahi et al. (2000) investigated the photometric near-infrared Fundamental Plane, and found that a strong correlation (p = -0.790) exists between the central surface brightness µ◦K and the Sersic number n. Both the central luminosity concentration and the total luminosity of elliptical galaxies appear correlated with the Sersic number. These same two relations are investigated for the mergers in the sample presented here. Figure 5 shows a plot of MK vs. n. The parameters show a very weak correlation (p = -0.235). Figure 6 shows a plot of µ◦K vs. n. The correlation between µ◦K vs. n is quite strong (p = -0.953). In these figures, objects with n = 10.0 are not plotted, nor used in the computation of the correlation coefficient because this value is an imposed limitation. As noted earlier, numerical instabilities set in when n ≥ 10 and produces unphysical values for the effective radius and central surface brightness. What this says is that for the mergers in this sample, the light is more centrally concentrated than expected. The deviation above a de Vaucouleurs profile is statistically significant and the central concentration of light correlates well with n. The “excess light” is real. There are three possible scenarios for the cause of the “excess light.” The first, is that this is an observational confirmation of the MH94 models. The “excess light” is produced by either a strong central starburst, or a post-starburst dense stellar core. The second possibility is that the light is produced by an AGN. Only one of the objects in the sample, UGC 8058 is known to be an AGN. The last possibility, is that the rise in the surface brightness profile is inherited from a cuspy profile from one of the progenitor galaxies in the merger, and is not produced as a result of the merger itself. As stated earlier, the MH94 models for mergers undergoing a starburst induced by gaseous – 18 – dissipation make predictions about the shape of the surface brightness profile near the nucleus. The presence of a starburst should affect the profile shape, depending on the strength of the burst. Mihos & Hernquist (1996) note that the intensity of the starburst is related to several factors, including the absence or presence of a bulge and encounter geometry. Once the starburst fades, it should leave behind a dense stellar core of more evolved stars. The presence of a dense stellar core can still affect the shape of the luminosity profile near the nucleus, but to a lesser degree. There should be some detectable presence of “excess luminosity” in the most advanced mergers and in seemingly “normal,” older elliptical galaxies. To date, there have been only two attempts to address these predictions. Hibbard & Yun (1999), attempted to address this issue in a numerical fashion. CO observations of the molecular gas of three mergers was converted to a stellar component (assuming all gas is converted to stars) and added to the surface brightness profiles. The goal was to create profiles containing an older stellar population superimposed with a younger population formed from a starburst of the gas. They chose NGC 3921, NGC 7252, and Arp 220 for their sample because their CO (1-0) observations of these objects had sufficient resolution to spatially sample the mergers on the scales of the hydrodynamical smoothing length used in the models. They converted the inferred H2 gas mass to a stellar component and allowed this stellar population to passively evolve. The light from the evolved population was then added to a B-band surface brightness profile for NGC 3921 and NGC 7252 and a K-band surface brightness profile for Arp 220. Their results did not show any rise in luminosity for NGC 3921, a modest rise of a factor of 2 for NGC 7252, and a factor of 7 rise for Arp 220. The two most advanced mergers with single nuclei, NGC 3921 and NGC 7252, did not show nearly as much “excess light” as might be expected from the MH94 models. Both NGC 3921 and NGC 7252 are part of the sample presented in this paper. The fits to these galaxies show the opposite result from Hibbard & Yun. NGC 3921 shows a rise in the profile, while NGC 7252 does not. A resolution to this discrepancy is not obvious. However, since the rise in luminosity “detected” by Hibbard & Yun is theoretical and not observed, it is possible that assumptions about the age and makeup of the stellar populations particular to each galaxy may have played a role. It is interesting to note that of the 6 shell ellipticals in the sample presented in this paper, one, NGC 5018, does show the presence of “excess light.” Kormendy (2004) discusses the MH94 models for a dissipative starburst in light of his work on elliptical light profiles. He suggests that the detection of “excess light” in these profiles may be in accordance with these models, and is currently pursuing this investigation (Kormendy 2004, private communication). Further, van der Marel & Zurek (2000) presented the preliminary results of a study of 11 mergers with NICMOS on HST and their findings indicated that at least one object, NGC 3921 shows a slight rise in its surface brightness profile in the F160W filter (≃ H-band) at a radial distance of ≃ 170 pc, while another, NGC 7727, shows no such rise. Both of these objects are in the merger sample presented here, and show the same results. It is important to discuss a notable discrepancy between the MH94 models and the actual data presented here. The models used by MH94 included progenitor spiral galaxies modeled with only – 19 – a disk and halo or modeled with a disk, bulge and halo. The models predict that for disk+halo progenitors, the rise in surface brightness should occur at ≃ 0.04Reff and for disk+bulge+halo progenitors at ≃ 0.02Reff . About half the mergers with “excess light” show the rise at ≃ 0.04Reff , while the other half are split between 3-5× and ≃ 10-30× further out. The predicted rise in surface brightness also differs somewhat from the actual data. MH94 predicts ≃ 3 magnitudes arcsec−2 rise for disk+halo models, and ≃ 2 magnitudes arcsec−2 rise for disk+bulge+hale models. The mergers in this sample only show ≃ 0.3-1.3 magnitudes arcsec−2 rise in surface brightness. The cause of this discrepancy is not apparent, but there are many factors which may come into play, such as whether the initial interaction was prograde or retrograde, the presence or absence of a bulge in one or both progenitors, unequal mass progenitors, choice of star-formation rates for the model and initial gas estimates, and most importantly whether the predicted rise in surface brightness from the models should be the same magnitude at K-band. The second explanation for the “excess light” is the presence of an AGN. While starburst and AGN are not mutually exclusive phenomenon, there are questions as to which is the dominant mechanism and particular wavelengths. UGC 8058 is known to have broad infrared emission lines at 2 µm (i.e. Goldader et al. 1995). It is likely that the AGN is contributing, if not the dominant mechanism responsible for the “excess light.” Unfortunately, K-band photometry alone is not sufficient to confirm or reject an AGN origin for the light. Using the unsharp mask and residual image techniques employed earlier, it is possible to examine the galaxy images for a bright central point-source. Taken in conjunction with the computed effective radius, this can provide some circumstantial evidence favoring or dis-favoring an AGN hypothesis. Only UGC 8058 has both a strong, intense point source, noticeable by the intense diffraction rings and an unrealistically small effective radius. While UGC 5101 has a small effective radius, the unsharp mask and residual images do show some extended central structure. The point source is not overwhelmingly dominant over the rest of the galaxy. Moreover, the spatial extent of the excess luminosity, including its extent over multiple isophotes in some objects, does seem to make the AGN explanation less likely. If the “excess light” were produced by UV photons from an AGN heating the surrounding dust, 10 then a dust grain exposed to a source of ≥ 10 L⊙ could not radiate if were more than a few parsecs from the source (Wright et al. 1988). Only a distribution of luminosity sources (such as stellar clusters) could create a spatially extended source. If the AGN was exposed, then the luminosity should be very centrally concentrated, limited to ≃ FWHM and not spatially extended either. The third possible explanation is that the steep luminosity profile is a result, not of the merger itself, but of the survival of the density profile from one of the progenitors in the merger. Using simulations, Barnes (1998) showed that for mergers of galaxies with different core profiles, the profile of the steep cuspy core survives the merger. Fulton & Barnes (2001) expanded this work by studying how the core affects the rest of the merger remnant. They found that the for equal-mass mergers, the inner cusp slope is preserved during the merger. If the progenitors have unequal cusps, then the more cuspy profile exerts influence on the merger remnant. Further, the profile of the merger remnant itself is heavily influenced by the cuspy profile of the core. The cuspy slope also has significant effects on the shape of the merger remnant. Shallow cusps produce prolate remnants, – 20 – and steep cusps produce oblate remnants. Thus the luminosity profile, which is a projection of the density profile, may simply be the result of the survival of the core profile from one of the progenitor galaxies. The K-band data is insufficient to completely rule out any of the above scenarios. Infrared spectroscopy, particularly within the K-band window, is an effective tool to determine the responsible mechanism. If a starburst is occurring, than strong emission lines, such as Brackett γ and H2 should be detectable. Models such as Starburst 99 (Leitherer et al. 1999) can be used to asses the age of the starburst (if present). It can also be used to constrain the type of stellar population present (i.e. post-starburst AGB or RSG) using stellar absorption lines such as CO at 1.63 and 2.29 µm. Detection of broad emission lines (v ≥ 1000 km/s) as well as certain forbidden lines such as [Al IX] at 2.04 µm are useful for confirming the presence of an AGN. If the underlying stellar population is old, than neither a starburst or an AGN is responsible and the luminosity profile may be a relic of one of the progenitor galaxies in the merger. Moderate resolution infrared spectroscopy will be presented in a subsequent paper to address this matter.

