Enhanced Warm H2 Emission in the Compact Group Mid-Infrared "Green
  Valley"
  We present results from a Spitzer, mid-infrared spectroscopy study of a
sample of 74 galaxies located in 23 Hickson Compact Groups, chosen to be at a
dynamically-active stage of HI depletion. We find evidence for enhanced warm H2
emission (i.e. above that associated with UV excitation in star-forming
regions) in 14 galaxies (~20%), with 8 galaxies having extreme values of L(H2
S(0)-S(3))/L(7.7micron PAH), in excess of 0.07. Such emission has been seen
previously in the compact group HCG 92 (Stephan's Quintet), and was shown to be
associated with the dissipation of mechanical energy associated with a
large-scale shock caused when one group member collided, at high velocity, with
tidal debris in the intragroup medium. Similarly, shock excitation or turbulent
heating is likely responsible for the enhanced H2 emission in the compact group
galaxies, since other sources of heating (UV or X-ray excitation from star
formation or AGN) are insufficient to account for the observed emission. The
group galaxies fall predominantly in a region of mid-infrared color-color space
identified by previous studies as being connected to rapid transformations in
HCG galaxy evolution. Furthermore, the majority of H2-enhanced galaxies lie in
the optical "green valley" between the blue cloud and red-sequence, and are
primarily early-type disk systems. We suggest that H2-enhanced systems may
represent a specific phase in the evolution of galaxies in dense environments
and provide new insight into mechanisms which transform galaxies onto the
optical red sequence.
