Lecturer: Prof. Sandra Faber
Affiliation: University of California, Santa Cruz,
Lick Observatory
Abstract:
Evidence indicates that galaxies live through a
life cycle in which they start out gas-rich,
consume their gas in star formation, and then
stop making stars (quench). Why star formation
falls so low is not clear. It is true that there is
less gas falling onto galaxies at late times, but
not much gas is needed to keep them blue, yet
observed star-formation rates after quenching
are very low. Recent observations at z ~ 2 and z
~ 0 have identified sequences of galaxies that
appear to be in the process of quenching, and
studying these sequences has shed light on the
physics of quenching. Galaxies at z ~ 2 appear to
fade on a "fast track" after experiencing
feedback from central starbursts and black hole
growth triggered by sudden "compaction"
events. Galaxies at z ~ 0 (today) in contrast
appear to quench on a "slow track" owing to
gradually increasing feedback from a much
more slowly growing central BH. Remarkably,
quenching in both cases appears to be
associated with reaching nearly the same
central threshold stellar density. Early gas-rich
galaxies cross this threshold rapidly due to
copious gaseous dissipation, whereas late, gas-
poor galaxies must depend on slowly acting
secular evolution processes, such as bars and
spiral arms, that gradually bring gas to the
center to make stars and (presumably) build the
black hole. On both tracks, it appears that
central stellar buildup and black hole growth
proceed together in nearly the same way
despite the large difference in speed and
physical processes.