Deep Surface Photometry Research Articles

Indications of the invalidity of the exponentiality of the disk within bulges of spiral galaxies

Context. A fundamental subject in extragalactic astronomy concerns the formation and evolution of late-type galaxies (LTGs). The standard scenario envisages a two-phase build-up for these systems, comprising the early assembly of the bulge followed by disk accretion. However, recent observational evidence points to a joint formation and perpetual coevolution of these structural components. Our current knowledge on the properties of the bulge and the disk is, to a large degree, founded on photometric decomposition studies, which sensitively depend on the adopted methodology and enclosed assumptions on the structure of LTGs. A critical assumption whose validity had never been questioned before is that galactic disks conserve their exponential nature up to the galactic center. This, although seemingly plausible, implies that bulge and disk coexist without significant dynamical interaction and mass exchange over nearly the entire Hubble time. Aims. Our goal is to examine the validity of the standard assumption that galactic disks preserve their exponential intensity profile inside the bulge radius (RB) all the way to the galactic center, as is generally assumed in photometric decomposition studies. Methods. We developed a spectrophotometric bulge-disk decomposition technique that provides an estimation for the net (i.e., disk-subtracted) spectrum of the bulge. Starting from an integral field spectroscopy (IFS) data cube, this tool computes the integrated spectrum of the bulge and the disk, scales the latter considering the light fraction estimated from photometric decomposition techniques, and subtract it from the former, thereby allowing for the extraction of the net-bulge spectrum. Considering that the latter depends on the underlying assumption for the disk luminosity profile, checking its physical plausibility (for instance, positiveness and spectral slope) places indirect constraints on the validity of the disk’s assumed profile inside the radius R⋆ < RB. In this pilot study, we tested the following three different disk configurations: the standard exponential disk profile as well as a centrally flattened or down-bending exponential disk profile. Results. A systematic application of our spectrophotometric bulge-disk decomposition tool to a representative sample of 135 local LTGs from the CALIFA survey yields a significant fraction (up to ∼30 (20)%) of unphysical net-bulge spectra when a purely exponential (centrally flattened) intensity profile is assumed for the disk. This never occurs for disks’ profiles involving a centrally decreasing intensity. Conclusions. The obtained results suggest that, for a significant fraction of LTGs, the disk component shows a down-bending beneath the bulge. If proven to be true, this result will call for a substantial revision of structural decomposition studies for LTGs and it will have far-reaching implications in our understanding of the photometric properties of their bulges. Given its major relevance, it appears worthwhile to explore the central stellar surface density of galactic disks further, through an improved version of the spectrophotometric decomposition tool presented here and its application combining deep surface photometry, spatially resolved spectral synthesis, and kinematical analyses.

Stellar Populations in the Outer Disk and Halo of the Spiral Galaxy M101

Abstract We use deep Hubble Space Telescope imaging in the outskirts of the nearby spiral M101 to study stellar populations in the galaxy’s outer disk and halo. Our Advanced Camera for Surveys (ACS) field lies 17.′6 (36 kpc) from the center of M101 and targets the blue “NE Plume” of M101's outer disk, while the parallel Wide Field Camera 3 (WFC3) field lies at a distance of 23.′3 (47 kpc) to sample the galaxy’s stellar halo. The WFC3 halo field shows a well-defined red giant branch characterized by low metallicity ([M/H] = −1.7 ± 0.2), with no evidence of young stellar populations. In contrast, the ACS disk field shows multiple stellar populations, including a young main sequence, blue and red helium-burning stars, and old RGB and asymptotic giant branch (AGB) populations. The mean metallicity of these disk stars is quite low: [M/H] = −1.3 ± 0.2 for the RGB population, and −1.15 ± 0.2 for the younger helium-burning sequences. Of particular interest is a bunching of stars along the BHeB sequence, indicative of an evolving cohort of massive young stars. We show that the young stellar populations in this field are well-described by a decaying burst of star formation that peaked ∼300–400 Myr ago, along with a more extended star formation history to produce the older RGB and AGB populations. These results confirm and extend the results from our previous deep surface photometry of M101's outer disk, providing an important cross-check on stellar population studies using resolved stellar populations versus integrated light photometry. We discuss our results in the context of halo formation models and the interaction history of M101 and its companions.

