Surprising Active Super Massive Black Holes in Old Globular Galaxy Clusters

Mo Xinhai

PureInsight | September 23, 2002

According to a piece of news on September 12 from NASA’s website, using NASA's Chandra X-ray Observatory, scientists at the Carnegie Observatories in Pasadena, Calif., have uncovered six times the expected number of active, super massive black holes in a single viewing of a cluster of galaxies. The finding has profound implications for theories as to how old galaxies fuel the growth of their central black holes.

Scientists are surprised by the finding because it suggests that voracious, central black holes might be as common in old, red galaxies as they are in younger, blue galaxies, a surprise to many astronomers. The team also used Carnegie's 6.5-meter Walter Baade Telescope at the Las Campanas Observatory in Chile for follow-up optical observations.

'This changes our view of galaxy clusters as the retirement homes for old and quiet black holes,' said Dr. Paul Martini, lead author on a paper describing the results that appears in the September 10 issue of The Astrophysical Journal Letters. "The question now is, how do these black holes produce bright X-ray sources, similar to what we see from much younger galaxies?"

"X rays can penetrate obscuring gas and dust as easily as they penetrate the soft tissue of the human body to look for broken bones," said co-author Dr. Dan Kelson. "So, with Chandra, we can peer through the dust and we have found that even ancient galaxies with 10-billion-year-old stars can have central black holes still actively pulling in copious amounts of interstellar gas. This activity has simply been hidden from us all this time. This means these galaxies aren't over the hill after all and our theories need to be revised."

Scientists say that super massive black holes — having the mass of millions to billions of suns squeezed into a region about the size of our Solar System — are the engines in the cores of bright active galaxies, often referred to as Active Galactic Nuclei, or AGN. Many astronomers think that all galaxies have central, super massive black holes, yet only a small percent show activity.

Galaxy clusters contain hundreds to thousands of galaxies. They are the largest known structures in the Universe and serve as a microcosm for the mechanics of the Universe at large. The galaxies in clusters are often old, reddish elliptically shaped galaxies, distinct from blue, spiral galaxies like our own.

The theory now in question is that as galaxies enter into clusters at high speeds, they are stripped of their interstellar gas, much as a strong wind strips leaves from a tree. Galaxies may also collide with one another and use up all of their gas in one huge burst of star formation triggered by this interaction. These processes remove most, if not all, of the interstellar gas. Since they no longer have the raw material to form new stars, the stellar population slowly gets old and the Galaxy appears red. No gas is left to fuel an AGN.

Previous surveys of galaxy clusters with optical telescopes have found that about only one percent of the galaxies in a cluster have AGN. This latest Chandra observation, if typical, bumps the count up to about 5 percent.

The team found six red galaxies with high x-ray activity during a nearly 14-hour Chandra observation of a galaxy cluster named Abell 2104, over 700 million light years from Earth. Based on previous optical surveys, only one was expected.

"If we relied on optical data alone, we would have missed these hidden monsters," said co-author Dr. John Mulchaey. Only one of the six AGN, in fact, had the optical spectral properties typical of AGN activity.

Recently, many modern cosmology theories are being challenged by more and more new discoveries in astronomy. By revising part of the assumptions in the theories, some of them might be able to cope with some specific discoveries. But this approach is likely to work only temporarily. Probably a fundamental change on our current concepts about time and space, life and universe is needed to solve these dilemma and the contradictions between cosmological theories and the observations.


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