ULTRAVIOLET+INFRARED STAR FORMATION RATES: HICKSON COMPACT GROUPS WITH<i>SWIFT</i>AND<i>SPITZER</i>

Abstract

We present Swift UVOT (1600-3000A) 3-band photometry for 41 galaxies in 11\nnearby (&lt;4500km/s) Hickson Compact Groups (HCGs) of galaxies. We use the\nuvw2-band (2000A) to estimate the dust-unobscured component, SFR_UV, of the\ntotal star-formation rate, SFR_T. We use Spitzer MIPS 24-micron photometry to\nestimate SFR_IR, the dust-obscured component of SFR_T. We obtain\nSFR_T=SFR_UV+SFR_IR. Using 2MASS K_s band based stellar mass, M*, estimates, we\ncalculate specific SFRs, SSFR=SFR_T/M*. SSFR values show a clear and\nsignificant bimodality, with a gap between low (&lt;~3.2x10^-11 / yr) and high\nSSFR (&gt;~1.2x10^-10 / yr) systems. All galaxies with MIR activity index a_IRAC\n&lt;= 0 (&gt;0) are in the high- (low-) SSFR locus, as expected if high levels of\nstar-formation power MIR emission from polycyclic aromatic hydrocarbon\nmolecules and a hot dust continuum. All elliptical/S0 galaxies are in the\nlow-SSFR locus, while 22 out of 24 spirals/irregulars are in the high-SSFR\nlocus, with two borderline cases. We divide our sample into three subsamples\n(I, II and III) according to decreasing HI-richness of the parent galaxy group\nto which a galaxy belongs. Consistent with the SSFR and a_IRAC bimodality, 12\nout of 15 type-I (11 out of 12 type-III) galaxies are in the high- (low-) SSFR\nlocus, while type II galaxies span almost the full range of SSFR values. Unlike\nHCG galaxies, galaxies in a comparison quiescent SINGS sub-sample are\ncontinuously distributed both in SSFR and a_IRAC. Any uncertainties can only\nfurther enhance the SSFR bimodality. These results suggest that an environment\ncharacterized by high galaxy number-densities and low galaxy\nvelocity-dispersions, such as the one found in compact groups, plays a key role\nin accelerating galaxy evolution by enhancing star-formation processes in\ngalaxies and favoring a fast transition to quiescence.(abridged)\n