The Lyman Alpha Reference Sample

Context. The Lyman α Reference Sample (LARS) of 14 star-forming galaxies offers a wealth of insight into the workings of these local analogs to high-redshift star-forming galaxies. The sample has been well-studied in terms of Lyα and other emission line properties, such as H I mass, gas kinematics, and morphology. Aims. We analyze deep surface photometry of the LARS sample in UBIK broadband imaging obtained at the Nordic Optical Telescope and the Canada-France-Hawaii Telescope, and juxtaposition their derived properties with a sample of local high-redshift galaxy analogs, namely, with blue compact galaxies (BCGs). Methods. We construct radial surface brightness and color profiles with both elliptical and isophotal integration, as well as RGB images, deep contours, color maps, a burst fraction estimate, and a radial mass-to-light ratio profile for each LARS galaxy. Standard morphological parameters like asymmetry, clumpiness, the Gini and M20 coefficients are also obtained and analyzed, as well as isophotal asymmetry profiles for each galaxy. In this context, we compare the LARS sample to the properties of the BCG sample and highlight the differences. Results. Several of these diagnostics indicate that the LARS galaxies have highly disturbed morphologies even at the level of the faintest outer isophotes, with no hint at a regular underlying population, as found in many BCG sample galaxies. The ground-based photometry reaches isophotes down to ~28 mag arcsec−2, while the space-based data reach only ~26 mag arcsec−2. The ground-based observations therefore reveal previously unexplored isophotes of the LARS galaxies. The burst fraction estimate suggests a spatially more extended burst region in LARS than in the BCGs. Comparison to stellar evolutionary models in color–color diagrams reveals complex behavior of the radial color profiles, often inconsistent with a single stellar population of any age and metallicity, but instead suggesting a mixture of at least two stellar populations with a typical young mass fraction in the range 0.1–1%. Conclusions. The galaxies in the LARS sample appear to be in earlier stages of a merger event compared to the BCGs. Standard morphological diagnostics like asymmetry, clumpiness, Gini and M20 coefficients cannot separate the two samples, although an isophotal asymmetry profile successfully captures the average difference in morphology. These morphological diagnostics do not show any correlation with the equivalent width or the escape fraction of Lyman Alpha.

Deep near-infrared surface photometry and properties of Local Volume dwarf irregular galaxies

We present deep H-band surface photometry and analysis of 40 Local Volume galaxies, a sample primarily composed of dwarf irregulars in the Cen A group, obtained using the IRIS2 detector at the 3.9m Anglo-Australian Telescope. We probe to a surface brightness of ~25 mag arcsec$^{-2}$, reaching a 40 times lower stellar density than the Two Micron All Sky Survey (2MASS). Employing extremely careful and rigorous cleaning techniques to remove contaminating sources, we perform surface photometry on 33 detected galaxies deriving the observed total magnitude, effective surface brightness and best fitting S\'ersic parameters. We make image quality and surface photometry comparisons to 2MASS and VISTA Hemispheric Survey (VHS) demonstrating that deep targeted surveys are still the most reliable means of obtaining accurate surface photometry. We investigate the B-H colours with respect to mass for Local Volume galaxies, finding that the colours of dwarf irregulars are significantly varied, eliminating the possibility of using optical-NIR colour transformations to facilitate comparison to the more widely available optical data sets. The structure-luminosity relationships are investigated for our `clean' sample of dwarf irregulars. We demonstrate that a significant fraction of the Local Volume dwarf irregular population have underlying structural properties similar to both Local Volume and Virgo Cluster dwarf ellipticals. Linear regressions to structure-luminosity relationships for the Local Volume galaxies and Virgo Cluster dwarf ellipticals show significant differences in both slope and scatter around the established trend lines, suggesting that environment might regulate the structural scaling relationships of dwarf galaxies in comparison to their more isolated counterparts.

