Observing Massive Galaxy Formation^*

Abstract

A major goal of contemporary astrophysics is understanding theorigin of the most massive galaxies in the universe, particularly nearbyellipticals and spirals. Theoretical models ofgalaxy formation have existed for many decades, although observationalevidence at both low and high redshifts is only beginning to put constraints on different ideas. We briefly describe theseobservations and how they are revealing the methods by which galaxies form by contrastingand comparing fiducial rapid collapse and hierarchical formation modelpredictions. The available data show that cluster ellipticals must haverapidly formed at z > 2, and that up to50% of all massive galaxies at z ~ 2.5 are involved in major mergers.While the former is consistent with the monolithic collapse picture, weargue that hierarchical formation is the only model that can reproduce all the available observations.