MORPHOLOGICAL EVOLUTION OF GALAXIES

We simulate the growth of large-scale structure for three different co smological models, an Einstein-de Sitter model (density parameter Omeg a(0) = 1), an open model (Omega(0) = 0.2), and a flat model with nonze ro cosmological constant (Omega(0) = 0.2, cosmological constant lambda (0) = 0.8), using a cosmological N-body code (particle-particle/partic le-mesh) with 64(3) dark matter particles in a comoving cubic volume o f present comoving size 128 Mpc. The calculations start at z = 24 and end at z = 0. We use the results of these simulations to generate dist ributions of galaxies at the present (z = 0), as follows: Using a Mont e Carlo method based on the present distribution of dark matter, we lo cated similar to 40,000 galaxies in the computational volume. We then ascribe to each galaxy a morphological type based on the local number density of galaxies in order to reproduce the observed morphology-dens ity relation. The resulting galaxy distributions are similar to the ob served ones, with most ellipticals concentrated in the densest regions , and most spirals concentrated in low-density regions. By ''tying'' e ach galaxy to its nearest dark matter particle, we can trace the traje ctory of that galaxy back in time by simply looking at the location of that dark matter particle at earlier time slices provided by the N-bo dy code. This enables us to reconstruct the distribution of galaxies a t high redshift and the trajectory of each galaxy from its formation e poch to the present. We use these galaxy distributions to investigate the problem of morphological evolution. Our goal is to determine wheth er the morphological type of galaxies is determined primarily by the i nitial conditions in which these galaxies form or by evolutionary proc esses (such as mergers or tidal stripping) occurring after the galaxie s have formed and eventually altering their morphology, or a combinati on of both effects. Our main technique consists of comparing the envir onments in which galaxies are at the epoch of galaxy formation (taken to be at redshift z = 3) with the environment in which the same galaxi es are at the present. Making the null hypothesis that the morphologic al types of galaxies do not evolve, we compare the galaxies that form in low-density environments but end up later in high-density environme nts to the ones that also form in low-density environments but remain in low-density environments. The first group contains a larger proport ion of elliptical and S0 galaxies than the second group. We assume tha t the initial galaxy formation process cannot distinguish a low-densit y environment that will always remain low density from one that will e ventually become high density. Therefore, these results are absurd and force us to discard the null hypothesis that morphological evolution does not occur. Our study suggests that similar to 75% of the elliptic al and S0 galaxies observed at present formed as such, while the remai ning similar to 25% of these galaxies formed as spiral galaxies and un derwent morphological evolution for all three cosmological models cons idered (the percentages might be smaller for elliptical than for S0 ga laxies). These numbers assume a morphological evolution process that c onverts one spiral galaxy into either a S0 or an elliptical galaxy. If the morphological evolution process involves mergers of spiral galaxi es, these numbers be would closer to 85% and 15%, respectively. We con clude that most galaxies did not undergo morphological evolution, but a nonnegligible fraction did.