 |
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. |
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.
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 |
|
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. |
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 |  |
 |
 |
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. |