A dynamical model for the Taffy galaxies UGC 12914/5
  The spectacular head-on collision of the two gas-rich galaxies of the Taffy
system, UGC 12914/15, gives us a unique opportunity to study the consequences
of a direct ISM-ISM collision. To interpret existing multi-wavelength
observations, we made dynamical simulations of the Taffy system including a
sticky particle component. To compare simulation snapshots to HI and CO
observations, we assume that the molecular fraction of the gas depends on the
square root of the gas volume density. For the comparison of our simulations
with observations of polarized radio continuum emission, we calculated the
evolution of the 3D large-scale magnetic field for our simulations. The
induction equations including the time-dependent gas-velocity fields from the
dynamical model were solved for this purpose. Our simulations reproduce the
stellar distribution of the primary galaxy, UGC 12914, the prominent HI and CO
gas bridge, the offset between the CO and HI emission in the bridge, the bridge
isovelocity vectors parallel to the bridge, the HI double-line profiles in the
bridge region, the large line-widths (~200 km/s) in the bridge region, the high
field strength of the bridge large-scale regular magnetic field, the projected
magnetic field vectors parallel to the bridge and the strong total power radio
continuum emission from the bridge. The stellar distribution of the secondary
model galaxy is more perturbed than observed. The observed distortion of the HI
envelope of the Taffy system is not reproduced by our simulations which use
initially symmetric gas disks. The model allows us to define the bridge region
in three dimensions. We estimate the total bridge gas mass (HI, warm and cold
H2) to be 5 to 6 10^9 M_sun, with a molecular fraction M_H2/M_HI of about unity
(abrigded).