4.4. Proﬁles Which “Turn-over”

In addition to the 16 mergers which show a rise in their surface brightness profiles, there are also 11 galaxies which show a “turn-over” in their profiles. They are noted with a “∩” symbol in column 10 of Table 2. The galaxies with a “turn-over” were identified in the same way as the galaxies with “excess light,” i.e. those that have at least a 2σ deficit in surface brightness compared with the best-fit model de Vaucouleurs model. The maximum radial extent of where the turnover occurs was found to be from ≃ 70-400 pc, with a median distance of 184 pc and an average distance of 199 pc. Once again, the Sersic n parameter is found to be well correlated with the luminosity. The Sersic fits show that 5/6 galaxies with n < 2.0 have a profile which “turn-over.” All of the galaxies which show a “turn-over” profile have a Sersic n < 4.0. As discussed earlier, the term “core” galaxy is a misnomer when applied to Sersic profiles. The Sersic profile is correlated with luminosity. In the case of mergers, the central luminosity in particular. Thus, galaxies which show a “turn-over” are those with a deficit of luminosity in their centers. The most extreme cases are n ≃ 1 galaxies such as NGC 4004 and NGC 6052 which are likely dominated by an exponential disk and have not undergone violent relaxation. The profiles which “turn-over” are in no way connected with the “core” galaxies described by Faber et al. (1997). The distances to the mergers in this sample are much greater than those in the Faber et al. sample, so the “turn-over” in the profile for the mergers is generally on scales much larger than in the Nuker profile. The radius at which a Nuker profile turns over would require either HST or ground-based adaptive optics to detect. – 21 –

4.5. Do Mergers make L∗ Elliptical Galaxies?

If spiral-spiral mergers form a significant number of elliptical galaxies, then they should be capable of producing at least some L∗ elliptical galaxies. To test this, the total absolute K-band magnitudes of the mergers were measured. The magnitudes were obtained by simply summing up the flux in circular apertures from the center to the edge of the array, after masking out foreground stars. This method makes no assumptions about the profile shape of the galaxy. One drawback, however, is that it assumes that the galaxy is smaller than the array. Nearly all of the galaxies appear to be smaller than the 3’.2 field of view. However, it should be noted that NGC 3256, NGC 7135, NGC 7252, and NGC 7727 have tidal features which do extend beyond the size of the QUIRC array. Fortunately, their main bodies lie well within the field of view of QUIRC, and the galaxies reach a µK ≃ 21 well before the edge of the array. Figure 7 is a histogram of the distribution of the total MK, with galaxies binned into integer units of absolute magnitude. The actual magnitudes are listed in column 5 of Table 3. An L∗ −1 −1 elliptical has an MK = -24.15 for H◦ = 75 km s Mpc (Kochanek et al. 2001). One note, Kochanek et al. use the Ks filter system, while the data here is in the Mauna Kea K filter. The filter difference between the two systems is ∆λ = 0.003 µm (Persson et al. 1998) which translates into a difference of less than a few milli-magnitudes for the standards used for calibration. This is smaller than the errors from the standards themselves and therefore it is assumed that Ks ≃ K. The clear bars show galaxies which have an MK fainter than MK∗ellip. The diagonally hatched bars are the galaxies with MK equal to or brighter than MK∗ellip. Note that for the MK = -24 bin, the bar is split between the two groups. This is because the threshold for an L∗ellip falls within this bar and there is one galaxy that lies below this threshold. The plot shows that 33/51 mergers in the sample have an MK equal to or brighter than MK∗ellip. The median MK = -24.65 and the average MK = -24.36. Further, 35/51 mergers in the sample have a luminosity equal to or in excess of 2 L∗spiral galaxies (equivalent to MK = -23.90). This suggests that the progenitor spirals for most of the mergers were each L∗ spiral galaxies. This appears to be in direct contrast to two earlier studies involving LIRG and ULIRG mergers and slightly different than the findings of Genzel et al. (2001). The first, Shier & Fischer (1998), used 2.3 µm CO velocity dispersions to infer the luminosities for 10 galaxies in their sample. They converted the velocity dispersions to a luminosity for each galaxy using the Faber-Jackson relation. The second, Colina et al. (2001), used the F160W filter on HST (≃ H-band) to measure total MH for 27 objects. They found that 52% of the sample had sub-L∗ luminosities. Both of these studies concluded that mergers did not produce L∗ elliptical galaxies. Colina et al. further commented that the progenitor galaxies must have each been sub-L∗spiral galaxies. Genzel et al. found that most of the ULIRGS in their sample were ≃ 1 L∗. However, these studies included a significant number of objects in the very early stages of merging (i.e. double nuclei and early stage interactions). The sample presented here contains 3 ULIRGs and 13 LIRGs. None of the mergers in the sample overlap with Colina et al. (2001). However, 3 objects from Shier & Fischer (1998) do overlap with the sample presented here. They are NGC 1614, NGC 2623, and IC 5298, all of which have MK brighter than MK∗ellip. Three ULIRG/LIRGs in the sample, UGC 6, NGC 2782, and NGC 4194 – 22 –

were found to have a sub-L∗ellip luminosity. There is one object in common between Genzel et al. and the sample presented here, AM 2055-425. Genzel et al. found MK = -24.15 (shifted to H◦ = −1 −1 75 km s Mpc ), which is equivalent to an L∗ellip, whereas the data presented here gives MK = -25.08. However, Genzel et al. concede that their absolute magnitude are “quite uncertain” owing to calibration problems and the fact that the magnitudes are measured within the effective radius only. The current total luminosity of the mergers is consistent with bright ellipticals. However, a significant fraction of the luminosity appears to be concentrated at small scales for a number of objects. This is consistent with the idea that a starburst or post-starburst population is responsible for the light. Over time, the light may fade, depending upon the age of the starburst or the age of the post-starburst population. Using Starburst 99 models (Leitherer et al. 1999), and assuming an instantaneous burst, the MK of the starburst can fade anywhere from 2-4 magnitudes, depending on the metallicity, and slope of the IMF of the stellar population as well as the age of the starburst itself. If the mergers are in a post-starburst phase, then the MK of the burst population may fade only 1-2 magnitudes. However, fading of 1-4 magnitudes within the central region of the galaxy results in a significantly smaller reduction of the total integrated luminosity of the galaxy. To test this, one magnitude of flux was removed from the central isophote of several mergers. This was done to simulate a decrease in luminosity of the central starburst or central stellar population. The spatial size of the aperture radius ranged from ≃ 0.2-1 kpc. First, the mK of the central isophote was computed, and then the brightness was reduced by one magnitude. The “new” flux was then computed for that central aperture. The difference between the original flux and the new flux in that aperture was derived and then subtracted from the total flux of the merger. The new MK of the merger was computed and compared to the original value. The difference was ≃ 0.04-0.2 magnitudes. This was repeated for a decrease in brightness of 4 magnitudes in the central aperture. The overall difference in MK was ≃ 0.04-0.35 magnitudes. It is unlikely that most or all of the mergers will undergo a drop of 4 magnitudes in the central region because of differences in metallicity and age of the central stellar population, as well as the age of the starburst (if one is occurring). Thus, the overall distribution of the MK of the sample is unlikely to change by more than a few tenths of a magnitude. However, if a continuous starburst is occurring, then the models do not predict a drop in luminosity, but an increase. This would likely increase the MK of the mergers by the same small amount.

