3. TIDAL TRAUMAS
Although the peculiar appearance of galaxies in a few double and
multiple systems had been noticed much earlier, it was
1959) who first called
attention to the enormous variety of extended
structures seen in such objects. Zwicky described these features as
``clouds, filaments and jets of stars which are ejected
massively from galaxies in collision'' by ``large scale tidal effects''.
Many astronomers, however, were skeptical that certain remarkably narrow
appendages could be blamed on tidal forces (e.g.
Gold & Hoyle 1959;
and Zwicky himself suggested that
``electromagnetic actions . . . contribute to the internal viscosity of
Bridges & Tails
Bridges and tails are common in interacting disk
galaxies. The former often appear to link large galaxies to small
companions, while the latter stretch far away from the galaxy causing
the perturbation. That ``old-fashioned gravity'' could extract these
narrow structures was illustrated by
Pfleiderer & Seidentopf (1961),
Toomre & Toomre (1972;
hereafter TT), Wright (1972),
and Eneev et al. (1973).
These calculations generally treated each galaxy as a point
mass surrounded by a disk of test particles, a simple numerical
method, but adequate for the job. Only close, relatively slow
passages - parabolic or even sub-parabolic - readily produce
convincing bridges and tails; faster passages produce smaller
disturbances. As TT noted, such
slow passages would naturally result
between pairs of galaxies which, bound gravitationally since
formation, have fallen together for the first time along extremely
With varying degrees of success, the calculations reproduced bridges
and tails as thin, curving ribbons yanked from disks by violent tidal
forces. Such features are clearly relics of recent collisions
rather than signs of ongoing interactions. The most impressive
examples arise in direct passages where the orbital angular speed of
the companion temporarily matches that of the stars within the disks,
creating a ``quite broad'' resonance.
The tidal hypothesis gained additional credibility from
deliberately simple models of four well-known interacting systems.
Further studies of these and other galaxies continue to support the
tidal picture. For example,
Schweizer (1978) and
Schombert et al. (1990)
have reiterated Zwicky's claim that bridges and tails are
composed largely of stars drawn from a population very similar to that
of the disks from which they extend. Kinematic studies of interacting
van der Hulst 1979a,
Combes et al. 1980;
Marcelin et al. 1987)
have revealed velocity fields quite consistent with tidal models
Early test particle models raised many questions which could only be
addressed with self-consistent calculations. One such question
concerns effects of self-gravity in structuring tidal bridges and
tails. Once launched, such features develop in an essentially
kinematic manner (TT),
and deep exposures of
NGC 4038 /
Schweizer (1978) reveal thick
tails consistent with free expansion
both parallel and perpendicular to the direction of extension. On the
other hand, self-gravity may create small-scale structure within
tails. Zwicky (1956) noticed a
concentration of luminosity near the
end of the southern tail of
NGC 4038 /
9 and speculated that such
objects might evolve into dwarf galaxies.
Schweizer (1978) reported
clumps of gas and young stars in the tidal tails of other interacting
Mirabel et al. (1991)
describe a beautiful system with at least
nine distinct knots spread along a total extent of 350 kpc.
Models indicate that dwarf galaxies formed in this manner can
incorporate considerable gas from the disks but very little material
from the halos of their parent galaxies, and so should have unusually
low M/L ratios
(Barnes & Hernquist 1992).
Another such question concerned the conjecture that tails extracted
from disks might be unable to climb out of deep halo potential wells
(e.g. White 1983b).
This raised the possibility that the long observed
tails could contradict the large masses proposed for invisible
halos. However, encounters between self-consistent disk/halo models
indicate that galaxies with halos of four times the mass of their
luminous components can nonetheless produce tails as long as those of
NGC 4038 /
(Barnes 1988). Since the energy
required to climb out is provided by falling in, mere length is probably
not an effective constraint on halo masses.
Wavelike Tidal Spirals
Self-gravity plays an important role in structuring the inner
disks of some interacting galaxies.
TT noted that galaxies with
bisymmetric spiral patterns often have close companions, and suggested
that these spirals have a tidal origin. N-body models of galactic
disks abundantly illustrate the development of tidally excited spirals
Barnes 1990). In a shearing,
self-gravitating disk of stars, perturbations can grow by factors
>> 10 while swinging around to become trailing spiral patterns
(Goldreich & Lynden-Bell 1965;
Julian & Toomre 1966). Such ``swing
amplification'' occurs because the shearing of the spiral pattern
temporarily matches the epicyclic motions of individual stars,
permitting a modified form of Jeans instability to develop (e.g.
Toomre 1981). This mechanism
can amplify tidal perturbations in situ,
rapidly bringing forth a trailing spiral over much of the disk. The
calculations show that such spirals, although manifested as density
waves as well as material arms, do not survive long but wind tightly
up over a few rotation periods.
Several groups have attempted to reproduce the inner spiral structure
M 51 with tidal interaction models
Howard & Byrd 1990;
Sundelius 1990). Perturbations
strong enough to generate the
large-scale tidal features originally modeled by
also give rise to a ``grand-design'' spiral density wave in the inner
disk. At present, however, none of the calculations offer a really
convincing reconstruction of
M 51's spiral structure; better models of
the pre-encounter disk of
NGC 5194 are probably required.
Some, although by no means all, galaxies with pronounced rings (e.g.
