Mergers of Gas-Rich Disk Galaxies

April 2001

Mergers may transform gas-rich disk galaxies into elliptical galaxies, and possibly into S0 galaxies as well. The outcome of a disk-galaxy merger depends on the relative sizes of the galaxies and on the behavior of the gas. These simulations illustrate the formation of extended gas disks in merger remnants.


Initial Conditions

I chose four encounter geometries, and ran each once with a galactic mass ratio of 1:1 and once with a mass ratio of 3:1. Subsequently a third series of experiments with 1:1 mass ratio and a larger pericentric separation were added. The table below lists inclinations i and pericentric arguments w for the four encounter geometries; in the 3:1 encounters i1 and w1 refer to the more massive galaxy. These experiments used the same bulge/disk/halo galaxy models as Barnes (1998, Ch. 4), but here each disk included a gaseous component amounting to 12.5% of the disk mass. In simulation units with G = 1, each galaxy in the 1:1 encounters, and each large galaxy in the 3:1 encounters, has total mass Mbulge + Mdisk + Mhalo = 0.0625 + 0.1875 + 1 = 1.25, half-mass radius rhalf = 0.28, rotation period t(rhalf) = 1.2, and binding energy E = -1.07; the gas has specific internal energy uint = 0.014. The simulations, each using a total of Ngas + Nstars + Nhalo = 24576 + 29696 + 32768 = 87040 particles, were run with a new N-body/SPH code featuring adaptive smoothing and time-stepping.

Geometry i1 w1 i2 w2
DIRect 0 0 71 30
RETrograde 180 0 -109 30
POLar 71 90 -109 90
INClined 71 -30 -109 -30

Thermodynamic Evolution

Evolution of all encounters. Falling curves (red) show Er, the energy lost to dissipation; rising curves show the first through seventh octiles of the gas density.

Gas Dynamics & Shocks

The animations below view each merger face-on to the orbital plane. Only gas is shown; colors indicate energy dissipated by shocks. Violent dissipation naturally occurs in inter-penetrating passages and final mergers; within disks, shocks often trace tidally-induced spirals and bars. Shocks also occur at the base of returning tidal tails; the reaccreted gas may rebuild tidally perturbed disks or form a new disk about a merger remnant.

1:1     rp = 0.2 1:1     rp = 0.4 3:1     rp = 0.2
DIR
RET
POL
INC

Disks in Merger Remnants

These animations view each remnant approximately edge-on, rotating it about the vertical axis. Gas is colored according to line-of sight velocity. In almost all remnants an extended gas disk is present; these disks are often strongly warped. Many remnants have distinct nuclear gas disks, which may exhibit counter-rotation or other forms of kinematic decoupling.

1:1     rp = 0.2 1:1     rp = 0.4 3:1     rp = 0.2
DIR
RET
POL
INC

Cumulative gas profiles for all remnants. Black curves show fraction of all gas within radius r plotted against log r. Red and blue curves show simular profiles for gas from disks 1 and 2, respectively.


Joshua E. Barnes (barnes@ifa.hawaii.edu)
Last modified: May 1, 2001
http://www.ifa.hawaii.edu/~barnes/research/gassy_mergers/index.html