In disk-disk collisions of galaxies, dynamical friction and subsequent relaxation may produce a mass distribution similar to that in classic elliptical galaxies. From the relative numbers of mergers and ellipticals in the New General Catalogue [26] estimated that a large fraction of ellipticals could be formed via merging. The first direct observational evidence for the transition from a disk-disk merger toward an elliptical was presented in the optical study of NGC7252 by [22]. The brightness distribution over most of the main body of this galaxy which is shown in Figure 3c is closely approximated by a de Vacouleurs (r-1/4) profile. However, NGC7252 still contains large amounts of interstellar gas and exhibits a pair of prominent tidal tails (see imilar proportion ( ~ 50%) of ULIGs whose K-band profiles are well fit by a r-1/4 law.
More recently, [9]
have proposed that ULIGs are elliptical
galaxies forming by merger-induced dissipative collapse. The
extremely large central gas densities
(~ 102-103
M
pc-3) observed in
many nearby ULIGs, and the large stellar velocity dispersions found in
the nuclei of Arp220 and NGC6240 are comparable to the
stellar densities and velocity dispersions respectively, in the
central compact cores of ellipticals.
Despite the K-band and CO evidence that LIGs may be forming ellipticals, we still need to account for two important additional properties of ellipticals: 1. the large population of globular clusters in the extended halos of elliptical galaxies, which cannot be accounted for by the sum of globulars in two preexisting spirals, and 2. the need to remove the large amounts of cold gas and dust present in infrared-luminous mergers in order to approximate the relative gas-poor properties of ellipticals. These two issues have been discussed by [18].