Ten billion years of galaxy evolution

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 greater than or equal to 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 similar to 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 similar to5% of E-Sa-Sab galaxies are peculiar at z similar to 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 similar to 0. Finally, a very tentative, and entirely empirical, scheme is proposed for the classification of objects with z > 2.