Deep multiband surface photometry on a sample of 24 blue compact galaxies

We present deep surface photometry of a volume--limited sample of 21 UM emission line galaxies in broadband optical UBVRI and near infra-red (NIR) HKs filters. The sample comprises 19 blue compact galaxies (BCGs) and two spirals. For some targets the exposure times are the deepest to date. For the BCG UM462 we observe a previously undetected second disk component beyond a surface brightness level of mu_B=26 mag arcsec^{-2}. This is a true low surface brightness component with central surface brightness mu_0=24.1 mag arcsec^{-2} and scale length h_r=1.5 kpc. All BCGs are dwarfs, with M_B>=-18, and very compact, with an average scale length of h_r~1 kpc. We separate the burst and host populations for each galaxy and compare them to stellar evolutionary models with and without nebular emission contribution. We also measure the A_{180} asymmetry in all filters and detect a shift from optical to NIR in the average asymmetry of the sample. This shift seems to be correlated with the morphological class of the BCGs. Using the color-asymmetry relation, we identify five BCGs in the sample as mergers, which is confirmed by their morphological class. Though clearly separated from normal galaxies in the concentration-asymmetry parameter space, we find that it is not possible to distinguish luminous starbursting BCGs from the merely star forming low luminosity BCGs.

Deep multiband surface photometry on a sample of 24 blue compact galaxies – I

[Abridged] We present deep optical and near-infrared UBVRIHKs imaging data for 24 blue compact galaxies (BCGs). The sample contains luminous dwarf and intermediate-mass BCGs which are predominantly metal-poor, although a few have near-solar metallicities. We have analyzed isophotal and elliptical integration surface brightness and color profiles, extremely deep (mu_B<29 mag arcsec^{-2}) contour maps and RGB images for each galaxy in the sample. The colors are compared to different spectral evolutionary models. We detect extremely extended low surface brightness (LSB) components dominant beyond the Holmberg radius as well as optical bridges between companion galaxies at the mu_V~28th mag arcsec^{-2} isophotal level. The central surface brightness mu_0 and scale length h_r are derived from two radial ranges typically assumed to be dominated by the underlying host galaxy. We find that mu_0 and h_r of the BCGs host deviate from those of dwarf ellipticals (dE) and dwarf irregulars (dI) solely due to a strong burst contribution to the surface brightness profile almost down to the Holmberg radius. Structural parameters obtained from a fainter region, mu_B=26-28 mag arcsec^{-2}, are consistent with those of true LSB galaxies for the starbursting BCGs in our sample, and with dEs and dIs for the BCGs with less vigorous star formation.

STELLAR POPULATIONS IN THE OUTER HALO OF THE MASSIVE ELLIPTICAL M49

We use deep surface photometry of the giant elliptical M49 (NGC 4472), obtained as part of our survey for diffuse light in the Virgo Cluster, to study the stellar populations in its outer halo. Our data trace M49's stellar halo out to ~ 100 kpc (7 Re), where we find that the shallow color gradient seen in the inner regions becomes dramatically steeper. The outer regions of the galaxy are quite blue (B-V ~ 0.7); if this is purely a metallicity effect, it argues for extremely metal poor stellar populations with [Fe/H] < -1. We also find that the extended accretion shells around M49 are distinctly redder than the galaxy's surrounding halo, suggesting that we are likely witnessing the buildup of both the stellar mass and metallicity in M49's outer halo due to late time accretion. While such growth of galaxy halos is predicted by models of hierarchical accretion, this growth is thought to be driven by more massive accretion events which have correspondingly higher mean metallicity than inferred for M49's halo. Thus the extremely metal-poor nature of M49's extended halo provides some tension against current models for elliptical galaxy formation.

