Hierarchical Structure Formation and Modes of Star Formation in Hickson Compact Group 31

Abstract

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.

Distribution of Faint Atomic Gas in Hickson Compact Groups

“…Such structures have been appropriately termed as tidaldwarf galaxies (TDGs ;Zwicky 1956;Schweizer 1978;Mirabel et al 1992;Duc & Mirabel 1999). Like other interacting galaxies, HCGs have been found to host multiple star-forming TDGs and individual star clusters outside the parent galaxies (e.g., Hunsberger et al 1996;Gallagher et al 2001Gallagher et al , 2010Iglesias-Páramo & Vílchez 2001;Xu et al 2005;Torres-Flores et al 2009;de Mello et al 2012;Konstantopoulos et al 2013;Eigenthaler et al 2015;Fedotov et al 2015). The young stars produce large amounts of ionizing photons,and given the right conditions (such as strong stellar winds; Vogt et al 2013;Borthakur et al 2014), a fraction of the ionizing flux may escape the molecular cloud cocoon where the young stars are embedded.…”

“…The two tidal-debris-dominated groups (HCG31 and HCG 92) show strong evidence for the presence of young stellar populations of age 4-10 Myr (Xu et al 2005;Torres-Flores et al 2009;Gallagher et al 2010). Herewe explore the possibility of photoionization of the low column density H I by the escaping ionizing radiation from such young star-forming regions in the vicinity of the H I in the IGrM.…”

“…The high neutral column densities and pressures resulting from tidal dynamics have been suggested as a potential cause for inducing star formation in tidal debris (Mullan et al 2013). Several studies have found large numbers of young stellar clusters being formed in the IGrM (Hunsberger et al 1996;Torres-Flores et al 2009;Gallagher et al 2010, and references therein). For example, the intergalactic star-forming regions in Stephanʼs Quintet appear to be devoid of older ( 10 year 9 > ) stellar populations, and their spectral energy distributionresembles that of dusty starforming regions in the galactic disks rather than dwarf starforming galaxies (Boquien et al 2010).…”

CHARACTERIZATION OF OPTICALLY SELECTED STAR-FORMING KNOTS IN (U)LIRGs

“…During the last two decades, young and massive compact clusters have been extensively studied in different environments including starburst galaxies (Meurer et al 1995), barred galaxies (Barth et al 1995), spiral disks (Larsen & Richtler 1999), interacting galaxies (Whitmore et al 1999(Whitmore et al , 2007Bik et al 2003;Weilbacher et al 2000Weilbacher et al , 2003Knierman et al 2003;Peterson et al 2009;Mullan et al 2011), and compact groups of galaxies (Iglesias-Páramo & Vílchez 2001;Gallagher et al 2010;Konstantopoulos et al 2010).…”

“…Star-forming galaxies become increasingly irregular at higher redshifts, with a blue clumpy structure, asymmetry, and lack of central concentration van den Bergh et al 1996;Im et al 1999). It has been claimed that clumps in clumpy galaxies represent star-forming complexes intrinsically more massive by one or two orders of magnitude than similar-size complexes in local galaxies (Efremov 1995;Elmegreen et al 2009;Gallagher et al 2010). Although most of the clumpy galaxies, observed at z = 0.7-2, do not show signs of interactions, given the extreme star formation properties in local (U)LIRGs, it is relevant to study the properties of their clumpy structure in order to establish the similarities to and differences from high-z clumpy galaxies.…”