Models of Starbursts in Merging Galaxies

Chris Mihos and Lars Hernquist, UC Santa Cruz

We present two short movies of starbursts induced by galaxy mergers. Each frame shows the star-forming morphology of the galaxies, color-coded such that red represents the regions of most intense star formation while blue represents low levels of star formation. These frames might be likened to unobscured H-alpha maps (if such things could ever exist!).

The disk galaxy models are composed of an exponential disk of stars and gas embedded in a massive dark halo. The disk:halo mass ratio is 1:5.8, and the gas represents 10% of the total disk mass. Each movie covers approximately 1.5 billion years, or approximately 10 disk half mass rotation periods, and the frames are stepped in 30 million year intervals.

The major merger shows the merging of two disk galaxies, initially in a parabolic orbit. The encounter is exactly prograde, meaning the disk planes coincide with the orbital plane and that each disk rotates in the same sense as the orbital motion. The strongest rates of star formation are achieved in the central regions of the merging galaxies, where an inflow of disk gas results in very high gas masses.

The satellite merger shows the reponse of a disk galaxy to an infalling satellite galaxy. The satellite has a mass of 10% of the disk mass, and falls in on a nearly prograde orbit, inclined 30 degrees to the disk plane. The satellite galaxy is free of gas, and therefore DOES NOT APPEAR in the movie, which shows only the star-forming gas in the disk galaxy. Again, the highest rates of star formation are found in the central regions of the disk, as a result of merger-driven inflow of disk gas. For more details, see the April 10 1994 issue of Astrophysical Journal Letters.

Each disk galaxy is modeled using 32768 particles each to represent the stellar disk and dark matter halo, while the disk gas is modeled using 16384 gas particles. The satellite galaxy is modeled using 8192 particles. As a result, the major merger uses a total of 163,840 particles, while the satellite merger uses 90,112 particles. (A Cray C90 at the San Diego Supercomputer Center took 35 hours to produce the satellite merger sequence.)