Gravitational collapse in tidal tails can produce bound structures. I study this process using numerical simulations of encounters between equal-mass disk galaxies, systematically changing encounter parameters to see which factors favor or inhibit the formation of these objects.
The simulations analyzed here are grouped into 11 ensembles, each containing 8 realizations. One ensemble serves as a reference; the other 10 ensembles are derived by varying parameters including the pericentric separation, disk orientations, and disk velocity dispersion.
Bound objects are found by a `friends-of-friends' algorithm. Lists of candidate objects are generated for a range of linking lengths bcrit. These are combined to produce two master catalogs of nearly virialized objects (0.3 < -T/U < 0.7); the `inclusive' and `exclusive' catalogs use the largest and smallest bcrit values which satisfy the above constraint on T/U.
The results of the dwarf-finding procedure are compared to a map of local potential wells in tidal tails. As one might expect, there is a good correspondence between local potential wells and bound objects.
Exclusive catalogs for all eight members of the reference ensemble are shown here. Bound objects are found along the entire lengths of the tidal tails.
These histograms summarize the `yields' of tidal objects in the various ensembles. Disk velocity dispersion has a strong effect on yield, with `cold' disks producing many more bound objects than do `warm' disks. Disk inclination i and pericentric separation rp also influence yield. Exclusive catalogs from the four ensembles plotted together in the upper right will be used to examine parameter correlations.
These plots show correlations between the mass of each bound object M and distance from primary D, virial velocity Vv, virial radius Rv, virial-to-true mass ratio Mv/M, half-mass density rhoh, and coarse-grained phase-space density fc. Mass correlates strongly with velocity dispersion and phase-space density, but only weakly with other parameters.
These plots show correlations between the distance of each bound object from the primary D and object mass M, virial velocity Vv, virial radius Rv, virial-to-true mass ratio Mv/M, half-mass density rhoh, and coarse-grained phase-space density fc. There is a weak correlation between distance and mass, with low-mass objects more numerous at large distances. Distance also correlates weakly with density, suggesting that tidal forces may disrupt low-density objects near their primary.
This animation shows the collapse of a fairly massive dwarf. Here red bodies are members of the dwarf in question, green bodies are members of other dwarfs in the same tail, blue bodies make up the tidal tail, and grey bodies are used for the rest of the disks.