The Origin of Two Galaxy Types

Avishai Dekel

Racah Institute of Physics, The Hebrew University of Jerusalem

The emergence of a concordance cosmological model based on cold dark matter and dark energy has triggered a new phase in the study of galaxy formation, in which the framework is known. A flood of multi-wavelength data from space and ground telescopes is revolutionizing our knowledge of axies, both today and in the distant past, introducing interesting theoretical challenges. This talk will address the fundamental question of physical cosmology: the origin of the two major types of galaxies. One population is the ``blue cloud" of star-forming disk galaxies in the field and the other is the ``red sequence" of dead ellipticals in clusters. We will explain the origin of the characteristic mass separating these populations, and address the main physical processes responsible for the properties of the two galaxy types.