The Morphological Evolution of Galaxies

Many galaxies have taken on their familiar appearance relatively recently. In the distant Universe, galaxy morphology deviates significantly (and systematically) from that of nearby galaxies at redshifts (z) as low as 0.3. This corresponds to a time ∼3.5 × 109 years in the past, which is only ∼25% of the present age of the Universe. Beyond z = 0.5 (5 × 109 years in the past), spiral arms are less well developed and more chaotic, and barred spiral galaxies may become rarer. At 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 have not changed much over time. The space density of luminous disk galaxies has not changed significantly sincez = 1, indicating that although the general appearance of these galaxies has continuously changed over time, their overall numbers have been conserved.