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Monster Black Holes: How Galactic Collisions Fed Them
By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
05 June 2001
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Variations on a chicken and an egg
The ESO findings, however, leave unanswered a huge chicken-and-egg question about the relationship between black holes and the earliest galaxies.
"Did galaxies form around primordial black holes?" Green wondered. And then perhaps stars formed, later combining with gas and dust near the black hole to ignite the quasars.
Or did galaxies form first? This would have led to the development of exploding stars called supernovae, which then collapsed and formed relatively small black holes. "Those black holes coalesced into the supermassive black holes of quasars, all the while with plenty of fuel around them," Green hypothesized.
Barger's work at the University of Hawaii has also provided clues that cloud the issue. She and her colleagues found that black hole growth might be a slower process than expected, and that many black holes didn't even get started until after the initial burst of galaxy formation. If this is true, then early galaxy collisions might have played less a role in building black holes than some think.
Which came first won't be answered, Green said, until more images of more distant quasars are made. Such observations should also determine how quasars became so powerful.
Slim pickings nowadays
Meanwhile, other researchers puzzle over the relative quietude of nearby supermassive black holes, which are in the later stages of their lives, having already evolved for billions of years.
Many, including one at the center of our own Milky Way Galaxy, emit few or no X-rays at all, though observations of star movement near the center of our galaxy clearly show the black hole is there, throwing around its colossal weight of 2.6 million Suns.
Why is our black hole so seemingly calm, despite its mass? Were black holes simply more voracious in the old days?
No, said Green. "They were just better fed."
But now their universe has expanded. Space is not so tight. Collisions are less frequent.
"Now that the rate of merging has decreased [and] most of the small galaxies have coalesced, the black holes in the centers of galaxies, including the Milky Way, are larger than ever," Green said. "But it's slim pickings on the banquet table."
Click here for the Science Tuesday archives and for more news and information about black holes. And return each week for another in-depth look at the world of space science.
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