4.6. Structural Parameters

4.6.1. Disky vs. Boxy isophotes

The ﬁrst parameter discussed is a4/a because it is the most important of the structural parameters. It is critical for investigating the properties and origins of mergers. There has been much review and discussion over whether the isophotal shapes of ellipticals are related to a speciﬁc type of formation mechanism, i.e. mergers preferentially form ellipticals with one shape (Kormendy & – 23 –

Bender 1996). On the other hand there is substantial numerical modeling which indicates that the isophotal shapes of mergers, and subsequently the ellipticals they produce, are dependent on the mass ratio of the progenitor galaxies (i.e. Naab, Burkert, & Hernquist 1999). Until now, there has been no analysis of the isophotal shapes of actual mergers, making it difficult to test the afore- mentioned hypotheses. The a4/a parameter has been measured for each isophote and is plotted in Figure 1 (bottom panel) against linear radius for all the galaxies in the sample, with the exception of AM 0956-282. Over-plotted vertically on each point are the errors generated by the ELLIPSE fitting program in IRAF. The dashed vertical line is the effective radius derived from the Sersic fitting. Each galaxy is characterized as either boxy or disky depending on the median value of the a4/a parameter. Hence a galaxy is “disky” if its median a4/a is positive, and “boxy” if its median a4/a is negative. Each galaxy’s contours were then visually checked to confirm the shape implied by the values. The isophotal type for the mergers are listed in column 2 of Table 6. The data show 35/50 mergers have preferentially disky isophotes, and 15/50 have boxy isophotes. One very unusual result is that NGC 6052 has distinctly boxy isophotes, even though its Sersic n parameter indicates it should have an exponential disk. In addition, the a4/a values indicate that NGC 1614 has boxy isophotes, although the image contours are not very convincing. Overall, there are no apparent characteris- tics that set the disky and boxy galaxies apart. The distribution of L∗ galaxies between the two isophotal shapes are similar, 24/35 (or ≃ 68%) for disky, 9/15 (or 60%) for boxy. The Sersic n doesn’t appear to be correlated either. The only significant things to note are that 7/10 of the n = 10 galaxies are disky, and a slightly larger fraction of the n = 2 galaxies are boxy. This is reversed from what might be expected, i.e. that galaxies with n < 4 are disky and n ≥ 4 are boxy (CCD93). The breakdown among galaxies with “excess light,” no deviation, and a “turnover” in their profiles are as follows, For disky galaxies: 28.6% have “excess light,” 20% have a “turnover” and 51.4% show no deviation. For boxy galaxies: 40% have “excess light,” 20% have a “turnover” and 40% show no deviation. It does seem that boxy galaxies may be more likely to have “excess light” than disky galaxies. Some interesting results do appear when the unsharp mask images are compared with the a4/a parameter. The technique removes much of the faint, low surface brightness features, leaving behind strong structures, particularly in the central regions. There are 10 objects in the sample, whose central regions revealed by this technique appear to be opposite from the results of the a4/a parameter. Four galaxies, NGC 1210, AM 0318-230, NGC 5018, and NGC 7135 show clearly rectangular centers, even though their overall isophotal shape is dominated by disky isophotes. Two of these galaxies are shell ellipticals (NGC 1210 and NGC 5018). Six galaxies, NGC 455, NGC 3921, UGC 10607, AM 2038-382, AM 2055-425, and AM 2246-490 show evidence of round or oval cores, even though their total isophotal shape is predominantly boxy. Looking at the plots of a4/a for the mergers, a significant fraction show evidence of isophotes varying between disky and boxy values. Changing isophotal shapes is not unheard of for elliptical galaxies. One case in point is NGC 3610. Scorza & Bender (1990) found the galaxy to have strongly peaked, or disky isophotes in the center, even though the outer isophotes were strongly spheroidal. Bender, Döbrereiner, & – 24 –

Möllenhoff (1989), whose paper first noted strong correlations between boxy isophotes and other observable factors such as radio and x-ray luminosities, noted that a non-trivial number of galaxies in their sample showed isophotes that varied between positive and negative a4/a values. There are a number of possibilities that may explain the variations of isophotal shape with radius: The dis- ruption caused by the merger itself may lead to variations in the isophotal shape. These variations may evolve over time and may be related to the completion of phase-mixing. Another possibility is that the shapes may be a result of the constraints placed on the stellar orbits by the encounter geometry and merging event (Barnes & Hernquist 1996). Or, the presence of disky isophotes may be due to the partial survival of one of the progenitor disks if the merger is of unequal mass. Finally, a new stellar disk may be formed in ellipticals and the oldest merger remnants if left-over gas not consumed in a starburst settles into a disk and eventually forms stars. The overall picture these results paint of how the isophotal shapes of mergers relate to elliptical galaxies is less than clear. The sample is clearly dominated by mergers with disky isophotes. However, many of these disky mergers are not analogous to low-intermediate luminosity ellipticals as might be expected. Approximately one-third have an Reff > 3 kpc, placing them in the bright family of ellipticals (Capaccioli, Caon, & D’Onofrio 1992). Moreover, most have a very high luminosity. This is exactly the opposite trend expected for disky ellipticals. Those galaxies with boxy isophotes don’t seem to follow the expected correlations either. Only 7/15 have an Reff > 3 kpc, are boxy and very bright. Boxy mergers are also more likely to have “excess light” in their surface brightness profiles rather than a “turn-over.” The best way to resolve these discrepancies is to measure the rotational velocities and compare them with the photometric properties. If the disky mergers show significant rotation, and the boxy mergers do not, then perhaps the luminosity fades over time and the mergers will begin to show the same correlations as elliptical galaxies. Next, the a3/a parameter is briefly addressed. This parameter is plotted against linear radius in Figure 1 for each galaxy. Bender, Döbrereiner, & Möllenhoff (1988, 1989) found that for most of the ellipticals in their sample, the a3/a parameter was negligible in comparison to the a4/a parameter. However, nearly two-thirds of the mergers in our sample show non-negligible a3/a values, in some cases larger in magnitude than the a4/a parameter. This is not surprising in light of the fact that the a3/a parameter is sensitive to isophotal irregularities. It is interesting to note that within the sub-sample of shell ellipticals, 4/6 show negligible a3/a values. Two of the shell ellipticals, Arp 230 and NGC 3656 have significant a3/a values. Arp 230 has two tidal arms and NGC 3656 has a prominent dust lane visible even at K-band.

4.6.2. Position Angle

It has been known for some time that elliptical galaxies often have twists in their isophotes. The term “isophotal twist” is often quite vague in the literature, with sometimes inconsistent definitions used. In some instances a physical definition is given (i.e. ∆θ from 0.5 Reff to 1.5 Reff ), in others, the largest ∆θ at any consecutive set of annuli. On first inspection, a number of mergers in this sample do show evidence of isophotal twisting. However, these twists are not all confined – 25 – to 1 or 2 effective radii, but occur spatially at different points and sometimes multiple times in a single merger. As a result, and in a more qualitative approach, we use a definition in which at least 3 consecutive isophotes change by ∆θ ≥ 15o for the merger to be defined as having “twisty” isophotes. Single discrepant points with large error bars are ignored. The galaxies with isophotal twists are listed in column 4 of Table 6. The top panel in Figure 1 for each galaxy displays the plot of position angle against linear radius. The results show 28/52 mergers have isophotal twists. The isophotal twists do not correlate with any other photometric parameters except for the slight difference in the number of boxy and disky galaxies with twists. Two-thirds of the boxy galaxies have twisty isophotes, compared to slightly more than half the disky galaxies.

