Galaxy Evolution in a Complex Environment: A Multi-Wavelength Study of HCG 7

Abstract

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.

Intragroup and Galaxy-Linked Diffuse X-Ray Emission in Hickson Compact Groups

“…Of the detected groups, diffuse emission in HCG 31 has not been reported in previous studies. Both of the two groups without detections, HCGs 7 10 and 22, have low to negligible star formation without much evidence of strong tidal interactions in the past few Gyr (e.g., Konstantopoulos et al 2010). In the detected groups, the temperatures are all fairly similar (0.6-0.72 keV, not including the hot component of HCG 62 or the anomalously low temperature of HCG 59, which is very poorly constrained), while the range in X-ray luminosities spans 10 40.37 -10 42.18 erg s −1 .…”

“…The most recent H i masses were taken from Verdes-Montenegro et al (2001) and Borthakur et al (2010). e Defined by Johnson et al (2007) using the ratio of the group H i mass to its dynamical mass, with subtypes A and B qualitatively assessing the location of the H i gas (localized to the group members or spread throughout the group, respectively; Konstantopoulos et al 2010).…”

“…12 Subtypes based on the morphology of the H i gas, which we qualitatively assessed from Very Large Array (VLA) observations, are included as part of the proposed evolutionary sequence of CGs (Konstantopoulos et al 2010). Type A groups are those in which the neutral gas is confined to the individual group members, while type B groups have H i gas distributed between the galaxies and not centered on any particular member(s).…”

“…Hypotheses concerning the continued existence of CGs have included that these systems are either recently formed (e.g., Mendes de Oliveira et al 2005), that they are chance alignments within larger structures (e.g., Mamon 1986), or that CGs are perpetually formed within collapsing rich groups (e.g., Diaferio et al 1994). However, all three proposed scenarios have evidence that contradicts them, such as the wide range in ages of star clusters in CGs (Konstantopoulos et al 2010(Konstantopoulos et al , 2012, the low surface brightness tidal debris and disturbed disk morphologies present in many groups (e.g., HCGs 16, 31, and 92;Mendes de Oliveira et al 1998;Gallagher et al 2010;Fedotov et al 2011), and indications that a small fraction of CG members are the result of mergers (Mendes de Oliveira et al 2005). Clearly these systems are much more complicated than first suspected, and understanding the origin and physical processes responsible for maintaining CGs over timescales in excess of several Gyr is important for refining cosmological simulations.…”

“…Previous papers have examined the star cluster populations in several HCGs (e.g., Gallagher et al 2001;Konstantopoulos et al 2010;Fedotov et al 2011) to study the consumption of gas by star formation; however, there has been no investigation of how the H i gas is heated to X-ray temperatures in these systems in the context of group evolution. In this work, as part of a multiwavelength campaign to study the evolution of both the galaxies in CGs and the group environment itself, we present the results of a study of the diffuse X-ray emission from nearby CGs (z 0.015) using Chandra observations.…”