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 comoving 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 may have formed at z > 1. 1. Scope of This Review This review is an attempt to summarize current observational constraints on evolution in disk galaxies. Because of space limitations, we will mostly touch upon recent highlights, and do so in a simplified way that treats morphological, luminosity/size, and dynamical evolution independently (even though they are probably deeply interlinked). Throughout this review we will espouse the philosophy that the best way to understand how galaxies evolve is to watch them evolve in situ at high redshifts. Since our remit is to focus on evolution in disks, it is necessary to direct most of our attention to observations capable of distinguishing between disk galaxies and other objects in the field at high redshifts. We therefore unapologetically emphasize a morphological point of view, and focus mostly on recent work undertaken with the Hubble Space Telescope (HST).