4.6.3. Ellipticity

The ellipticity (ǫ) measures the elongation of the isophotes of a galaxy. Table 6 lists the average, median and maximum ellipticities, as well as the ellipticity at 1.5 Reff for each object. Results are presented for 1.5 Reff in order to compare with earlier numerical simulations of mergers. The ellipticity at this radius is called the “effective ellipticity.” Since the isophotal points are in steps of the seeing, which correspond to a specific radius, 1.5 Reff may not correspond to a specific isophote. In such cases where it does not (and this is the case most of the time), the data from the spatially nearest isophote is used. The median ellipticity for the sample is 0.29, and the median ellipticity at 1.5 Reff is 0.30. Figure 8 is a histogram of the effective ǫ for the sample. Noted in Table 6 are the ellipticity values for four objects. The first two, UGC 5101 and UGC 8058 have very small Reff and the points tabulated are well within the first isophote. The next two objects, Arp 156 and NGC 7135, have Reff greater than the radius of the last isophote. The values listed for UGC 5101 and UGC 8058 are the ellipticities of the first isophote, and for Arp 156 and NGC 7135 the ellipticities of the last isophote. These values are not included in the median values for the ellipticity at 1.5 Reff , nor the histogram in Figure 8 and the subsequent analysis. The histogram in Figure 8 shows a peak at ǫeff = 0.2-0.3. The distribution is approximately Gaussian with the mode, median and average all equal to 0.30. The standard deviation is 0.14. Naab & Burkert (2003) modeled 1:1, 2:1, 3:1, and 4:1 mass mergers taking into account 500 random viewing projections and found a trend in ellipticity with the merger mass. 1:1 mass mergers showed a peak in the distribution at ǫeff = 0.3, 2:1 mass mergers showed two peaks at ǫeff = 0.25 and a stronger peak at ǫeff = 0.45, while 3:1 and 4:1 mass mergers showed peaks at ǫeff = 0.55 and ǫeff = 0.60 respectively. Roughly speaking, the distribution of ǫeff indicates a tendency towards 1:1 and some 2:1 mass mergers. The ellipticity does not appear to correlate with any of the other photometric parameters, such as n, MK, Reff , µeff or µ◦. Boxy and disky isophotes do appear to have somewhat different ellipticities. Mergers with boxy isophotes have an average ǫ ≃ 0.30, while those with disky isophotes have an average ǫ ≃ 0.27. This the reverse of the trend found by Bender, Döbrereiner, & Möllenhoff (1989). Their results show boxy ellipticals have an average ǫ ≃ 0.24 and disky ellipticals have an average ǫ ≃ 0.32. Moreover, the data for the mergers does not replicate the correlation they found – 26 –

between ǫ and a4/a. Boxy and disky mergers have the same spread of ǫ values. They also found that rounder ellipticals had a smaller dispersion in the values of a4/a. This trend is not found among the mergers. Finally, CCD93 noted a correlation between the maximum ǫ and the Sersic parameter n. Ellipticals with large ǫ had smaller n values. A similar trend is not seen in the merger sample presented here.

5. Summary and Discussion

K-band photometry for a large sample of advanced mergers has been presented here. The deep integration times coupled with the large field of view of the infrared detector have been useful for investigating the photometric properties of these mergers over a large dynamic range, and out to large spatial radii. This is important for testing whether mergers have undergone complete, incomplete, or no violent relaxation on larger scales than previous K-band studies of mergers. The excellent seeing conditions under which the data were obtained has provided the opportunity to study the mergers on very small spatial scales as well. As a result, the luminosity profiles have been probed on very small scales, revealing the presence of “excess light” in the surface brightness profiles of a number of mergers in the sample. The high resolution and large spatial scale of the data set has also been used to qualitatively investigate the extent to which phase mixing has occurred. Finally, the data have also been used to extract important structural parameters. For the first time, structural comparisons can be made between real mergers and numerical simulations. Summarized below are the important results of the paper: 1) The K-band photometry fit with both the de Vaucouleurs profile and Sersic profiles indicates that, for most of the mergers, the older stellar populations have undergone violent relaxation as a result of the merging event. The profiles suggest complete relaxation for 38/51 galaxies; 7/51 show evidence of incomplete relaxation; 3/51 show evidence for relaxation out to spatial scales of ≤ 3 kpc, which is smaller than the scales on which a bulge may contaminate the profile; 2/51 show little or no evidence of having undergone violent relaxation; and the evidence for any violent relaxation is questionable for 1/51. 2) Most of the mergers and shell ellipticals in the sample show no evidence of complete phase- mixing. This result is not surprising as the time scales for phase-mixing are quite long. 3) Mergers appear to have strong concentrations of luminosity in their centers. 16/51 of the mergers show evidence of “excess light” in their surface brightness profiles. Three possible explanations have been presented to account for this phenomenon. The first, is that the excess luminosity may confirm the models of MH94 for a starburst triggered by dissipative collapse in mergers. A starburst or post-starburst central stellar population may be the source of the “excess light.” Although the amount of “excess light” does not match the predictions made by the models, the fact that the light is spatially extended and not a point source, as well as the correlation between the Sersic n parameter and central surface brightness provides strong circumstantial evidence in favor of this explanation. However, one object is known to harbor an AGN, which may be responsible for the “excess – 27 – light.” It is possible that the “excess light” in all or some of the objects may be the result of an AGN, rather than a starburst. Finally, a third possibility is that the shape of the luminosity profile is a relic from the central core of one of the progenitor galaxies, and not formed from the merging event itself. The core profiles can survive the merger and can influence the shape of the luminosity profile of the merged system, as shown in simulations by Fulton & Barnes (2001). The photometry alone cannot completely rule out or confirm any one of these possibilities. Follow-up infrared spectroscopy can provide important evidence to confirm one of the three possible sources for the excess luminosity. 4) The K-band data show that mergers can produce galaxies with a luminosity ≥ an L∗ elliptical galaxy. Close to two-thirds of the sample have a luminosity ≥ a 1 L∗ elliptical galaxy and more than two-thirds have a luminosity ≥ 2 L∗ spirals. This is consistent with a formation mechanism in which two gas-rich spirals merge. The data further suggest that it is unlikely that the total luminosity of the mergers will fade by more than a few tenths of a magnitude if a central starburst is present. These results are in contrast to earlier infrared studies of mergers. 5) While the isophotal shapes of many of the mergers in the sample show evidence of both disky and boxy shapes, most of the mergers show a predominance towards disky isophotal shapes (a4/a > 0). Most of the mergers with disky isophotes are also quite luminous. However, those with boxy isophotes are more likely to have “excess light,” and thus show a rise in the surface brightness profiles near the nucleus. The photometric properties of mergers with disky and boxy isophotes do not seem to correlate with the photometric properties of their respective disky and boxy elliptical counterparts. It is possible that the isophotal shapes have yet to “settle down,” possibly due to incomplete phase mixing. Comparison of the isophotal shapes with kinematic properties such as rotational velocities and Vm/σo may help to provide a clearer picture of what is occurring. The results of the K-band photometry for this large sample suggest that mergers exhibit many of the same photometric properties as elliptical galaxies. Where departures do occur, they are likely due to the effects of a formation process that is still ongoing. However, the results of the K-band photometry by no means conclusively prove that mergers make elliptical galaxies. The photometry requires follow-up spectroscopic investigations to answer a number of questions raised by the results presented in this paper. These questions will be addressed in subsequent papers.

