Observations of Disk Galaxy Evolution

The morphologies of disk galaxies begin to deviate systematically from those of nearby galaxies at surprisingly low redshifts, possibly as low as z=0.3. This corresponds to a time ~3.5 Gyr in the past, which is only one quarter of the present age of the Universe. Beyond z = 0.5 (a look-back time of 5 Gyr) the effects of evolution on spiral structure are rather obvious: spiral arms are less well-developed and more chaotic, and barred spiral galaxies seem 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, the co-moving space density and the sizes of luminous disks have not changed significantly since z=1. Tully-Fisher measurements indicate that the dynamical state and luminosities of large disk systems are also consistent with passive evolution out to a redshift of unity. We conclude that the general appearance of luminous disk galaxies has continuously changed with cosmic epoch, but their overall numbers have been conserved since z=1, and the bulk of the stars in these systems probably formed at z>1.