DEEPKs-NEAR-INFRARED SURFACE PHOTOMETRY OF 80 DWARF IRREGULAR GALAXIES IN THE LOCAL VOLUME

We present deep near-infrared (K_s) images and surface photometry for 80 dwarf irregular galaxies (dIs) within ~5 Mpc of the Milky Way. The galaxy images were obtained at five different facilities between 2004 and 2006. The image reductions and surface photometry have been performed using methods specifically designed for isolating faint galaxies from the high and varying near-infrared sky level. Fifty-four of the 80 dIs have surface brightness profiles which could be fit to a hyperbolic-secant (sech) function, while the remaining profiles could be fit to the sum of a sech and a Gaussian function. From these fits, we have measured central surface brightnesses, scale lengths, and integrated magnitudes. This survey is part of a larger study of the connection between large-scale structure and the global properties of dIs, the hypothesized building-blocks of more massive galaxies.

DIFFUSE TIDAL STRUCTURES IN THE HALOS OF VIRGO ELLIPTICALS

We use deep V-band surface photometry of five of the brightest elliptical galaxies in the Virgo cluster to search for diffuse tidal streams, shells, and plumes in their outer halos (r > 50 kpc). We fit and subtract elliptical isophotal models from the galaxy images to reveal a variety of substructure, with surface brightnesses in the range mu_V= 26-29 mag/arcsec^2. M49 possesses an extended, interleaved shell system reminiscent of the radial accretion of a satellite companion, while M89's complex system of shells and plumes suggests a more complicated accretion history involving either multiple events or a major merger. M87 has a set of long streamers as might be expected from stripping of low luminosity dwarfs on radial orbits in Virgo. M86 also displays a number of small streams indicative of stripping of dwarf companions, but these comprise much less luminosity than those of M87. Only M84 lacks significant tidal features. We quantify the photometric properties of these structures, and discuss their origins in the context of each galaxy's environment and kinematics within the Virgo cluster.

Removing Internal Reflections from Deep Imaging Data Sets

ABSTRACTWe present a means of characterizing and removing internal reflections between the CCD and other optical surfaces in an astronomical camera. The stellar reflections appear as out-of-focus images and are not necessarily axisymmetric about the star. Using long exposures of very bright stars as calibration images, we are able to measure the position, size, and intensity of reflections as a function of their position on the field. We also measure the extended stellar point-spread function out to 1°. Together this information can be used to create an empirical model of the excess light from bright stars and reduce systematic artifacts in deep surface photometry. We then reduce a set of deep observations of the Virgo cluster with our method to demonstrate its efficacy and to provide a comparison with other strategies for removing scattered light.

How the extinction of extragalactic background light affects surface photometry of galaxies, groups and clusters

The faint regions of galaxies, groups and clusters hold important clues about how these objects formed, and surface photometry at optical and near-infrared wavelengths represents a powerful tool for studying such structures. Here, we identify a hitherto unrecognized problem with this technique, related to how the night sky flux is typically measured and subtracted from astronomical images. While most of the sky flux comes from regions between the observer and the target object, a small fraction - the extragalactic background light (EBL) - comes from behind. We argue that since this part of the sky flux can be subject to extinction by dust present in the galaxy/group/cluster studied, standard reduction procedures may lead to a systematic oversubtraction of the EBL. Even very small amounts of extinction can lead to spurious features in radial surface surface brightness profiles and colour maps of extended objects. We assess the likely impact of this effect on a number of topics in extragalactic astronomy where very deep surface photometry is currently attempted, including studies of stellar halos, starburst host galaxies, disc truncations and diffuse intragroup/intracluster light. We argue that EBL extinction may provide at least a partial explanation for the anomalously red colours reported for the halos of disc galaxies and the hosts of local starburst galaxies. EBL extinction effects also mimic truncations in discs with unusually high dust opacities, but are unlikely to be the cause of such features in general. Failure to account for EBL extinction can also give rise to a non-negligible underestimate of intragroup and intracluster light at the faintest surface brightness levels currently probed. (Abridged)

Deep surface photometry of edge-on spirals in Abell galaxy clusters

Context. There is a clear scarcity of structural parameters for stellar thick discs, especially for spiral galaxies located in high-density regions, such as galaxy clusters and compact groups.

