2002PASP..114..797V

Observations in the Hubble Deep Fields have been used to study the evolution of galaxy morphology over time. The majority of galaxies with z<1 are seen to be disklike, whereas most objects with z>2 appear to be either chaotic or centrally concentrated ``blobs.'' Such blobs might be the ancestral objects of ellipticals or of galaxy bulges. About (1)/(3) of objects with z>2 appear to be in the process of merging. The region with 1<z<2 marks an important transition in the global history of star formation from a merger-dominated regime at z>2 to one at z<1 in which most star formation takes place in galactic disks. It is speculated that the break in the Madau plot at z∼1.5 might be related to the transition from merger-dominated star formation at z>2 to disk-dominated star formation at z<1. The Hubble tuning-fork diagram provides a satisfactory framework only for galaxy classification at z≤0.5. At z>0.5, barred spirals become increasingly rare. Possibly very young disks are still too hot (chaotic) to become unstable in global barlike modes. Furthermore, it becomes increasingly difficult to shoehorn galaxies at z≥0.5 into the Hubble classification sequence E-Sa-Sb-Sc. The fraction of ``peculiar'' spirals, i.e., those that do not fit naturally into the Hubble tuning-fork diagram, increases from 12% at z∼0.0 to 46% at 0.6<z<0.8. Early-type galaxies appear to approach their normal morphology faster than do late-type spirals. As a result, only ∼5% of E-Sa-Sab galaxies are peculiar at z∼0.7, compared to 69% peculiars among objects of types Sbc-Sc. With increasing redshift, the spiral-arm pattern in late-type spirals is seen to become ever more chaotic. In early-type galaxies, the spiral arms appear to be less well developed at large redshifts than they are at z∼0. Finally, a very tentative, and entirely empirical, scheme is proposed for the classification of objects with z>2.