Volume 547, November 2012
|Number of page(s)||27|
|Published online||24 October 2012|
A dynamical model for the Taffy galaxies UGC 12914/5⋆
1 CDS, Observatoire astronomique, UMR 7550, 11 rue de l’université, 67000 Strasbourg, France
2 Univ. Bordeaux, Laboratoire d’Astrophysique de Bordeaux, UMR 5804, 33270 Floirac, France
3 CNRS, LAB, UMR 5804, 33270 Floirac, France
4 Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
Received: 24 May 2012
Accepted: 20 September 2012
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-1) 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 × 109 M⊙, with a molecular fraction MH2/MHI of about unity. Despite the enormous mass of molecular gas in the bridge, very little star formation is present, similar to other systems with extraplanar gas and broad CO lines. The structure of the model gas bridge is bimodal: on kpc-scales there is a dense (≳0.01 M⊙ pc-3) component with a high velocity dispersion >100 km s-1 and a less dense (~10-3 M⊙ pc-3) component with a smaller, but still high, velocity dispersion ~50 km s-1. The synchrotron lifetime of relativistic electrons is only long enough to be consistent with the existence of the radio continuum bridge for the less dense component. On the other hand, only the high-density gas undergoes a high enough mechanical energy input to produce the observed strong emission of warm H2. We propose that, despite the high local gas densities, this high input of mechanical energy drives strong turbulence and quenches star formation in the bridge gas except for the giant Hii region near UGC 12915. Our model suggests that we observe this galaxy head-on collision near the time of maximum CO and H2 emission.
Key words: galaxies: interactions / galaxies: ISM / galaxies: kinematics and dynamics
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2012
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