7.2. Properties of merged galaxies
Motivated by the near coincidence of the expected number of merged remnants with the total number of elliptical galaxies in the NGC catalogue and the expectation that the merging of spiral discs would probably lead to "spheroidal heaps of stars," Toomre speculated that elliptical galaxies form from mergers. This idea has received some observational support. Schweizer (1982, 1983) has studied the merger remnant NGC 7252. The two long tails seen in this galaxy indicate a merger between two disc galaxies of about equal masses. The mean surface brightness profile, however, follows the de Vaucouleurs r1/4 law quite closely. This result suggests that merging has rearranged the disc material into something resembling an elliptical galaxy. Malind and Carter (1980) have observed shell-like structure around many elliptical galaxies. Quinn (1982, 1983) has shown that merging a cold disc with a more massive elliptical galaxy results in shells. In addition, merging can also account for the observed pattern between the radial spacings of the shells which other mechanisms, such as galactic winds (Fabian, Nulsen and Steward, 1980) could probably not explain.
Although the idea that many elliptical galaxies have formed from the merging of discs is still highly controversial (Ostriker, 1980; Tremaine, 1981), the suggestion has stimulated several theoretical investigations into the properties of merged remnants. In most cases the studies have been highly idealized, e.g. collisions between purely stellar systems of comparable masses. Even so, some very interesting results have emerged.
White (1978,
1979b)
has studied the merging of equal mass spheroidal
stellar systems using Aarseth's N-body program. The models are quite
crude since each "galaxy" is represented by only 250 particles. This
reduces the effective dynamic range because a large softening length
must be used in the gravitational potential in order to reduce the
effects of two-body relaxation (cf.
Section 6.2). White has found that
the merger remnants all have similar density profiles with
(r)
r-3
independent of the initial density profiles of the progenitors. It is
almost certain that this result is due to merging rather than two-body
relaxation since White finds that "galaxies" evolved in isolation
maintain density profiles that are quite different than the
r-3
behaviour of the merger remnants. Although encouraging, the results
are not applicable to the formation of ellipticals from spirals since
the N-body calculations start with spheroidal systems.
The much harder problem of disc mergers has only been tackled recently
(Gerhard, 1981;
Farouki and Shapiro, 1982;
Quinn, 1982;
Negroponte, 1982;
Negroponte and White,
1983).
Much of the problem of
simulating encounters between disc galaxies stems from their
instability towards forming bars
(Ostriker and Peebles,
1973).
There are several ways of suppressing the bar instability. The discs may be
set up with large random motions,
~
vrot (where vrot is the rotational
velocity) but this conflicts with the kinematics of stars in the solar
neighbourhood where
vrot
250 km sec-1 and
~ 40 km sec-1
(Toomre, 1974,
and references therein). The bar instability may be suppressed
by using a very large softening parameter in the gravitational potential
(Erickson, 1974;
Quinn, 1982)
but this is artificial and
significantly limits the resolution of the models. Alternatively, one
can adopt the method advocated by Ostriker and Peebles and include a
hot halo component. Using a halo has the disadvantage that the
already meagre number of particles used in the simulations must be
shared between the halo and the disc components. There is also the
additional problem of two-body relaxation in the disc component
(Rybicki, 1971).
These problems have still not been fully resolved and
it is clear that more work needs to be done. Nevertheless, the results
from numerical simulations show that both the halo and "luminous"
components acquire Hubble-type density profiles after merging. A
numerical simulation of a merger between two disc-halo systems is
shown in Figure 7.2
(Negroponte and White,
1983).
An important objection to the idea that ellipticals form from
merging spirals has been discussed by
Ostriker (1980).
In a somewhat modified form the argument runs as follows. Consider a cold
self-gravitating exponential disc with scale length
-1 and mass
MD. Its total energy is
ED = - 0.15GMD2
(Fall, 1979b,
Eq. 6.24). Now,
as indicated by the merger experiments, if two such identical discs
merge, the resulting object should be spheroidal with a density
profile close to the de Vaucouleurs r1/4
law. Thus the total internal energy of the merged remnant is
EE
- 0.17GME2 / RE. During the
merger, the total energy and mass are fairly well conserved, so if the
discs merge with orbital energy E0,
![]() |
(7.4a) |
Equation (7.4a) may be written in terms of the peak projected velocity
dispersion of the remnant
p,
![]() |
(7.4b) |
where vm is the peak rotational velocity of the
exponential disc. The
orbital energy E0 is usually negligible in Eq. (7.4b),
hence in order to obtain a giant elliptical with a velocity dispersion of
300
km/sec one requires
vm
400 km/sec which is a higher rotational
velocity than is seen in any spiral. Inclusion of heavy halos may
modify the above argument. However, the results of Gerhard's
simulations give
p
vm
/ 21/2 in good agreement with Eq. (7.4b). It is
possible that dissipative effects can help resolve this problem. The
numerical simulations of
Negroponte and White
(1983)
include a "gas"
component and they find that a substantial fraction of the gas falls
to the central regions during a merger.