Surface photometry of cD envelopes

We have carried out deep BVR surface photometry of 6 cD and cD-like galaxies using the 2.1-m telescope at San Pedro Mártir, cD galaxies are supergiant galaxies (M&gt;1013M⊙) with enormous halos (&gt;100 kpc in radius) surrounding a giant elliptical galaxy, found generally at the centre of rich clusters (Oemler 1976, Schombert 1988). The surface brightness profiles of their halos (envelopes) break from the r1/4 law, containing more light at large radii (Schombert (1988)), although a detailed 1 and 2 dimensional analysis of their morphology has yet to be carried out. There have been four main theories as to the origin of cD envelopes (Schombert (1988) and references within), a) stripping of stars from other cluster member galaxies, b) formation of galaxy and envelope at the same time during the formation of the cluster, c) mergers of cluster members, which do not easily explain the high velocity dispersions in the envelopes (~ 1000 km s−1), d) cooling flows: accumulation of cooling X-ray-emitting ICM gas around the central galaxy. Very red envelopes have been found around some cD's, and star formation biased towards lowmass stars in cooling flows were invoked to explain these red halos, but the expected very bright near-IR halos were not detected (Joy et al. 1995 and references within). Previous detailed studies of cD galaxies (e.g. Mackie 1992) found a range of colour gradients.

Deep Surface Photometry of M87 with 13 Optical Bands

Multicolor surface photometry of M87 was undertaken with the Beijing-Arizona-Taiwan-Connecticut photometric system, which contains 13 optical bands covering a range from 3800 to 10000 A. Radial profiles have been derived for surface brightness, ellipticity, position angle, and the residuals from the fitted ellipses. The results show a good agreement with pure elliptical isophotes for M87, and no obvious substructure or dust is detected out to r = 500'', corresponding to 40 kpc from the center. The surface brightness profile at r < 190'' is well fitted by the de Vaucouleurs law. Moreover, a cD envelope is detected outside that radius. Given the surface brightness profiles of the 13 bands, the spectral energy distribution (SED) of M87 is obtained as a function of semimajor axis (SMA). By fitting the observed SEDs to synthetic ones derived from the PEGASE model, we explored the star formation history (SFH) of this giant elliptical galaxy. In the hypothesis that the evolution time T of M87 is the same as the mean age of its globular clusters, about 14 Gyr, we find that the characteristic timescale τ of the SFH increases monotonically from 2 to 7 Gyr as a function of SMA, while the luminosity-weighted mean stellar ages range from about 11 to 9 Gyr in a mild decrease, and the mean stellar metallicity Z drops from 0.017 to 0.014 as the SMA increases to r = 500''. In the range 40'' < r < 190'', the metallicity gradient Δ[Fe/H]/Δ[log r] = -0.04, while in the range 190'' < r < 400'', the metallicity gradient Δ[Fe/H]/Δ[log r] = -0.17.

