![]() | Annu. Rev. Astron. Astrophys. 1996. 34:
749-792 Copyright © 1996 by Annual Reviews. All rights reserved |
Recent theoretical models have been quite successful in explaining
several important features that appear to be common to most gas-rich mergers.
For example, theorists reacted rapidly to the observational discovery of
high concentrations
of cold gas in the central regions of some mergers and developed
new dynamic models with gas+stars that have provided much needed
insight into how such gas concentrations are produced
(e.g. Barnes &
Hernquist 1992,
Barnes 1995).
During collisions, the gas readily losses angular momentum due to
dynamical friction,
decouples from the stars, and inflows
rapidly toward the merger nuclei. The typical half-mass radius of the
gas is only 2.5% that of the stars, which scaled to Milky Way units
implies ~ 5 x 109 M
of neutral gas within a radius of ~ 140 pc and a gas density
of ~ 103 M
pc-3 (Barnes 1995).
Although all gas-rich disk-disk mergers appear to end up with enormous
gas densities at their centers, independent of the specific orbital
parameters, the rate of gas inflow may depend on the structure of the
progenitor galaxies, specifically, on the size of a dense central bulge
(Mihos & Hernquist
1994).
Galaxies without bulges develop bars that produce a more steady inflow that
may last ~ 1.5 x 108 years. By contrast, in objects
with large bulges the disk tends to be stabilized against strong inflow
until just before the galaxies finally
coalesce, when strong dissipation finally drives the contained gas to
the center in 5 x
107 years. If the level of infrared activity depends on the
rate of gas infall into what is to be the maximum central gas density
configuration, or if some other mechanism such as a binary black hole is
required to create an extreme nuclear starburst (e.g.
Taniguchi & Wada
1996), then it may be the case that not all mergers of gas-rich
spirals will produce ULIGs.
Noninteracting spirals typically have large quantities of H I beyond
their optical disks.
For instance, in the Milky Way most of the H I mass is beyond the solar
circle, whereas most of the molecular gas is at galactocentric radii
< 0.7 R.
Computer models predict that a large fraction of the H I gas in the
outer regions of the pre-encounter disks will be pulled out to large
radii in the form of tidal tails,
out of which dwarf irregular galaxies can be formed
(Barnes &
Hernquist 1992,
Elmegreen 1993).
Elmegreen et
al. (1993) have proposed that the
heating of the interstellar medium due to the encounter increases the
Jeans' mass and that the H I
gas that leads the matter launched into intergalactic space will form
self-gravitationally bound cloud complexes, which may collapse and appear as
detached irregular dwarf galaxies. This model forms large H I clouds
with masses as large as 109 M
at the end of the tidal tails
opposite the companion
(Mirabel et
al. 1992,
Duc & Mirabel
1994). In the
Barnes & Hernquist
(1992) model, however, the clumps form from
collapse of the stellar population, with
mass fluctuations on all scales, producing also ``failed dwarfs'' of only old
stars due to the fact that the potential wells are too shallow to
capture enough H I to form new stars. This result is consistent with the
wide range of B - V colors of the
condensations along the tidal tails of some ULIGs
(Mirabel et
al. 1991).