Summer 2004 -- Vol. 21

Link Between Black Holes and Galaxies Discovered in Our Own Back Yard

By studying more than 120,000 nearby galaxies observed as part of the Sloan Digital Sky Survey (SDSS), a team of astronomers from Germany and the United States, including Donald Schneider, professor of astronomy and astrophysics at Penn State and chairman of the SDSS Quasar Science Team, has been able to show that the growth of supermassive black holes is closely linked with the birth of new stars in their host galaxies.

This discovery—a first direct glimpse of the connection between galaxy formation and black-hole formation—was announced at the International Astronomical Union's Maps of the Cosmos Symposium in Sydney, Australia. The paper, titled “The Host Galaxies of the Active Galactic Nuclei,” also was submitted to the Monthly Notices of the Royal Astronomical Society.

One of the most remarkable discoveries of recent years has been the demonstration that every large galaxy harbors, at its core, a black hole weighing many million times as much as the Sun, explained research team leader Guinevere Kauffmann of the Max Planck Institute for Astrophysics in Garching, Germany.

Furthermore, Kauffmann said the mass of this central black hole is very closely related to the properties of the galaxy in which it is embedded. This implies that the formation of the black hole is intimately entwined with that of its galaxy, but the nature of this link remains obscure.

Does the black hole control the growth of its host, or does the galaxy limit the growth of its central black hole? Do black holes and galaxy growth form some kind of a symbiotic relationship? These questions can be answered only by careful study of the growth process, she said.

Co-team leader Timothy Heckman of the Johns Hopkins University in
Baltimore, Maryland, explained that as black holes grow they release prodigious amounts of energy, in extreme cases outshining their host galaxy, to produce a bright quasar. The main epoch of quasar activity, and perhaps of black-hole growth, occurred when the universe was between a third and a tenth of its present age of 14 billion years.

Heckman said large galaxies are thought to have formed through the
collapse and merging of smaller systems during this same time period. Black-hole growth is still detectable in galaxy nuclei today, however, and stars still form in these inner regions. “Since nearby galaxies can be studied much more easily than their distant and more spectacular ancestors, it is no surprise that the link between black-hole growth and galaxy growth first became apparent in our own backyard,” he said. The light from these nearby galaxies, which the team studied, took less than one billion years to reach us—compared to almost ten billion years for most quasars. These galaxies are close enough for researchers to study in some detail but we see them as they were long after the rapid building process for both black holes and galaxies has subsided to a lower level.

By searching for tell-tale features in the spectra of more than 120,000 galaxies, the SDSS team was able to show that more than 20,000 of them contain black holes that are currently growing. The growth rate of the black hole is inferred from the strength of characteristic emission lines known to be correlated with how much material is falling onto the black hole.

These growing black holes are located almost exclusively in galaxies more
massive than the Milky Way. Massive galaxies where black-hole growth
currently is weak or absent typically have the structure and star content of old
elliptical galaxies, which finished making stars long ago, researchers explained. Galaxies where black-hole growth is currently strong have similar mass and structure, but show evidence for substantial recent star formation.

In its conclusion, the team said that as the rate of black-hole growth increases, so does the amount of star formation within the past 100 million years, which is recent in astronomical terms. In the most extreme objects, the black hole is growing as fast as in bright quasars and the galaxy is dominated by young stars. The researchers say that this observation probably means that the black hole is growing by swallowing some of the same supply of relatively cold and dense gas from which stars are forming elsewhere in the galaxy. They also say that the stellar mass of these galaxies and the masses of their central black holes are clearly growing together. Like chicken and egg, neither black hole nor galaxy can be said to come first; each is necessary for the other.

The authors of the black hole findings are Guinevere Kauffmann, Stephane Charlot, Jarle Brinchmann, and Simon White from the Max-Planck-Institute for Astrophysics; Timothy Heckman and Susan Ridgway from Johns Hopkins University; Zeljko Ivezic, Gordon Richards, and Patrick B. Hall from Princeton University; Christy Tremonti from the University of Arizona; Jon Brinkmann from Apache Point Observatory; Masataka Fukugita of the University of Tokyo; and Donald Schneider of Penn State.

SDSS /Penn State

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