Few & Madore 1986) appear to be
the result of collisions. As shown in simple N-body simulations
(Lynds & Toomre 1976;
Theys & Spiegel 1977), rings
develop when a companion galaxy makes a close and nearly
normal-incidence passage through the plane of the victim, exciting
large epicyclic oscillations in the target disk. Initially, all these
oscillators are in phase, but since their periods increase with mean
radius, the oscillations drift out of phase and orbits crowd together
radially to produce an expanding ring. Indeed, kinematic studies of
several ring galaxies find velocities consistent with the collision
Fosbury & Hawarden 1977;
Few et al. 1982). Even the
``folded ring'' galaxy
Arp 144, once interpreted as a collision between
a galaxy and an intergalactic H I cloud
(Freeman & de Vaucouleurs
is now known to include two bodies with
infrared colors typical of evolved stellar populations
(Joy et al. 1988).
To be sure, not all ring-making collisions are described by the
simplest models. The system studied by
Taylor & Atherton (1984)
exhibits a rather complex velocity field. More remarkable yet is the
extreme range of velocities, exceeding 1000 km/s, observed in
the messy H II ring of
(Hippelen 1989); many
features of this system are nicely reproduced by a test particle
calculation, but the M/L ratios implied seem uncomfortably high.
The ``Cartwheel galaxy'' itself presents a modest
puzzle or two; the
putative companion does not seem massive enough to produce the very
strong ring observed
(Davies & Morton 1982), and the
features giving this galaxy its name are not well understood.
Self-consistent three-dimensional models might help account for all
these systems (e.g.
Appleton & James 1990).
Only head-on collisions leave the bulge of the victim at the center
of the ring; if the perturber is somewhat off-center, the bulge can
be yanked to one side, producing an empty ring
(Lynds & Toomre 1976).
More off-center passages, although presumably more common, may not
retain their ring-like appearance for as long as a direct hit; the
resulting morphology changes smoothly from a ring to an open
tidally-induced spiral as the pericentric separation is increased
(Toomre 1978). Ring-like shapes
resulting from off-center passages
are more common than generally thought. One example is the very
extended H I tail of
(Appleton et al. 1986), which
presents an almost circular outline beautifully illustrated in VLA
maps (Rots et al. 1990).
Tidal interactions involving elliptical galaxies are more subtle than
those involving spirals, since encounters in rich clusters tend to be
fast and disturbed ellipticals produce diffuse sprays of stars instead
of narrow filaments. Nonetheless, a number of interacting ellipticals
have been identified by their luminosity profiles
distorted and off-center isophotes (e.g.
Lauer 1986), and peculiar
Borne & Hoessel 1988).
The effects of hyperbolic encounters on the luminosity profiles of
spherical galaxies have been studied both analytically (e.g.
Knobloch 1978) and numerically
(e.g. Dekel et al. 1980;
Aguilar & White 1985,
McGlynn 1990). Such encounters
do not tidally truncate
the target; on the contrary, they promote stars to loosely-bound
orbits, creating extended halos with
profiles, closely following a de Vaucouleurs law (e.g.
Aguilar & White (1986) found
that stars which have not yet phase-mixed
produce transient, outward-moving luminosity excesses. These results
provide a natural explanation for the distended profiles of galaxies
with nearby companions
Photometric decomposition techniques provide further evidence of
interactions among elliptical galaxies (e.g.
Hoessel et al. 1985;
1988). In these reductions a
luminosity model is simultaneously fit to each galaxy in the field;
the residuals reveal non-concentric isophotes and elongated features
referred to as ``dynamical friction wakes''. These studies find
little evidence for
strong interactions among most
multiple-nucleus cluster galaxies, in general accord with the view
that such systems are either optical doubles or extremely fast,
Tonry 1984). Some points of
Hoessel et al. (1985)
report that only those pairs
with velocity differences smaller than their internal dispersions show
signs of interaction, while
Lauer (1988) sees evidence for
interactions in pairs with velocity differences in excess of 1000 km/s.
Surface photometry and long-slit velocity data of several interacting
elliptical galaxies have been used to construct semi-restricted
Borne et al. 1988;
Balcells et al. 1989).
These models do a fairly convincing job of reproducing the tidally
disturbed morphology and ``U-shaped'' velocity profiles (e.g.
Borne & Hoessel 1988) of their
subjects, but it is difficult to assess their
claimed uniqueness since they do not seem to be overconstrained by the
data. Fully self-consistent models including dark halos are probably
needed to reliably predict the future evolution of these systems.
An interesting exception to most of the rules for interacting
ellipticals are ``dumbbell'' systems (e.g.
Valentijn & Casertano 1988).
These are comparable pairs of giant elliptical galaxies with
projected separations of ~ 10 h-1 kpc, found at the
centers of some rich clusters. The distorted morphologies and
relatively small pairwise velocity differences observed in these
objects imply that many are bona fide interacting systems.
Tremaine (1990) suggested that
dumbbell galaxies are the last stage in the
merger of rich clusters, each containing a D or cD galaxy.
This proposal provides a reasonable account of many features of
dumbbell systems, including their morphology, separations, velocity
differences, and overall frequency.
Rix & White (1989) have
constructed self-consistent equilibria for dumbbell galaxies and used
N-body calculations to show that at least some of these models are
free from violent dynamical instabilities; it is not clear if systems
with extensive common envelopes can also turn out to be stable or if
real dumbbell systems can get themselves into such slowly-evolving