The low-luminosity galaxy population in the NGC 5044 Group

We present multicolour imaging for 33 dwarf and intermediate-luminosity galaxies in the field of the NGC 5044 Group, complemented with mid-resolution spectroscopy for a subsample of 13 objects. With these data, a revised membership and morphological classification is made for the galaxies in the sample. We were able to confirm all but one of the "definite members" included in the spectroscopic subsample, which were originally classified based on morphological criteria; however, an important fraction of background galaxies is probably present among "likely" and "possible" members. The presence of a nucleus could be detected in just five out of the nine galaxies originally classified as dE,N, thus confirming the intrisic difficulty of photographic-plate morphological classification for this kind of object. Our deep surface photometry provided clear evidences for disc structure in at least three galaxies previously catalogued as dE or dS0. Their transition-type properties are also evident from the colour-magnitude diagram, where they lie near the late-type galaxies locus, suggesting an evolutionary connection between a parent disc-galaxy population and at least part of present-day dEs. Six new dSph candidates were also found, most of them at small projected distances from NGC 5044, the central galaxy of the Group. The NGC 5044 Group appears clearly defined in redshift space, with a mean heliocentric radial velocity, <v_r>=2461 +/- 84 km/s (z=0.0082), and a moderate dispersion, \sigma_{v_r}=431 km/s. Our data show no luminosity segregation for early-type galaxies: both dwarf and bright E/S0 systems show very similar velocity distributions (\sigma_{v_r} ~ 290 km/s), in contrast to late-type galaxies that seem to display a broader distribution (\sigma_{v_r} ~ 680 km/s).

Spiral galaxies observed in the near-infraredK band

Deep surface photometry in the K band was obtained for 54 normal spiral galaxies, with the aim of quantifying the percentage of faint bars and studying the morphology of spiral arms. The sample was chosen to cover a wider range of morphological types while inclination angles and distances were limited to allow a detailed investigation of the internal structure of their disks and future observations and studies of the disk kinematics. An additional constraint for a well defined subsample was that no bar structure was seen on images in the visual bands. Accurate sky projection parameters were determined from the K maps comparing several different methods. The surface brightness distribution was decomposed into axisymmetric components while bars and spiral structures were analyzed using Fourier techniques. Bulges were best represented by a Sersic r l/n law with an index in the typical range of 1-2. The central surface brightness of the exponential disk and bulge-to-disk ratio only showed weak correlation with Hubble type. Indications of a central point source were found in many of the galaxies. An additional central, steep, exponential disk improved the fit for more than 80% of the galaxies suggesting that many of the bulges are oblate. Bars down to the detection level at a relative amplitude of 3% were detected in 26 of 30 galaxies in a subsample classified as ordinary SA spirals. This would correspond to only 5% of all spiral galaxies being non-barred at this level. In several cases, bars are significantly offset compared to the starting points of the main spiral pattern which indicates that bar and spiral have different pattern speeds. A small fraction (∼10%) of the sample has complex central structures consisting of several sets of bars, arcs or spirals. A majority of the galaxies (∼60%) displays a two-armed, grand-design spiral pattern in their inner parts which often breaks up into multiple arms in their outer regions. Phase shifts between the inner and outer patterns suggest in some cases that they belong to different spiral modes. The pitch angles of the main two-armed symmetric spiral pattern in the galaxies have a typical range of 5-30°. The sample shows a lack of strong, tight spirals which could indicate that such patterns are damped by non-linear, dynamical effects due to their high radial force perturbations.

Spiral arms in near-infrared bands

We investigate the contribution of Br and H2 emission due to young objects in the arms of spiral galaxies observed in the K 0 lter. Out of a sample of disk galaxies for which we obtained deep surface photometry in broad- and narrow-band near{infrared lters, we selected two grand design spirals (NGC 5861, NGC 7412), which clearly have sharp knots along their arms both in optical and NIR images. For these galaxies we estimate the amount of light coming from Br and H2 emission and we conclude that it represents only a few percent of the observed K 0 light. For comparison we used the spiral galaxy NGC 4603, which has high recessional velocity. In this case the emission lines we study are practically shifted out of the narrow{band lter. Comparing its flux with what we found in the two former cases, we conclude that a major contribution from young objects in K 0 comes from continuum radiation which in the arm regions can amount to 20%.

Sky Subtraction for Deep Surface Photometry of the Intergalactic Light from Clusters of Galaxies11Based on observations obtained at the European Southern Observatory, La Silla, Chile.

