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Which Came First: Galaxy or Black Hole?

September 20, 2011 3:00:00 AM

Irene Klotz

The relationship between a galaxy and its black hole is as mystifying as any of those found among families on Earth.

Scientists don’t even know which came first — galaxies or their black holes, those regions of space so dense with matter that even photons of light fall prey to their gravitational jaws.

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Scrambling the cosmic conundrum anew is a discovery of small dwarf galaxies with giant black holes, a finding that upends currently held theories of galaxy formation.

Previous studies showed that as a galaxy grows and evolves, its black hole seems to grow and evolve too, at least for the big clusters.

Using data from the Hubble Space Telescope, astronomers now find that some small galaxies in the distant universe have disproportionately sized black holes.

“They seem to be out of sync in some fundamental way,” astronomer Sandra Faber, with California’s Lick Observatory, told Discovery News. “These black holes are too massive given their star content at that time,” Faber said. “They’ve grown up too fast. The holes have gotten ahead of the teen-age galaxies.”

Even more puzzling: the telltale bulge of stars associated with super-massive black holes in large galaxies is noticeably absent in these smaller siblings, suggesting there may be more than one way to grow a black hole.

The finding is based on studies of 28 dwarf galaxies huddled together some 10 billion light years from Earth. While black holes, by definition, cannot be seen, astronomers look at the radiation streaming from stars surrounding the black hole to figure out its size and features.

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“People might have thought that only once a galaxy gets to its old age and grows this bulge could you grow a super-massive black hole,” lead researcher Jonathan Trump, with the University of California at Santa Cruz, told Discovery News.

He expects the research will shift scientists’ thinking about the environments in which galaxies grow and when they can sport black holes.

“It’s potentially a paradigm-changing observation,” said Faber.

The research will be published in an upcoming edition of the Astrophysical Journal.