  Spitzer space telescope observations led to the surprising detection of a
diverse set of extragalactic sources whose infrared spectra are dominated by
line emission of molecular hydrogen (H2). The absence or relative weakness of
typical signs of star formation (like dust features, lines of ionized gas)
suggest the presence of large quantities of H2 gas with no (or very little)
associated star formation. We use the Stephan's Quintet (SQ) galaxy collision
to define a physical framework to describe the H2 formation and emission in
galaxy-wide shocks. SQ observations show that exceptionally turbulent H2 gas is
coexisting with a hot, X-ray emitting plasma. The extreme mid-IR H2 emission
from the shock exceeds that of the X-rays. These observations set a new light
on the contribution of H2 to the cooling of the interstellar medium, on the
relation between molecular gas and star formation, and on the energetics of
galaxy formation.
  These observations can be interpreted by considering that the shock is moving
through an inhomogeneous medium. They suggest that most of the shock energy is
transferred to bulk kinetic energy of the H2 gas. The turbulent energy of the
post-shock gas drives a mass cycle across the different gas phases where H2 is
forming out of the hot/warm gas. This interpretation puts the H2 emission into
a broader context including optical and X-ray observations. We propose that the
turbulence in the clouds is powered by a slow energy and momentum transfer from
the bulk motion of the gas and that the dissipation of this turbulent energy in
turn is powering the H2 emission.