The authors would like to extend a special thanks to Josh Barnes and John Hibbard for taking time to read the paper in several of its incarnations, and taking the time to discuss the results and analysis. The suggestions and comments provided by them were extraordinarily useful. The authors would also like to thank Gabriela Canalizo, Alister Graham, and Chris Mihos for additional comments, discussions and suggestions, as well as T. B. Wall, oﬀ of whom many ideas and suggestions were bounced. We also acknowledge the anonymous referee for his/her detailed reading of the original manuscript and useful suggestions which notably improved the paper. The authors wish to thank Michael Connelley for observing one of the objects in the sample and Sandrine Bot- tinelli for assisting with a number of observations. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California In- – 28 – stitute of Technology, under contract with the National Aeronautics and Space Administration. This research is supported in part, by a Fellowship from the NASA Graduate Student Researchers Program, Grant # NGT5-50396. – 29 –

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This preprint was prepared with the AAS LATEX macros v5.2. Table 1. Merger Sample & Observation Log

Object Names

NGC Arp/Arp-Madore UGC VV IC/other R.A. Dec. V⊙ Integration Time seeing scale notes (J2000) (J2000) (kms−1) (sec) (′′) (pc)

′ ′′ ··· ··· UGC 6 VV 806 · · · 00h 03m 09s 21◦ 57 37 6583 3210 0.9 386 LIRG ′ ′′ NGC 34 ··· ··· VV 850 · · · 00h 11m 06s -12◦ 06 26 5931 3300 0.9 362 LIRG ′ ′′ · · · Arp 230 ··· ··· IC51 00h 46m 24s -13◦ 26 32 1742 2325 1.4 164 shell ′ ′′ NGC455 Arp164 UGC815 ··· ··· 01h 15m 57s 05◦ 10 43 5269 2250 0.9 309 ′ ′′ NGC 828 · · · UGC 1655 ··· ··· 02h 10m 09s 39◦ 11 25 5374 2580 1.1 414 LIRG ′ ′′ ··· ··· UGC 2238 ··· ··· 02h 46m 17s 13◦ 05 44 6436 3480 0.9 409 LIRG ′ ′′ NGC 1210 AM 0304-255 ··· ··· ··· 03h 06m 45s -25◦ 42 59 3928 2820 0.6 173 shell ′ ′′ · · · AM 0318-230 ··· ··· ··· 03h 20m 40s -22◦ 55 53 10699 3660 0.9 680 ′ ′′ NGC 1614 Arp 186 ··· ··· ··· 04h 33m 59s -08◦ 34 44 4778 2730 0.6 198 LIRG ′ ′′ · · · Arp 187 ··· ··· ··· 05h 04m 53s -10◦ 14 51 12291 3150 0.9 736 ′ ′′ · · · AM 0612-373 ··· ··· ··· 06h 13m 47s -37◦ 40 37 9734a 2160 1.0 684 ′ ′′ NGC2418 Arp165 UGC3931 ··· ··· 07h 36m 37s 17◦ 53 02 5052 3600 0.6 222 ′ ′′ ··· ··· UGC 4079 ··· ··· 07h 55m 06s 55◦ 42 13 6108 3600 0.5 231 ′ ′′ NGC2623 Arp243 UGC4509 VV79 · · · 08h 38m 24s 25◦ 45 17 5535 3390 0.6 230 LIRG ′ ′′ ··· ··· UGC 4635 ··· ··· 08h 51m 54s 40◦ 50 09 8684 3480 0.6 361 ′ ′′ NGC2655 Arp225 UGC4637 ··· ··· 08h 55m 37s 78◦ 13 23 1404 1800 0.9 84 shell ′ ′′ NGC 2744 · · · UGC 4757 VV 612 · · · 09h 04m 38s 18◦ 27 37 3424 3600 0.6 134 ′ ′′ NGC2782 Arp215 UGC4862 ··· ··· 09h 14m 05s 40◦ 06 49 2562 3000 0.6 103 LIRG ′ ′′ NGC2914 Arp137 UGC5096 ··· ··· 09h 34m 02s 10◦ 06 31 3151 1800 0.8 165 ′ ′′ ··· ··· UGC 5101 ··· ··· 09h 35m 51s 61◦ 21 11 11809 2700 1.0 779 ULIRG ′ ′′ · · · AM 0956-282 · · · VV 592 · · · 09h 58m 46s -28◦ 37 19 980 3600 0.6 40 ′ ′′ NGC 3256 AM 1025-433 · · · VV 65 · · · 10h 27m 51s -43◦ 54 14 2783 2925 1.0 181 LIRG ′ ′′ NGC3310 Arp217 UGC5786 VV356 · · · 10h 38m 45s 53◦ 30 05 993 2940 0.9 58 ′ ′′ · · · Arp156 UGC5814 ··· ··· 10h 42m 38s 77◦ 29 41 10778b 2400 0.8 606 ′ ′′ NGC 3597 AM 1112-232 ··· ··· ··· 11h 14m 41s -23◦ 43 39 3504 3300 0.8 184 ′ ′′ NGC3656 Arp155 UGC6403 VV22a · · · 11h 23m 38s 53◦ 50 30 2869 3150 0.5 109 shell Table 1—Continued

Object Names

NGC Arp/Arp-Madore UGC VV IC/other R.A. Dec. V⊙ Integration Time seeing scale notes (J2000) (J2000) (kms−1) (sec) (′′) (pc)

′ ′′ NGC3921 Arp224 UGC6823 VV31 · · · 11h 51m 06s 55◦ 04 43 5838 3645 0.8 335 ′ ′′ NGC 4004 · · · UGC 6950 VV 230 · · · 11h 58m 05s 27◦ 52 44 3377 1560 0.8 190 ′ ′′ · · · AM 1158-333 ··· ··· ··· 12h 01m 20s -33◦ 52 36 3027 3600 0.6 137 ′ ′′ NGC4194 Arp160 UGC7241 VV261 · · · 12h 14m 09s 54◦ 31 36 2506 3600 0.6 101 LIRG ′ ′′ NGC 4441 · · · UGC 7572 ··· ··· 12h 27m 20s 64◦ 48 06 2674 1440 0.8 150 ′ ′′ ··· ··· UGC 8058 · · · Mrk 231 12h 56m 14s 56◦ 52 25 12642 1170 0.6 510 ULIRG ′ ′′ · · · AM 1255-430 ··· ··· ··· 12h 58m 08s -43◦ 19 47 9026 3420 0.9 529 ′ ′′ · · · AM 1300-233 ··· ··· ··· 13h 02m 52s -23◦ 55 18 6446 1200 0.6 260 LIRG ′ ′′ NGC 5018 · · · UGCA 335 ··· ··· 13h 13m 00s -19◦ 31 05 2794 2025 0.6 119 shell ′ ′′ · · · Arp193 UGC8387 VV821 IC883 13h 20m 35s 34◦ 08 22 7000 2625 0.6 282 LIRG ′ ′′ · · · AM 1419-263 ··· ··· ··· 14h 22m 06s -26◦ 51 27 6709 3600 0.7 344 ′ ′′ ··· ··· UGC 9829 VV 847 · · · 15h 23m 01s -01◦ 20 50 8492 3600 0.6 342 ′ ′′ NGC6052 Arp209 UGC10182 VV86 · · · 16h 05m 12s 20◦ 32 32 4716 3600 0.7 219 ′ ′′ ··· ··· UGC 10607 VV 852 IC4630 16h 55m 09s 26◦ 39 46 10359 3360 0.7 506 ′ ′′ ··· ··· UGC 10675 VV 805 · · · 17h 03m 15s 31◦ 27 29 10134 3600 0.6 409 ′ ′′ NGC 6598 · · · UGC 11139 ··· ··· 18h 08m 56s 69◦ 04 04 8308 2700 0.6 355 ′ ′′ · · · AM 2038-382 ··· ··· ··· 20h 41m 13s -38◦ 11 36 6057 1920 0.6 274 ′ ′′ · · · AM 2055-425 ··· ··· ··· 20h 58m 26s -42◦ 39 00 12840 2880 1.0 847 LIRG ′ ′′ NGC 7135 AM 2146-350 ··· ··· IC 5136 21h 49m 46s -34◦ 52 35 2640 2520 0.6 119 ′ ′′ ··· ··· UGC 11905 ··· ··· 22h 05m 54s 20◦ 38 22 7522 3000 0.8 395 ′ ′′ NGC 7252 Arp 226, AM 2217-245 ··· ··· ··· 22h 20m 44s -24◦ 40 41 4688 3360 0.9 275 ′ ′′ · · · AM 2246-490 ··· ··· ··· 22h 49m 39s -48◦ 50 58 12884 2520 1.1 976 ULIRG ′ ′′ ··· ··· ··· ··· IC 5298 23h 16m 00s 25◦ 33 24 8197 3240 0.6 330 LIRG ′ ′′ NGC 7585 Arp 223 ··· ··· ··· 23h 18m 01s -04◦ 39 01 3447 2040 0.9 206 shell ′ ′′ NGC7727Arp222 VV67 ··· ··· 23h 39m 53s -12◦ 17 35 1855 3240 0.8 100 Note. — (a) Velocity obtained from measurement CaT lines at λ≃ 8542 A˚ obtained with the ESI spectrograph on Keck-2. (b) The primary recession velocity listed on NED of 1881 km/s, based on H I measurements, is incorrect. The recessional velocity obtained from the CaT lines agrees with the secondary recession velocity listed on NED of V = 10778 km s−1. Table 2. Properties of the K-band de Vaucouleurs and Sersic Proﬁle Fits

