Saturday, October 21, 2006

Stellar birth control by supermassive black holes

This news item, which was mentioned in our article on black holes, suggests that the supermassive black hole at the center of the relatively nearby galaxy M87 might be responsible for a low rate of star birth in the galaxy.

There is also new evidence from the very early universe that the same suppression of star formation was already happening 11 billions years ago:

Stellar birth control in the early universe
Every year only a handful of new stars are born out of the gas that fills the space between the stars in galaxies like the Milky Way. To account for the large number of stars in the Universe today, about 400 billion in the Milky Way alone, it was thought that the "stellar birth rate" must have been much higher in the past.

Surprisingly, in this study appearing in the October 2 issue of Astrophysical Journal, astronomers using the 8.1m Gemini telescope in Chile report that many of the largest galaxies in the Universe had a very low stellar birth rate even when the Universe was only about 20 percent of its present age.

"Our new results imply that the stars in many large galaxies were born when the Universe was in its infancy, in the first few billion years after the Big Bang," said team leader Mariska Kriek, a PhD student from Leiden University and Yale. "The results confirm what some astronomers had suspected -- galaxies seem to have some method of 'birth control' that is very effective."

These new findings add to growing evidence that in big galaxies the formation of new stars was significantly suppressed after an initial period of vigorous activity. "These galaxies had a very violent early youth, but rose into stable adulthood well before many galaxies like the Milky Way were even in kindergarten," said Kriek."

And why might that have been?
One suggestion is that enormous black holes in the centers of large galaxies may be responsible for suppressing star formation. When material spirals into a black hole, huge amounts of energy are released and are rapidly injected into the galaxy's gas. This energy injection may dilute the gas sufficiently to prevent future star birth.

Another study of about 800 nearby elliptical and lenticular galaxies of various sizes has come to the same conclusion:

Huge Black Holes Stifle Star Formation
Supermassive black holes play a stealthy role in two major types of galaxies in the universe, bulking up until they are big enough to effectively shut down the formation of new stars, scientists have found.

The new results explain why scientists have observed in the past that massive galaxies have fewer young stars. Black holes, monstrous heaps of dense matter, grow at a different rate than the galaxies that surround them. But once a black holes reach a critical mass and become too large for its host galaxy, it zaps away nearly all the gas needed for young stars to form.

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