Clusters of old galaxies emit more x-rays and are more active than expected, according to new data that runs contrary to existing theories on how galaxies and stars evolve.
Astronomers looking at galaxies in a cluster called Abell 2104, about 700 million light-years away, found six x-ray sources instead of the one expected. The radiation is emitted by supermassive black holes in the centers of galaxies. As gas and dust are attracted by the black holes, which can have billions of times the mass of the Sun, the material heats up and energy is released in the form of x-rays.
The findings are surprising because old clusters like this one should not have enough gas and dust to fuel the supermassive black holes, also known as Active Galactic Nuclei (AGN). If they did, that material ought to also go into making new stars. But the stars in the galaxies appear to be mostly older.
Current theories hold that the gas and dust is either stripped or used up as the galaxies collide over time to form the clusters.
"Basically, I think it just tells us we don't quite understand the dynamics of these big structures," John Mulchaey, a part of the observing team, told SPACE.com. "To really understand how the universe has evolved and where it's going, one of the things you need to do is to understand structure -- how large concentrations of galaxies like clusters form."
Further observations, scheduled for three weeks from now, may also help scientists understand more about black holes.
"There are potentially enough active black holes in these clusters to use the cluster environment itself as a laboratory to study black holes," said Paul Martini, who headed the group from the Carnegie Observatories in Pasadena, California.
"The environment of clusters of galaxies is quite different from the typical galaxy environment, such as the one around our own Milky Way," Martini said. "The great thing about finding active black holes in such different environments is that differences in the frequency of active black holes between these environments can teach us about the processes that make black holes active in the first place."
Martini and his colleagues used images from an optical telescope at Las Campanas observatory in Chile and from the Chandra X-ray Observatory in space. These findings, published in the recent issue of Astrophysical Journal Letters, were possible only because of the clarity of the Chandra satellite, launched in 1999.
"A lot of the gas and dust that are fueling the AGN are also shielding them [in the optical] from our own line of site," Mulchaey said. But, just as in hospitals, "x-rays penetrate through very significant amounts of material."
Although scientists have been putting x-ray observatories into space (x-rays cannot penetrate the atmosphere so are invisible from Earth) for 40 years, they have not had sufficient resolution to study individual galaxies in clusters. "They're like someone who has bad vision and then taking off his glasses," Mulchaey said.
Also, astronomers may not have been that interested in studying the individual galaxies.
"If you look at that picture, you see the dominant blue in the middle. People have basically been paying attention to the middle of the cluster and have been ignoring the galaxies themselves," Mulchaey said. "We have to get over the obvious fact that the first thing you see is this big blue blob."
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