Studying the evolution of galaxies in compact groups over the past 3 Gyr. I. The nuclear activity

We present the first -- of a series -- study of the evolution of galaxies in compact groups over the past 3 Gyr. This paper focuses on the evolution of the nuclear activity and how it has been affected by the dense environment of the groups. Our analysis is based on the largest multiwavelength compact group sample to-date, containing complete ultraviolet-to-infrared (UV-to-IR) photometry for 1,770 isolated groups (7,417 galaxies). We classified the nuclear activity of the galaxies based on optical emission line and mid-infrared diagnostic methods, as well as using spectral energy distribution fitting. We observe a 15% increase on the number of the AGN-hosting late-type galaxies found in dynamically old groups, over the past 3 Gyr, accompanied by the corresponding decrease of their circumnuclear star formation. Comparing our compact group results with those of local isolated field and interacting pair galaxies, we find no differences in the AGN at the same redshift range. Based on both optical and mid-IR colour classifications, we report the absence of Seyfert 1 nuclei and we attribute this to the low accretion rates, caused by the depletion of gas. We propose that the observed increase of LINER and Seyfert 2 nuclei (at low-zs), in the early-type galaxies of the dynamically young groups, is due to the morphological transformation of lenticular into elliptical galaxies. Finally, we show that at any given stellar mass, galaxies found in dynamically old groups are more likely to host an AGN. Our findings suggest that the depletion of gas, due to past star formation and tidal stripping, is the major mechanism driving the evolution of the nuclear activity in compact groups of galaxies.

Herschel observations of Hickson compact groups of galaxies: Unveiling the properties of cold dust

We present a Herschel far-IR and sub-mm study of a sample of 120 galaxies in 28 Hickson Compact Groups. Fitting their UV to sub-mm spectral energy distributions with the model of da Cunha et al. (2008), we accurately estimate the dust masses, luminosities and temperatures of the individual galaxies. We find that nearly half of the late-type galaxies in dynamically old groups, those with more than 25% of early-type members and redder UV-optical colours, have also significantly lower dust-to-stellar mass ratios compared to those of actively star-forming galaxies of the same mass found both in HCGs and the field. Examining their dust-to-gas mass ratios we conclude that dust was stripped out of these systems as a result of the gravitational and hydrodynamic interactions, experienced due to previous encounters with other group members. About 40% of the early-type galaxies (mostly lenticulars), in dynamically old groups, display dust properties similar to those of the UV-optical red late-type galaxies. Given their stellar masses, star formation rates and UV-optical colours, we suggest that red late-type and dusty lenticular galaxies represent transition populations between blue star-forming disk galaxies and quiescent early-type ellipticals. [...ABRIDGED...] Our deep Herschel observations also allow us to detect the presence of diffuse cold intragroup dust in 4 HCGs. We also find that the fraction of 250micron emission which is located outside of the main bodies of the red late-type galaxies as well as of the dusty lenticulars is 15-20% of their integrated emission at this band. All these findings are consistent with an evolutionary scenario in which gas dissipation, shocks and turbulence in addition to tidal interactions, shape the evolution of galaxies in compact groups.

A mid-IR study of Hickson Compact Groups II. Multi-wavelength analysis of the complete GALEX-Spitzer Sample

We present a comprehensive study on the impact of the environment of compact galaxy groups on the evolution of their members using a multi-wavelength analysis, from the UV to the infrared, for a sample of 32 Hickson compact groups (HCGs) containing 135 galaxies. Fitting the SEDs of all galaxies with the state-of-the-art model of da Cunha (2008) we can accurately calculate their mass, SFR, and extinction, as well as estimate their infrared luminosity and dust content. We compare our findings with samples of field galaxies, early-stage interacting pairs, and cluster galaxies with similar data. We find that classifying the groups as dynamically old or young, depending on whether or not at least one quarter of their members are early-type systems, is physical and consistent with past classifications of HCGs based on their atomic gas content. [...ABRIDGED...] We also examine their SF properties, UV-optical and mid-IR colors, and we conclude that all the evidence point to an evolutionary scenario in which the effects of the group environment and the properties of the galaxy members are not instantaneous. Early on, the influence of close companions to group galaxies is similar to the one of galaxy pairs in the field. However, as the time progresses, the effects of tidal torques and minor merging, shape the morphology and star formation history of the group galaxies, leading to an increase of the fraction of early-type members and a rapid built up of the stellar mass in the remaining late-type galaxies.

A mid-IR study of Hickson Compact Groups I : Probing the Effects of Environment in Galaxy Interactions

Hickson Compact Groups (HCGs) are among the densest galaxy environments of the local universe. To examine the effects of the environment on the infrared properties of these systems, we present an analysis of Spitzer and ISO mid-infrared imaging as well as deep ground based near-infrared imaging of 14 HCGs containing a total of 69 galaxies. Based on mid-infrared color diagnostics we identify the galaxies which appear to host an active nucleus, while using a suite of templates, we fit the complete infrared spectral energy distribution for each group member. We compare our estimates of galaxy mass, star formation rate, total infrared luminosities, and specific star formation rates (sSFR) for our HCG sample, to samples of isolated galaxies and interacting pairs and find that overall there is no discernible difference among them. However, HCGs which can be considered as dynamically old, host late-type galaxies with a slightly lower sSFR than the one found in dynamically young groups. This could be attributed to multiple past interactions among the galaxies in old groups, that have led to the build up of their stellar mass. It is also consistent with our prediction for the presence of diffuse cold dust in the intergalactic medium of 9 of the dynamically old groups.