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| CfA Colloquium Schedule Spring 2004 | ||
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6 May 2004Speaker: Avishai Dekel (The Hebrew University, Israel) Title: The Characteristic Scale of Galaxy Formation Abstract: Recent data, e.g. from SDSS and 2dF, reveal a robust bi-modality of the galaxy population, which is divided into two major classes at a characteristic stellar mass of M_* ~3x10^{10} Msun (near L_*). Less massive galaxies tend to be blue, star-forming disks residing in low-density environments. Their properties are correlated along a "fundamental line" of decreasing surface brightness, internal velocity and metallicity with decreasing luminosity down to the smallest dwarf galaxies. Galaxies above the critical mass are dominated by spheroids of red, old stars, with high surface brightness and metallicity independent of luminosity. They tend to reside in high-density environments and to host AGNs. It will be shown in simple terms how this bi-modality and critical scale can arise as the combined imprint of three different physical processes. In galaxies below the critical mass, the stellar disks are built by cold flows, with supernova feedback regulating star formation and driving the fundamental line. In galaxies above the critical mass, the infalling gas is heated by an extended shock to the dark-matter halo virial temperature. This hot, dilute gas is vulnerable to AGN feedback, which prevents it from ever cooling to form stars. Galaxies near the critical mass are thus the most efficient star formers, yielding a minimum in the halo mass-to-light ratio at the same critical scale. The implications on galaxy-formation modeling will be discussed, and the main discrepancies between theory and observation will be addressed in view of these new ideas. References for students:
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