The Astrophysical Journal, 471: 115-142, 1996 November 1

Transformations of Galaxies. II. Gas
Dynamics in Merging Disk Galaxies

Joshua E. Barnes

Institute for Astronomy, University of Hawai`i,
2680 Woodlawn Drive, Honolulu, Hawai`i 96822

Lars Hernquist

Lick Observatory, University of California,
Santa Cruz, California 95064


In mergers of disk galaxies, gas plays a role quite out of proportion to its relatively modest contribution to the total mass. To study this behavior, we have included gas dynamics in self-consistent simulations of collisions between equal-mass disk galaxies. The large-scale dynamics of bridge- and tail-making, orbit decay, and merging are not much altered by the inclusion of a gaseous component. However, tidal forces during encounters cause otherwise stable disks to develop bars, and the gas in such barred disks, subjected to strong gravitational torques, flows toward the central regions where it may fuel the kpc-scale starbursts seen in some interacting disk systems. Similar torques on the gas during the final stages of a collision yield massive gas concentrations in the cores of merger remnants, which may be plausibly identified with the molecular complexes seen in objects such as NGC 520 and Arp 220. This result appears insensitive to the detailed microphysics of the gas, provided that radiative cooling is permitted. The inflowing gas can dramatically alter the stellar morphology of a merger remnant, apparently by deepening the potential well and thereby changing the boundaries between the major orbital families.


selected frames; MPEG movie

1. Encounter Overview. This sequence shows the time-evolution of encounter A, viewed along the orbital axis. Here dark halo matter is shown in red, bulge stars are shown in yellow, disk stars are shown in blue, and the gas is shown in green.

selected frames; MPEG movie

2. Gas Only. This sequence also views encounter A along the orbital axis, but at a larger scale. Moreover, here only the gas in the two galaxies is shown.

selected frames; MPEG movie

3. Disk Response. In this sequence the direct disk from encounter A is viewed along its spin axis, using the same color scheme as in Video 1.

selected frames; MPEG movie

4. Final Encounter. This sequence presents up a close-up of the latter part of Video 2, using the same color scheme.

selected frames; MPEG movie

5. Dwarf Formation. This sequence shows the formation of the most massive bound object in the tidal tails of encounter A. Here stars and gas which wind up in the ``dwarf'' are shown in blue and green, respectively, while other disk and bulge stars are shown in red; halo matter is not shown.

Computer simulations and video production were performed at the Pittsburgh Supercomputing Center (PSC). We thank Joel Welling of PSC for creating the MPEG movie files.

Joshua E. Barnes (

Last modified: June 11, 1997