1/4 1/4 2 1/4 2 Name R ﬁt Radius last isophote R χν R r.m.s. Sersic n Sersic χν Sersic r.m.s. Violent Relaxation Proﬁle Shape tolastisophote (kpc) (magarcsec−2) (magarcsec−2) (a)

UGC6 Yes 7.7 2.29 0.34 10.00 1.11 0.24 Yes \ NGC34 Yes 9.4 1.18 0.21 10.00 0.21 0.09 Yes \ Arp230 Yes 3.2 1.63 0.29 1.56 0.38 0.14 Notenoughinfo ∩ NGC455 Yes 13.9 0.84 0.13 6.21 0.59 0.11 Yes \ NGC828 Yes 16.1 0.50 0.11 2.99 0.27 0.08 Yes ∩ UGC 2238 No 10.2 3.66 0.39 1.46 0.47 0.14 Questionable ∩ NGC1210 Yes 7.7 0.31 0.08 4.08 0.31 0.08 Yes AM0318-230 Yes 13.5 0.36 0.13 5.01 0.33 0.13 Yes NGC1614 Yes 12.1 4.09 0.26 10.00 2.59 0.20 Yes \ Arp187 Yes 19.8 0.23 0.09 4.10 0.22 0.09 Yes AM0612-373 Yes 23.2 0.27 0.09 3.44 0.24 0.08 Yes NGC2418 Yes 15.5 1.08 0.12 2.85 0.44 0.07 Yes ∩ UGC4079 Yes 12.0 0.67 0.11 3.08 0.50 0.09 Yes NGC2623 Yes 9.1 2.42 0.24 10.00 1.05 0.16 Yes \ UGC4635 Yes 13.3 0.99 0.16 5.07 0.90 0.15 Yes NGC2655 Yes 6.4 0.68 0.09 3.01 0.15 0.04 Yes ∩ NGC 2744 No 6.5 1.00 0.14 2.35 0.33 0.08 Incomplete NGC2782 Yes 7.9 2.67 0.18 6.68 2.02 0.16 Yes \ NGC2914 Yes 7.2 2.48 0.24 6.97 2.09 0.22 Yes UGC5101 Yes 12.4 0.43 0.17 10.00 0.04 0.05 Yes \ AM0956-282 Yes 2.6 1.87 0.16 2.45 1.10 0.13 Notenoughinfo ∩ NGC 3256 No 8.4 2.35 0.22 2.17 0.71 0.12 Incomplete NGC3310 Yes 3.3 3.37 0.24 2.41 2.13 0.19 Notenoughinfo Arp156 Yes 18.7 0.76 0.15 9.64 0.44 0.12 Yes \ NGC 3597 No 5.5 2.94 0.31 1.87 0.94 0.18 Incomplete ∩ NGC3656 Yes 9.2 1.47 0.13 4.47 1.42 0.13 Yes NGC3921 Yes 10.7 0.49 0.12 5.03 0.43 0.11 Yes \ NGC4004 Yes 5.5 3.52 0.35 1.53 1.78 0.25 No ∩ Table 2—Continued

1/4 1/4 2 1/4 2 Name R ﬁt Radius last isophote R χν R r.m.s. Sersic n Sersic χν Sersic r.m.s. Violent Relaxation Proﬁle Shape tolastisophote (kpc) (magarcsec−2) (magarcsec−2) (a)

AM1158-333 Yes 4.7 0.98 0.17 3.75 0.97 0.16 Yes ∩ NGC4194 Yes 4.9 1.75 0.19 4.59 1.69 0.18 Yes NGC4441 Yes 5.4 0.54 0.12 6.83 0.32 0.09 Yes \ UGC8058 No 9.6 2.54 0.37 10.00 0.83 0.21 Incomplete \ AM1255-430 Yes 10.5 1.19 0.25 1.87 0.49 0.16 Yes AM1300-233 No 14.5 2.02 0.19 4.99 1.93 0.18 Incomplete NGC5018 Yes 13.2 0.67 0.07 4.39 0.61 0.07 Yes \ Arp193 No 9.3 1.01 0.17 2.74 0.69 0.13 Incomplete AM1419-263 Yes 17.5 0.10 0.04 4.12 0.09 0.04 Yes UGC9829 No 12.6 2.61 0.26 2.34 2.10 0.24 Incomplete NGC6052 No 7.8 4.66 0.36 1.13 0.80 0.15 No ∩ UGC10607 Yes 12.1 0.42 0.13 2.94 0.27 0.10 Yes UGC10675 Yes 8.5 1.90 0.30 10.00 0.76 0.19 Yes \ NGC6598 Yes 19.8 0.13 0.05 3.65 0.11 0.04 Yes AM2038-382 Yes 7.9 2.19 0.27 10.00 0.87 0.17 Yes \ AM2055-425 Yes 11.8 0.81 0.25 2.34 0.66 0.22 Yes NGC7135 Yes 10.1 3.72 0.21 10.00 1.89 0.15 Yes \ UGC11905 Yes 8.6 0.36 0.13 5.24 0.31 0.12 Yes NGC7252 Yes 10,7 0.73 0.13 3.32 0.65 0.13 Yes AM2246-490 Yes 19.5 2.00 0.32 10.00 1.05 0.23 Yes \ IC5298 Yes 11.2 1.23 0.19 3.93 1.23 0.19 Yes NGC7585 Yes 11.7 0.43 0.08 3.53 0.39 0.08 Yes NGC7727 Yes 8.7 0.79 0.09 3.41 0.63 0.08 Yes ∩ Note. — (a) Proﬁle Shapes: \ = excess light, ∩ = turnover, = no deviation. – 38 –

Table 3. K-band Photometric Properties

a a a b Name µ◦ µeﬀ Log Reﬀ MK (mag arcsec−2) (mag arcsec−2) (kpc) (mag)

