What do black holes (which are very small) and galaxies (which are very large) have to do with each other?

In the past five years, astronomers have learned that there is a strong connection between galaxies and black holes. Most galaxies, not just active galaxies like quasars, appear to have a black hole at their core. Even our Milky Way Galaxy has one. Although it remains unseen to optical telescopes, the source radio astronomers call Sagittarius A* (A-star) is almost certainly a black hole. A wonderful video of stars circling around the center of our galaxy can be found at http://www.eso.org/outreach/press-rel/pr-2002/pr-17-02.html . This video (found about halfway down the page) shows a summary of 10 years of observations, revealing stars circling around what seems to be an empty patch of sky - the black hole. Using a wide variety of measurement techniques, astronomers have found supermassive (more than 1,000,000 times the mass of our Sun) black holes at the centers of many galaxies. A good summary of many of these observations can be found at http://www.cosmiclight.com/imagegalleries/blackholes.htm . In fact, finding a galaxy that might not have a black hole has become enough of a curiosity to deserve publication (http://www.sciencemag.org/cgi/content/abstract/1063896).

Even more remarkably, the black holes at the centers of galaxies appear to have had an influence on the galaxy formation. Two research groups independently discovered that the mass of the central hub (or bulge) of each galaxy is approximately 500 times the mass of the black hole, no matter how large or small the galaxy. One of these reports can be found at http://hubblesite.org/newscenter/newsdesk/archive/releases/2000/22/
Such a connection may seem puzzling. Galaxies are huge collections of stars, gas, and dust. Black holes may be extremely massive (more than 1,000,000,000 times the mass of our Sun) but are physically small (even the largest black hole would fit within our Solar System with plenty of room to spare). The effective gravitational influence of even a supermassive black hole extends only a few light years, while the bulge of a galaxy may extend to 20.000 light years from the center. The only explanation seems to be that the black hole and the galaxy co-evolved. In some way, the growth of the black hole influenced the growth of the galaxy.

One possibility is that black holes form early at the centers of galaxies, where the mass is most concentrated. The black hole can then go through a quasar phase (quasars are known to be short-lived phenomena), in which it spews out powerful jets and winds. These blast waves spread far beyond the gravitational sphere of influence of the black hole and can blow away the gas that is needed both for star formation and for continued growth of the black hole. This process puts a limit on the mass of the black hole and on the mass of stars in the bulge of the galaxy, leaving the masses at a constant ratio as observed. Recent X-ray observations (http://www.nasa.gov/centers/marshall/news/news/releases/2005/05-017.html) seem to be consistent with such a picture. The details of how this self-limiting growth might work remain unclear and will be studied further in the future.

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This week's question comes from Dr. Dave Thompson. Dr. Thompson is an astrophysicist who studies gamma rays in the Exploration of the Universe Division. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2007. His particular scientific interest is gamma-ray pulsars.