Ten Billion Years of Galaxy Evolution

Observations in the Hubble Deep Fields have been used to study the evolutionof galaxy morphology over time. The majority of galaxies with z < 1 are seen tobe disk like, whereas most objects with z > 2 appear to be either chaotic orcentrally concentrated ``blobs''. Such blobs might be the ancestral objects ofellipticals or of galaxy bulges. About 1/3 of objects with z > 2 appear to bein the process of merging. The region with 1 < z < 2 marks an importanttransition in the global history of star formation from a merger dominatedregime at z > 2, to one at z < 1 in which most star formation takes place ingalactic disks. It is speculated that the break in the Madau plot at z sim 1.5might be related to the transition from merger-dominated star formation at z >2 to disk-dominated star formation at at z < 1.Comment: 19 pages. 1 figure. To be published in the August 2002 issue of PAS