Low star formation efficiency due to turbulent adiabatic compression in
  the Taffy bridge
  The Taffy system (UGC 12914/15) consists of two massive spiral galaxies which
had a head-on collision about 20 Myr ago. New sensitive, high-resolution
CO(1-0) observations of the Taffy system with the IRAM PdBI are presented.
About 25% of the total interferometric CO luminosity stems from the bridge
region. Assuming a Galactic N(H2)/ICO conversion factor for the galactic disks
and a third of this value for the bridge gas, about 10% of the molecular gas
mass is located in the bridge region. The giant HII region close to UGC 12915
is located at the northern edge of the high-surface brightness giant molecular
cloud association (GMA), which has the highest velocity dispersion among the
bridge GMAs. The bridge GMAs are clearly not virialized because of their high
velocity dispersion. Three dynamical models are presented and while no single
model reproduces all of the observed features, they are all present in at least
one of the models. Most of the bridge gas detected in CO does not form stars.
We suggest that turbulent adiabatic compression is responsible for the
exceptionally high velocity dispersion of the molecular ISM and the suppression
of star formation in the Taffy bridge. In this scenario the turbulent velocity
dispersion of the largest eddies and turbulent substructures/clouds increase
such that giant molecular clouds are no longer in global virial equilibrium.
The increase of the virial parameter leads to a decrease of the star formation
efficiency. Most of the low-surface density, CO-emitting gas will disperse
without forming stars but some of the high-density gas will probably collapse
and form dense star clusters, such as the luminous HII region close to UGC
12915. We suggest that globular clusters and super star clusters formed and
still form through the gravitational collapse of gas previously compressed by
turbulent adiabatic compression during galaxy interactions.