UGC6 -7.47 13.87 -0.33 -24.01 NGC34 -8.78 12.55 -0.45 -24.61 Arp230 14.39 17.44 -0.01 -21.75 NGC455 5.28 18.41 0.73 -24.64 NGC828 10.40 16.55 0.50 -25.36 UGC2238 12.85 15.69 0.32 -24.58 NGC1210 8.88 17.39 0.29 -23.72 AM0318-230 6.34 16.87 0.44 -25.09 NGC1614 -4.77 16.56 0.34 -24.74 Arp187 9.50 18.05 0.82 -25.25 AM0612-373 10.91 18.04 0.89 -25.63 NGC2418 11.09 16.92 0.54 -25.31 UGC4079 12.21 18.55 0.65 -23.78 NGC2623 -4.60 16.73 0.25 -24.22 UGC4635 5.91 16.56 0.35 -24.71 NGC2655 10.00 16.20 0.11 -23.70 NGC2744 13.67 18.43 0.43 -22.83 NGC2782 4.63 18.78 0.59 -23.83 NGC2914 3.54 18.33 0.44 -23.51 UGC5101 -10.80 10.53 -0.58 -25.50 AM0956-282 14.57 19.66 0.13 -20.50 NGC3256 11.49 15.85 0.27 -24.72 NGC3310 12.00 16.88 -0.08 -22.07 Arp156 -0.08 20.60 1.36 -25.81 NGC3597 11.73 15.45 0.01 -23.72 NGC3656 9.00 18.34 0.51 -23.70 NGC3921 5.48 16.05 0.23 -25.13 NGC4004 14.68 17.65 0.32 -22.89 AM1158-333 9.25 17.05 0.04 -22.61 NGC4194 5.66 15.27 -0.21 -23.21 NGC4441 4.50 18.98 0.50 -22.98 UGC8058 -18.00 3.33 -1.49 -27.55 AM1255-430 13.14 16.85 0.44 -24.93 AM1300-233 8.37 18.87 0.96 -24.65 NGC5018 7.58 16.76 0.49 -25.15 Arp193 10.50 16.10 0.27 -24.40 AM1419-263 8.96 17.57 0.64 -24.94 Table 3—Continued

a a a b Name µ◦ µeﬀ Log Reﬀ MK (mag arcsec−2) (mag arcsec−2) (kpc) (mag)

UGC9829 13.43 18.17 0.73 -24.96 NGC6052 15.57 17.67 0.47 -23.55 UGC10607 9.45 15.49 0.26 -25.20 UGC10675 -8.95 12.35 -0.50 -24.80 NGC6598 10.27 17.87 0.75 -25.51 AM2038-382 -7.00 14.34 -0.17 -24.70 AM2055-425 12.08 16.82 0.49 -25.08 NGC7135 0.28 21.62 1.27 -23.95 UGC11905 5.29 16.33 0.22 -24.51 NGC7252 9.64 16.51 0.34 -24.84 AM2246-490 -2.16 19.17 1.00 -25.52 IC5298 7.65 15.83 0.24 -24.92 NGC7585 9.81 17.14 0.51 -24.98 NGC7727 9.66 16.72 0.29 -24.23

Note. — (a) Extracted from Sersic ﬁts. (b) Measured using the summed ﬂux in circular apertures out to the edge of the array. Table 4. Phase-Mixing Results

Name unsharp mask structure residual image

UGC 6 round core, otherwise featureless diffuse loops & shells near center NGC 34 round core, potential clusters surrounding, otherwise featureless positive intensity to NW Arp 230 round core with clearly visible tidal arms bright tidal arms, quadrupole feature around nucleus NGC 455 oval core, small, diffuse extension to NW & SE featureless (w/exception of tidal tails) NGC 828 inclined central disk with strong spiral features central inclined disk, quadrupole feature UGC 2238 edge-on disk with upturn on SE side large residuals in central region NGC 1210 rectangular core, diffuse extension to NW & SE faint streamers emanating from center every 45◦ AM0318-230 rectangularcore,otherwisefeatureless featureless (w/exception of tidal tails) NGC1614 centralspiralarms,elongatedstructureSW largeresiduals in center due to spiral arms Arp 187 elongated central structure, diffuse extension to N apparent streamers to N & SE emanating from center AM0612-373 roundcore,diffuseextensiontoNE&SW quadrupole feature surrounding nucleus NGC 2418 rhombus shaped core, otherwise featureless faint patches surrounding nucleus, otherwise featureless UGC 4079 patches in central region, stellar structure N large residuals in center NGC 2623 oval core, otherwise featureless variations in residual in center, positive to NW, negative to SE UGC 4635 round core, diffuse extension to NE & SW patches of varying intensity surrounding nucleus NGC2655 rectangularcore,otherwisefeatureless faintcentral quadrupole feature NGC 2744 diffuse patchy core, otherwise featureless large residuals of varying positive and negative intensity NGC2782 patchycore,possiblespiralshaped largeresiduals in possible disk, patches of intensity, plumes NGC 2914 oval core, possible disk patches of intensity surrounding nucleus, non-uniform residuals UGC 5101 oval core, faint thin structure running N to S faint central quadrupole feature AM 0956-282 diffuse patchy core w/no structure poor model subtraction, large residuals & intensity fluctuations NGC 3256 central face-on damaged spiral, bright elongated patch S of nucleus large residuals and intensity variations due to central disk NGC3310 centralcorew/surroundingring,armsN&S largeresiduals and intensity variations due to central disk Arp 156 oval core, faint extension to NW & SE very faint central quadrupole feature NGC 3597 oval core, thin structure E & NE, bright patch W of nucleus large residuals, opposite intensity variations N and S of center NGC 3656 patchy core, vertical dust lane large residuals, dust lane in center, plumes extending from center NGC 3921 oval core, otherwise featureless large residuals, N tail negative intensity, S tail positive intensity NGC 4004 thin, vertical bar-like structure, second thin bar structure at 30◦ large positive residuals along arms, negative residuals NE & SE AM 1158-333 thin patchy diffuse structure at 45◦ poor model subtraction, negative residuals W, positive residuals E Table 4—Continued

Name unsharp mask structure residual image

NGC 4194 patchy structure, possible dust lanes large residuals, residuals wrapped around center NGC4441 roundcore,otherwisefeatureless featureless UGC 8058 very bright core showing diffraction spikes bright core, diffraction spikes, tidal tail shows positive residuals AM 1255-430 round core w/surrounding diffuse flaring every 45◦ large residuals, residuals wrapped around center AM 1300-233 thin possible edge-on disk at 60◦ large residuals around center in spiral pattern NGC5018 rectangularcore,otherwisefeatureless centralquadrupole feature, diffuse patches Arp 193 thin possible edge-on disk at 45◦ large residuals, positive intensity NE, negative intensity SW AM1419-263 roundcorewithflaringtoNE&SW largeresiduals, positive intensity SW, negative intensity NE UGC 9829 thin bar across nucleus w/tidal extensions to N & S large residuals at center, tidal arms all positive intensity NGC 6052 thin diffuse patchy structure very poor model subtraction, large residuals UGC10607 roundcore,w/faintflaringtoN&S largeresiduals, positive intensity along tail, negative intensity E UGC10675 odd-shapedcore,otherwisefeatureless largeresiduals, negative intensity N, positive intensity S NGC6598 roundcorew/flarestoN&S largeresidualsaroundcenter in spiral pattern AM 2038-382 elongated (NW to SE) core w/faint flaring faint residuals near center, otherwise featureless AM2055-425 roundcorewithsomeflaring largeresiduals,negative to N, positive to S NGC 7135 rectangular core, otherwise featureless tail to NE extends to nucleus, plumes to E and S UGC 11905 oval core at 45◦, otherwise featureless large residuals around center in spiral pattern NGC 7252 central core in face-on disk w/bright possible SSCs surrounding plumes and shells extend outward from nucleus AM 2246-490 round core, otherwise featureless faint plumes E & N of nucleus, negative intensity around plumes IC 5298 round core w/faint extensions to NE & SW, bright spot NE tail positive intensity along arms and E, negative intensity W NGC 7585 oval core w/flaring, otherwise featureless plumes extending outwards from nucleus NGC 7727 core surrounded by inclined disk, several potential clusters central quadrupole feature, plumes extending from nucleus Table 5. Mergers with “excess light”

