A dynamical model for the Taffy galaxies UGC 12914/5

Abstract

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 H 2 ) to be 5 to 6× 10 9 M , with a molecular fraction M H 2 /M HI 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 H 2 . 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 H 2 emission.

X-Ray Emission From the Taffy (Vv254) Galaxies and Bridge

“…These values 27 , which are comparable with, or greater than the age of the universe, suggest very inefficient star formation in the bridge (the star formation efficiency is the inverse of t del ). A similar conclusion, that the bridge exhibits unusually low star formation for its gas mass, was found by Vollmer et al (2012) using Spitzer data alone. Long depletion times have been found in a sample of warm H 2 -rich Hickson Compact Group galaxies (Alatalo et al 2015a), and in NGC 1266 (Alatalo et al 2015b).…”

“…It is striking that the CO (1-0) molecular gas in the Taffy bridge seems to be anticorrelated with the soft X-rays, whereas the H I, like the dust and [C II] emission, is better correlated. In the models of Vollmer et al (2012), the offset between the diffuse H I and the denser CO-emitting gas was explained in terms of the impact parameter in the collision. The X-ray emission seems to follow this trend, being more correlated with the diffuse components of the bridge than the denser colder molecular gas.…”

“…The large amounts of gas found in the bridge (∼7 × 10 9 M e , Zhu et al 2007;Peterson et al 2012;Vollmer et al 2012) has to survive the violence of the collision (Braine et al 2003). Nevertheless, in the gas and stellar dynamical modeling of the Taffy collision by Vollmer et al (2012), it was shown that momentum transfer between the gas clouds in the galaxies during the collision was sufficient to transport large quantities of gas into the bridge. The model also provided a possible explanation for the spatial offset in the distribution of the dense molecular gas, and the diffuse H I in the bridge (Gao et al 2003b).…”

Effects of environmental gas compression on the multiphase ISM and star formation

“…In most cases shear motions can be detected in the Hi velocity field (e.g., NGC 4535, Chung et al 2009). Therefore, a ridge of enhanced polarized emission located at the outer edge of the gas disk in the absence of a sheared velocity field is a clear sign of gas compression (Vollmer et al 2007(Vollmer et al , 2012(Vollmer et al , 2013. We identified such ridges in the Virgo spiral galaxies NGC 4501 and NGC 4568.…”

“…We used the N-body code described in Vollmer et al (2001), which consists of two components: a noncollisional component that simulates the stellar bulge/disk and the dark halo, and a collisional component that simulates the ISM. A scheme for star formation was implemented where stars were formed during cloud collisions and then evolved as noncollisional particles (see Vollmer et al 2012). The non-collisional component consists of 81 920 particles that simulate the galactic halo, bulge, and disk.…”