Diminished Competition

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Science  06 Feb 2009:
Vol. 323, Issue 5915, pp. 689
DOI: 10.1126/science.323.5915.689c

Most galaxies harbor giant black holes at their centers. For the most part, the black holes just sit quietly, but if their neighborhoods contain enough gas, they become active by accreting the gas, releasing large amounts of radiation in the process. Over time, the gas surrounding an active black hole gets heated and may even be expelled from the galaxy, a process that depletes the reserves of cold gas that feed star formation. In order to understand how black holes affect the galaxies that host them, Schawinski et al. studied a sample of local galaxies with active black holes and moderate luminosities, selected by virtue of their x-ray emission to avoid missing galaxies that would otherwise be obscured by dust. Color comparisons revealed that this sample falls in between galaxies that are actively forming stars and those that are quiescent, irrespective of the amount of dust. The colors suggest that star formation has stopped in these galaxies long before the effects of black hole growth can be detected—a challenge to scenarios where substantial levels of star formation coexist with black-hole growth. — MJC

Astrophys. J. 692, L19 (2009).

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