The hierarchical origin of galaxy morphologies

Abstract

We report first results from a series of N-body/gasdynamical simulations\ndesigned to study the origin of galaxy morphologies in a cold dark\nmatter-dominated universe. The simulations include star formation and feedback\nand have numerical resolution sufficiently high to allow for a direct\ninvestigation of the morphology of simulated galaxies. We find, in agreement\nwith previous theoretical work, that the presence of the main morphological\ncomponents of galaxies--disks, spheroids, bars--is regulated by the mode of gas\naccretion and intimately linked to discrete accretion events. In the case we\npresent, disks arise from the smooth deposition of cooled gas at the center of\ndark halos, spheroids result from the stirring of preexisting disks during\nmergers, and bars are triggered by tides generated by satellites. This\ndemonstrates that morphology is a transient phenomenon within the lifetime of a\ngalaxy and that the Hubble sequence reflects the varied accretion histories of\ngalaxies in hierarchical formation scenarios. In particular, we demonstrate\ndirectly that disk/bulge systems can be built and rebuilt by the smooth\naccretion of gas onto the remnant of a major merger and that the present-day\nremnants of late dissipative mergers between disks are spheroidal stellar\nsystems with structure resembling that of field ellipticals. The perplexing\nvariety of galaxy morphologies is thus highly suggestive of--and may actually\neven demand--a universe where structures have evolved hierarchically.\n