ABSTRACTA new observing procedure and method of analysis of CCD observations are presented. The observing procedure maximizes the on‐source time without the need for observing separate blank sky fields. It is shown that a substantial improvement in the correction of sky and detector systematics can be achieved by explicitly separating the sky subtraction from the calibration of the spatial gain variations of the CCD, or flat‐fielding. When we perform the sky subtraction before flat‐fielding, the requirements on the latter are not as stringent. We test this method with deep I‐band images obtained under extremely challenging observing conditions. Because of the presence of illumination gradients due to moonlight, a flat field with an accuracy of better than 10% could not be obtained. Nevertheless, a sky subtraction with an accuracy of approximately 0.1% of the sky brightness was achieved over the whole mosaic.

Deep CCD Surface Photometry of the Edge-on Spiral NGC 4244

We have obtained deep surface photometry of the edge-on spiral galaxy NGC 4244. Our data reliably reach 27.5 R magnitude arcsec^{-2}, a significant improvement on our earlier deep CCD surface photometry of other galaxies. NGC 4244 is a nearby Scd galaxy whose total luminosity is approximately one magnitude fainter than the peak of the Sc luminosity function. We find that it has a simple structure: a single exponential disk, with a scale height h_Z = 246 +/- 2 pc, a scale length h_R = 1.84 +/- 0.02 kpc and a disk cutoff at a radius R(max) = 10.0 kpc (5.4 scale lengths). We confirm a strong cutoff in the stellar disk at R(max), which happens over only 1 kpc. We do not see any statistically significant evidence for disk flaring with radius. Unlike the more luminous Sc galaxies NGC 5907 and M 33, NGC 4244 does not show any evidence for a second component, such as a thick disk or halo, at mu(R) < 27.5 magnitude arcsec^{-2}.

HI, HII, and R-Band Observations of a Galactic Merger Sequence

bWe present high-quality aperture synthesis observations of the neutral hydrogen distribution in a sample of five galactic systems believed to be involved in progressive stages of merging: Arp 295, NGC 4676, NGC 520, NGC 3921, and NGC 7252. This is the first time that the atomic hydrogen in such a broad range of disk mergers has been imaged. These data are supplemented by wide-field images taken through a narrowband Hα filter, and by deep (μ_R_ &gt; 26.5 mag arcsec^-2^) R-band surface photometry. We identify several trends along the merging sequence. In the early stages, large amounts of HI still exist within the galactic disks and star formation is widespread. The ionized gas emission often takes on the appearance of plumes and arcs emanating from the nuclear regions, which are presumably the sites of interaction-induced starbursts. In the final stages there is little if any H I within the remnant bodies, and tidal material is seen moving inward. We conclude that as the merger rearranges the light profiles of the progenitor disk galaxies into r^1/4^ profiles, it leads to an efficient conversion of the atomic gas into other forms within the main bodies of the merger remnants. This suggests that these remnants will evolve into elliptical galaxies in their atomic gas contents as well as their photometric properties. However, the observations of NGC 520 reveal an extensive rotating gaseous disk, suggesting that perhaps some mergers will not destroy the atomic gas disks of the progenitors. The morphological similarity between the gaseous and stellar tails and the smooth gas kinematics confirm that gravity plays the dominant role in producing them. There are, however, some striking differences between the faint gas and light distributions. H I mapping often reveals gaseous extension not at all apparent optically, and tidal features of different optical morphologies have different gas characteristics: The edge-brightened tails are gas rich, while the featureless plumes and halos are gas poor. Some of these features may be explained by the different velocity dispersions of the gas and stars, some by different gas contents in the progenitors, and some remain unexplained. Overall, large quantities of both gas and starlight (M_H I_ &gt; 10^9^h^-2^ M_sun_, L_R_ &gt; 10^9^h^-2^ L_sun_) are seen at large radii (r &gt; 50 h^-1^ kpc). Since this material evolves on very long time scales, it may leave observable signatures for many Gyr.