Name Radius Max. ∆µK Excess Luminosity −2 (pc) (mag arcsec ) Log L⊙

UGC6 386 1.04 10.13 NGC34 362 0.68 10.21 NGC455 617 0.48 9.82 NGC1614 397 1.31 10.49 NGC2623 230 0.85 9.85 NGC2782 206 0.63 9.21 UGC5101 779 0.41 10.48 Arp156 606 0.44 9.83 NGC3921 335 0.30 9.61 NGC4441 150 0.45 8.79 UGC 8058a 510 0.88 11.77 NGC5018 239 0.31 9.55 UGC10675 408 0.64 10.15 AM2038-382 274 0.86 10.13 NGC7135 358 0.81 9.53 AM2246-490 976 0.89 10.48

Note. — (a) Known AGN Table 6. K-band Structural Parameters

Name Isophotal Shape Median a4/a Twisty Isophotes Average ǫ Median ǫ Max ǫ Eﬀective ǫ

UGC6 Disky 0.01282 Yes 0.24 0.25 0.32 0.17 NGC34 Disky 0.02038 Yes 0.14 0.16 0.23 0.03 Arp230 Disky 0.05607 Yes 0.17 0.16 0.36 0.29 NGC455 Boxy -0.00549 No 0.38 0.42 0.47 0.44 NGC828 Disky 0.00347 No 0.32 0.31 0.42 0.28 UGC2238 Disky 0.02371 No 0.43 0.49 0.61 0.59 NGC1210 Disky 0.00840 No 0.23 0.22 0.36 0.35 AM0318-230 Disky 0.00131 Yes 0.15 0.16 0.24 0.16 NGC1614 Boxy -0.00469 Yes 0.36 0.39 0.52 0.45 Arp187 Disky 0.01753 No 0.40 0.40 0.48 0.47 AM0612-373 Disky 0.02972 No 0.40 0.44 0.51 0.45 NGC2418 Boxy -0.00242 Yes 0.16 0.17 0.23 0.21 UGC4079 Boxy -0.00242 Yes 0.49 0.50 0.61 0.52 NGC2623 Disky 0.01283 Yes 0.27 0.29 0.35 0.25 UGC4635 Disky 0.01438 No 0.30 0.29 0.45 0.44 NGC2655 Boxy -0.00231 No 0.27 0.28 0.34 0.33 NGC2744 Boxy -0.04094 No 0.45 0.46 0.51 0.46 NGC2782 Boxy -0.00064 Yes 0.15 0.15 0.32 0.14 NGC2914 Disky 0.01387 Yes 0.35 0.39 0.49 0.39 UGC5101 Disky 0.03601 No 0.30 0.31 0.36 0.13a NGC3256 Disky 0.01708 Yes 0.21 0.24 0.42 0.30 NGC3310 Boxy -0.00386 Yes 0.19 0.19 0.43 0.10 Arp156 Disky 0.01556 No 0.25 0.29 0.34 0.34b NGC3597 Disky 0.01206 No 0.33 0.37 0.53 0.37 NGC3656 Disky 0.01206 Yes 0.17 0.17 0.31 0.18 NGC3921 Boxy -0.00097 Yes 0.13 0.13 0.26 0.20 NGC4004 Disky 0.00652 Yes 0.50 0.49 0.77 0.37 AM1158-333 Disky 0.00664 Yes 0.36 0.32 0.91 0.27 NGC4194 Disky 0.00668 Yes 0.33 0.36 0.41 0.14 – 44 – ǫ a b Effective ǫ Max ǫ Median ǫ eff R 1.5 < Twisty Isophotes Average Table 6—Continued /a 4 a 1st isophote, (b) Last isophote < eff R Name Isophotal Shape Median Note. — (a) 1.5 AM1255-430AM1300-233NGC5018 DiskyArp193 BoxyAM1419-263UGC9829 DiskyNGC6052 0.00091 DiskyUGC10607 -0.00675 DiskyUGC10675 DiskyNGC6598 0.01315 BoxyAM2038-382 0.00346 Yes BoxyAM2055-425 0.00891 No DiskyNGC7135 0.02383 Disky BoxyUGC11905 -0.02101 No Boxy -0.00188NGC7252 0.29 No 0.00506AM2246-490 No 0.65 DiskyIC5298 0.02999 Yes -0.00419 DiskyNGC7585 0.31 Yes -0.00809 0.25 Disky No BoxyNGC7727 0.65 0.47 Yes 0.37 0.00812 0.45 0.00193 Yes 0.26 0.36 0.72 No Disky Disky 0.32 Yes 0.00018 0.48 -0.00793 0.36 Disky 0.49 0.30 0.34 0.69 0.24 0.40 No 0.57 Yes 0.32 0.01669 0.56 0.00093 0.08 0.18 0.25 0.31 0.59 Yes 0.01335 0.27 No 0.25 0.45 0.52 0.50 0.39 0.07 0.19 0.31 0.25 0.35 0.12 0.32 Yes 0.29 No 0.21 0.30 Yes 0.25 0.23 0.13 0.23 0.38 0.24 0.13 0.15 0.18 0.31 0.24 0.13 0.36 0.31 0.26 0.33 0.16 0.44 0.18 0.36 0.29 0.04 0.26 0.26 0.41 0.16 0.45 0.32 0.36 0.27 0.22 0.20 NGC4441UGC8058 Disky Disky 0.00584 0.00407 No Yes 0.29 0.06 0.30 0.07 0.37 0.11 0.30 0.00 – 45 –

Fig. 1.— Figure 1 displays for each merger 4 vertical panels: the reverse grey-scale K-band image displayed with a log stretch and overplotted with a metric scale bar, the surface brightness profile fitted with a de Vaucouleurs r1/4 law plotted against radius1/4 and linear radius (top axis), the surface brightness profile fitted with a Sersic r1/n law plotted against radius1/4 and linear radius (top axis), and the structural parameters a4/a, a3/a, Position angle, and ellipticity plotted against the linear radius in kpc. The data points in the surface brightness profiles and structural parameter plots are all plotted out to a 3σ detection limit. Data points in the surface brightness profiles marked with an “×” are 2σ or greater deviations from the r.m.s. of the best-fit de Vaucouleurs profile. The vertical dashed line overlaid on the structural plots indicates the effective radius of the galaxy. – 46 – – 47 – – 48 – – 49 – – 50 – – 51 – – 52 – – 53 – – 54 – – 55 – – 56 – – 57 – – 58 – – 59 – – 60 – – 61 – – 62 – – 63 –

Fig. 2.— Sersic surface brightness profiles for a range of index n for Reff = 2 kpc (sample median is 2.2 kpc). Profiles with n > 2 are very similar at large radii. Figure taken from Surace & Sanders (2000). – 64 –

Fig. 3.— Histogram of the distribution of the Sersic parameter n values binned by integer. – 65 –

Fig. 4.— Displayed for each merger in pairs are the K-band unsharp mask image (left) and the K-band residual image created by subtracting a model galaxy image from the real image (right). – 66 – – 67 – – 68 – – 69 – – 70 – – 71 – – 72 – – 73 – – 74 –

Fig. 5.— Plot of MK vs. n. The plot shows the a weak correlation (a Pearson linear correlation coeﬃcient of p = -0.235) between the absolute magnitude and the Sersic parameter n. – 75 –

Fig. 6.— Plot of the K-band central surface brightness µ◦ vs. n. The plot shows the a strong correlation (a Pearson linear correlation coeﬃcient of p = -0.953) between the central luminosity and the Sersic parameter n. – 76 –

Fig. 7.— Distribution of MK for the sample derived from the Sersic ﬁts to the proﬁles. The bars with the diagonal lines represent the mergers that are ≥ 1 L∗ellip. The clear bars represent the mergers < 1 L∗ellip. – 77 –

Fig. 8.— Histogram showing the distribution of the ǫeﬀ of the mergers in the sample.