Are Old Galaxies Really Red and Dead?


Tuesday, October 20 2015 - 12:00 pm, PDT
Leo Blitz
UC Berkeley

Galaxies are broadly divided into two classes: spiral and elliptical. Unlike the spirals, the ellipticals, often referred to as early-type galaxies, are largely composed of old stars that give them a reddish color, They typically have little interstellar material with which to form new stars; these galaxies are often referred to as “red and dead.” We will see, however, that a substantial fraction of these galaxies contain surprising amounts of neutral hydrogen and these do form stars, albeit at a reduced rate compared to their spiral cousins. Early-type galaxies outside of clusters can be seen to be accreting gas from their surroundings, which is the source of at least some of the gas fueling star-formation taking place within them. In addition, the galaxies are seen to contain super-massive black holes in their centers. The black holes appear to be responsible for ejecting much of the gas that falls into the nuclei of these galaxies, reenergizing the intergalactic medium.

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