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Register a new userUV+IR Star Formation Rates: Hickson Compact Groups with Swift and Spitzer
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Added by
Panayiotis Tzanavaris
Publication date
2010
fields
Physics
and research's language is
English
Authors
P. Tzanavaris
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We present Swift UVOT (1600-3000A) 3-band photometry for 41 galaxies in 11 nearby (<4500km/s) Hickson Compact Groups (HCGs) of galaxies. We use the uvw2-band (2000A) to estimate the dust-unobscured component, SFR_UV, of the total star-formation rate, SFR_T. We use Spitzer MIPS 24-micron photometry to estimate SFR_IR, the dust-obscured component of SFR_T. We obtain SFR_T=SFR_UV+SFR_IR. Using 2MASS K_s band based stellar mass, M*, estimates, we calculate specific SFRs, SSFR=SFR_T/M*. SSFR values show a clear and significant bimodality, with a gap between low (<~3.2x10^-11 / yr) and high SSFR (>~1.2x10^-10 / yr) systems. All galaxies with MIR activity index a_IRAC <= 0 (>0) are in the high- (low-) SSFR locus, as expected if high levels of star-formation power MIR emission from polycyclic aromatic hydrocarbon molecules and a hot dust continuum. All elliptical/S0 galaxies are in the low-SSFR locus, while 22 out of 24 spirals/irregulars are in the high-SSFR locus, with two borderline cases. We divide our sample into three subsamples (I, II and III) according to decreasing HI-richness of the parent galaxy group to which a galaxy belongs. Consistent with the SSFR and a_IRAC bimodality, 12 out of 15 type-I (11 out of 12 type-III) galaxies are in the high- (low-) SSFR locus, while type II galaxies span almost the full range of SSFR values. Unlike HCG galaxies, galaxies in a comparison quiescent SINGS sub-sample are continuously distributed both in SSFR and a_IRAC. Any uncertainties can only further enhance the SSFR bimodality. These results suggest that an environment characterized by high galaxy number-densities and low galaxy velocity-dispersions, such as the one found in compact groups, plays a key role in accelerating galaxy evolution by enhancing star-formation processes in galaxies and favoring a fast transition to quiescence.(abridged)
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The formation of ultra-compact dwarf galaxies (UCDs) is believed to be interaction driven, and UCDs are abundant in the cores of galaxy clusters, environments that mark the end-point of galaxy evolution. Nothing is known about the properties of UCDs in compact groups of galaxies, environments where most of galaxy evolution and interaction is believed to occur and where UCDs in intermediate state of evolution may be expected. The main goal of this study is to detect and characterize, for the first time, the UCD population of compact groups. For that, 2 groups in different evolutionary stages, HCG 22 and HCG 90, were targeted with VLT/FORS2/MXU. We detect 16 and 5 objects belonging to HCG 22 and HCG 90, respectively, covering the magnitude range -10.0 > M_R > -11.5 mag. Their colours are consistent with old ages covering a broad range in metallicities. Photometric mass estimates put 4 objects in HCG 90 and 9 in HCG 22 in the mass range of UCDs (>2x10^6 M_Sun) for an assumed age of 12 Gyr. These UCDs are on average 2-3 times larger than typical Galactic GCs, covering a range of 2 >~ r_h >~ 21 pc. The UCDs in HCG 22 are more concentrated around the central galaxy than in HCG 90, at the 99% confidence level. They cover a broad range in [alpha/Fe] abundances from sub- to super-solar. The spectra of 3 UCDs show tentative evidence for intermediate age stellar populations. We calculate the specific frequency (S_N) of UCDs for both groups, finding that HCG 22 has about three times higher S_N than HCG 90. The ensemble properties of the detected UCDs supports 2 co-existing formation channels: a star cluster origin and an origin as tidally stripped dwarf nuclei. Our results imply that the UCDs detected in both groups do not, in their majority, originate from relatively recent galaxy interactions. Most of the detected UCDs have likely been brought into the group with their host galaxies.[abridged]
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.
We present deep H{alpha} imaging of seven Hickson Compact Groups (HCGs) using the 4.1m Southern Astrophysics Research (SOAR) Telescope. The high spatial resolution of the observations allow us to study both the integrated star-formation properties of the main galaxies as well as the 2D distribution of star-forming knots in the faint tidal arms that form during interactions between the individual galaxies. We derive star-formation rates and stellar masses for group members and discuss their position relative to the main sequence of star-forming galaxies. Despite the existence of tidal features within the galaxy groups, we do not find any indication for enhanced star-formation in the selected sample of HCGs. We study azimuthally averaged H{alpha} profiles of the galaxy disks and compare them with the g and r surface-brightness profiles. We do not find any truncated galaxy disks but reveal that more massive galaxies show a higher light concentration in H{alpha} than less massive ones. We also see that galaxies that show a high light concentration in r, show a systematic higher light concentration in H{alpha}. TDG candidates have been previously detected in R-band images for 2 groups in our sample but we find that most of them are likely background objects as they do not show any emission in H{alpha}. We present a new tidal dwarf galaxy (TDG) candidate at the tip of the tidal tail in HCG 91.
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.
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