UV+IR Star Formation Rates: Hickson Compact Groups with Swift and
  Spitzer
  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)
