The Morphological Evolution of Galaxies

Abstract

Many galaxies appear to have taken on their familiar appearance relatively recently. In the distant Universe, galaxy morphology started to deviate significantly (and systematically) from that of nearby galaxies at redshifts, z, as low as z = 0.3. This corresponds to a time ~3.5 Gyr in the past, which is only ~25% of the present age of the Universe. Beyond z = 0.5 (5 Gyr in the past) spiral arms are less well-developed and more chaotic, and barred spiral galaxies may become rarer. By z = 1, around 30% of the galaxy population is sufficiently peculiar that classification on Hubble's traditional tuning fork system is meaningless. On the other hand, some characteristics of galaxies do not seem to have changed much over time. The co-moving space density of luminous disk galaxies has not changed significantly since z = 1, indicating that while the general appearance of these objects has continuously changed with cosmic epoch, their overall numbers have been conserved. Attempts to explain these results with hierarchical models for the formation of galaxies have met with mixed success.