HIERARCHICAL STRUCTURE FORMATION AND MODES OF STAR FORMATION IN HICKSONCOMPACT GROUP 31

摘要

The handful of low-mass, late-type galaxies that comprise Hickson Compact Group 31 (HCG 31) is in the midstof complex, ongoing gravitational interactions, evocative of the process of hierarchical structure formation athigher redshifts. With sensitive, multicolor Hubble Space Telescope imaging, we characterize the large populationof < 10 Myr old star clusters (SCs) that suffuse the system. From the colors and luminosities of the youngSCs, we find that the galaxies in HCG 31 follow the same universal scaling relations as actively star-forminggalaxies in the local universe despite the unusual compact group environment. Furthermore, the specific frequencyof 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 maingalaxies and the tidal-dwarf candidate, F, stellar complexes, which are sensitive to the magnitude of diskturbulence, have both sizes and masses more characteristic of z = 1-2 galaxies. After subtracting the lightfrom compact sources, we find no evidence for an underlying old stellar population in F—it appears to be atruly new structure. The low-velocity dispersion of the system components, available reservoir of H I, and currentstar formation rate of ~10 M⊙ yr~(-1)indicate that HCG 31 is likely to both exhaust its cold gas supply andmerge within -1 Gyr. We conclude that the end product will be an isolated, X-ray-faint, low-mass elliptical.