The environment where galaxies are found heavily influences their evolution. Close groupings, like the ones in the cores of galaxy clusters or compact groups, evolve in ways far more dramatic than their isolated counterparts. We have conducted a multi-wavelength study of Hickson Compact Group 7, consisting of four giant galaxies: three spirals and one lenticular. We use Hubble Space Telescope (HST ) imaging to identify and characterize the young and old star cluster populations. We find young massive clusters (YMC) mostly in the three spirals, while the lenticular features a large, unimodal population of globular clusters (GC) but no detectable clusters with ages less than a few Gyr. The spatial and approximate age distributions of the ∼ 300 YMCs and ∼ 150 GCs thus hint at a regular star formation history in the group over a Hubble time. While at first glance the HST data show the galaxies as undisturbed, our deep ground-based, wide-field imaging that extends the HST coverage reveals faint signatures of stellar material in the intra-group medium. We do not, however, detect the intra-group medium in H i or Chandra X-ray observations, signatures that would be expected to arise from major mergers. Despite this fact, we find that the H i gas content of the individual galaxies and the group as a whole are a third of the expected abundance. The appearance of quiescence is challenged by spectroscopy that reveals an intense ionization continuum in one galaxy nucleus, and post-burst characteristics in another. Our spectroscopic survey of dwarf galaxy members yields a single dwarf elliptical galaxy in an apparent stellar tidal feature. Based on all this information, we suggest an evolutionary scenario for HCG 7, whereby the galaxies convert most of their available gas into stars without the influence of major mergers and ultimately result in a dry merger. As the conditions governing compact groups are reminiscent of galaxies at intermediate redshift, we propose that HCGs are appropriate for studying galaxy evolution at z ∼ 1 − 2.

We have searched for compact stellar structures within 17 tidal tails in 13 different interacting galaxies using F606W -and F814W -band images from the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST ). The sample of tidal tails includes a diverse population of optical properties, merging galaxy mass ratios, Hi content, and ages. Combining our tail sample with Knierman et al. (2003), we find evidence of star clusters formed in situ with M V < -8.5 and V -I < 2.0 in 10 of 23 tidal tails; we are able to identify cluster candidates to M V = -6.5 in the closest tails. Three tails offer clear examples of "beads on a string" star formation morphology in V -I color maps. Two tails present both tidal dwarf galaxy (TDG) candidates and cluster candidates. Statistical diagnostics indicate that clusters in tidal tails may be drawn from the same power-law luminosity functions (with logarithmic slopes ≈ -2 --2.5) found in quiescent spiral galaxies and the interiors of interacting systems. We find that the tail regions with the largest number of observable clusters are relatively young ( 250 Myr old) and bright (V 24 mag arcsec −2 ), probably attributed to the strong bursts of star formation in interacting systems soon after periapse. Otherwise, we find no statistical difference between cluster-rich and cluster-poor tails in terms of many observable characteristics, though this analysis suffers from complex, unresolved gas dynamics and projection effects.

Superstar clusters are young, compact star clusters found in the central regions of interacting galaxies. Recently, they have also been reported to preferentially form in certain tidal tails, but not in others. In this paper, we have used 21‐cm H i maps and the Hubble Space Telescope Wide Field Planetary Camera 2 images of eight tidal tail regions of four merging galaxy pairs to compare the kiloparsec scale H i distribution with the location of superstar clusters found from the optical images. For most of the tails, we find that there is an increase in superstar cluster density with increasing projected H i column density, such that the star cluster density is highest when log NH i≳ 20.6 cm−2, but equal to the background count rate at lower H i column density. However, for two tails (NGC 4038/39 Pos A and 3921), there is no significant star cluster population despite the presence of gas at high column density. This implies that the NH i threshold is a necessary but not sufficient condition for cluster formation. Gas volume density is likely to provide a more direct criterion for cluster formation, and other factors such as gas pressure or strength of encounter may also have an influence. Comparison of H i thresholds needed for formation of different types of stellar structures await higher resolution H i and optical observations of larger numbers of interacting galaxies.

The handful of low-mass, late-type galaxies that comprise Hickson Compact Group 31 is in the midst of complex, ongoing gravitational interactions, evocative of the process of hierarchical structure formation at higher redshifts. With sensitive, multicolor Hubble Space Telescope imaging, we characterize the large population of < 10 Myr old star clusters that suffuse the system. From the colors and luminosities of the young star clusters, we find that the galaxies in HCG 31 follow the same universal scaling relations as actively star-forming galaxies in the local Universe despite the unusual compact group environment. Furthermore, the specific frequency of the globular cluster system is consistent with the low end of galaxies of comparable masses locally. This, combined with the large mass of neutral hydrogen and tight constraints on the amount of intragroup light, indicate that the group is undergoing its first epoch of interaction-induced star formation. In both the main galaxies and the tidal-dwarf candidate, F, stellar complexes, which are sensitive to the magnitude of disk turbulence, have both sizes and masses more characteristic of z = 1-2 galaxies. After subtracting the light from compact sources, we find no evidence for an underlying old stellar population in F -it appears to be a truly new structure. The low velocity dispersion of the system components, available reservoir of H i, and current star formation rate of ∼10 M ⊙ yr −1 , indicate that HCG 31 is likely to both exhaust its cold gas supply and merge within ∼ 1 Gyr. We conclude that the end product will be an isolated, X-ray-faint, low-mass elliptical.

We measure the non-axisymmetry in the luminosity distribution in the inner few kpc of the remnants of advanced mergers of galaxies with a view to understand the relaxation in the central regions. For this, we analyse the images from the Two-Micron All-Sky Survey (2MASS) archival data for a selected sample of 12 merging galaxies, which show signs of interaction but have a single nucleus. The central regions are fitted by elliptical isophotes whose centres are allowed to vary to get the best fit. The centres of isophotes show a striking sloshing pattern with a spatial variation of up to 20-30 per cent within the central 1 kpc. This indicates mass asymmetry and a dynamically unrelaxed behaviour in the central region. Next, we Fourieranalyse the galaxy images while keeping the centre constant and measure the deviation from axisymmetry in terms of the fractional Fourier amplitudes (A 1 , A 2 , etc.) as a function of radius. All the mergers show a high value of lopsidedness (up to A 1 ∼ 0.2) in the central 5 kpc. The m = 2 asymmetry is even stronger, with values of A 2 up to ∼0.3, and in three cases these are shown to represent bars. The corresponding values denoting non-axisymmetry in inner regions of a control sample of eight non-merger galaxies are found to be several times smaller.Surprisingly, this central asymmetry is seen even in mergers where the outer regions have relaxed into a smooth elliptical-like r 1/4 profile or a spiral-like exponential profile. Thus, the central asymmetry is long lived, estimated to be ∼1 Gyr, and hence lasts for over 100 local dynamical time-scales. These central asymmetries are expected to play a key role in the future dynamical evolution of the central region of a merger, and can help in feeding of a central active galactic nucleus.