The Chicken or the Egg?

Having already drawn in and consumed more than 3 million stars, Sagittarius A* is certainly far more massive than stellar and midsized black holes (not to mention mini–black holes). Yet mounting evidence suggests that this giant’s growth cycle is far from finished. As Keel points out, “Even at a mass of 3 million suns, this black hole proves quite modest by the standards of other galaxies.”

Indeed, astronomers have intensified their studies of galactic cores and continue to discover truly enormous supermassive black holes in many distant galaxies. The nearby Andromeda galaxy, for instance, harbors a 30-million-solar-mass black hole in its core. A galaxy named NGC 4486B has a central black hole measuring about 500 million solar masses, and the core of a galaxy designated NGC 4261 features a stupendous object of some 1.2 billion solar masses. This suggests that there may be no physical limit to the size of a supermassive black hole.

Also, the fact that these giants seem to be integral features of galaxies and that they are eating their way through the galactic cores is surely significant. It now appears certain that supermassive galactic black holes must strongly affect the structure, evolution, and ultimate fate of galaxies. Says science writer Steve Nadis, “New evidence strongly suggests a much more intimate connection than astronomers ever thought possible between galaxies and the supermassive black holes that dominate their cores.”

But the nature of this grand cosmic connection is for the moment problematic for scientists. Central to the present debate on the topic is a variation of the old “chicken or the egg” question, in this case, Which came first, galaxies or giant black holes? Some astronomers think that galaxies and their central black holes form from the “outside in.” In other words, swirling masses of gases and dust condense to form spinning galaxies of stars, and over time some of the giant stars in the core collapse into black holes, which in turn merge to become one really massive black hole.

In contrast, others argue for the “inside out” hypothesis. In this version, as Asimov says, “The black hole may have come first and then served as a ‘seed,’ gathering stars about itself as super-accretion disks that become clusters and galaxies.” As for where these initial seed black holes came from, no one knows. They may have been created somehow in the Big Bang along with mini–black holes.

Whichever came first galaxies or large black holes the two seem to grow and develop together in step, so to speak. Late in 2000, astronomer Michael Merrifield and his colleagues at the University of Nottingham, in England, found a telling correlation between the age of galaxies and the masses of the supermassive black holes at their cores. Simply put, the older the galaxy, the more massive its central hole. “We’re measuring the time scale over which black holes grow,” Merrifield explains, “and it appears to be comparable to the age of the host galaxies. So they really are developing together.”

This new finding raises an important question. If giant black holes continue to grow at the expense of their host galaxies, why do astronomers not see some galaxies in their death throes, almost totally absorbed by the cosmic monsters within? The most obvious answer is that the universe is not yet old enough. Indeed, present-day humans probably exist at a time in the life cycle of the universe when most galactic black holes are still relative youngsters possessing

Stars in the center of the galaxy called M87 are tightly packed and moving very fast, suggesting they are orbiting a massive black hole.

from a few million to a few billion solar masses. According to this view, if humans could somehow travel far ahead in time, they would see many galactic black holes with tens and hundreds of billions of solar masses devouring the last remains of their parent galaxies.

Sagittarius A* Rips a Star Apart

In this excerpt from an article in the October 2001 issue of Astronomy magazine, science writer Robert Zimmerman describes the possible origins of Sagittarius East. It is now believed to be the remnants of an unusual supernova created by the immense gravitational effects of the black hole Sagittarius A*.

Sagittarius East is now believed to be a large bubble, possibly one of the largest supernova remnants known, that formed fewer than 100,000 years ago and maybe as recently as 10,000 years ago. Although it engulfs Sagittarius West [a cloudlike region nearby] and Sagittarius A*, it lies mostly behind both. Astronomers think that the energy required to punch out this shell of gas in such a dense region would have to be as much as 50 times greater than the most powerful supernova explosion. What could have produced this much energy still puzzles astronomers. Some theorize that Sagittarius East was created when a star approached within 50 million miles of the central black hole and was torn apart by the strong gravity.

At the center of this mass of gaseous clouds lies Sagittarius A*, which astronomers believe to be a giant black hole.