Tracing Ram-Pressure Stripping with Warm Molecular Hydrogen Emission
  We use the Spitzer Infrared Spectrograph (IRS) to study four infalling
cluster galaxies with signatures of on-going ram-pressure stripping. H$_2$
emission is detected in all four; two show extraplanar H$_2$ emission. The
emission usually has a warm (T $\sim$ $115 - 160$K) and a hot (T $\sim$ 400 $-$
600K) component that is approximately two orders of magnitude less massive than
the warm one. The warm component column densities are typically $10^{19} -
10^{20}$ cm$^{-2}$ with masses of $10^6 - 10^8 M_\odot$. The warm H$_2$ is
anomalously bright compared with normal star-forming galaxies and therefore may
be excited by ram-pressure. In the case of CGCG 97-073, the H$_2$ is offset
from the majority of star formation along the direction of the galaxy's motion
in the cluster, suggesting it is forming in the ram-pressure wake of the
galaxy. Another galaxy, NGC 4522, exhibits a warm H$_2$ tail approximately 4
kpc in length. These results support the hypothesis that H$_2$ within these
galaxies is shock-heated from the interaction with the intracluster medium.
Stripping of dust is also a common feature of the galaxies. For NGC 4522, where
the distribution of dust at 8 $\mu$m is well resolved, knots and ripples
demonstrate the turbulent nature of the stripping process. The H$\alpha$ and 24
$\mu$m luminosities show that most of the galaxies have star formation rates
comparable to similar mass counterparts in the field. Finally, we suggest a
possible evolutionary sequence primarily related to the strength of
ram-pressure a galaxy experiences to explain the varied results observed in our
sample.
