Yale Bulletin and Calendar

October 6, 2006|Volume 35, Number 5

Astronomers find evidence
of galactic 'birth control'

An international team of astron,omers based at Yale and Leiden University in The Netherlands found that "old stars" dominated many large galaxies in the early universe, raising the new question of why these galaxies progressed into "adulthood" so early in the life of the 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, astronomers using the 8.1m Gemini telescope in Chile found that many of the largest galaxies had a very low stellar birth rate even when the universe was only about 20% of its present age. The study appeared in the Oct. 2 issue of Astrophysical Journal.

"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," says team leader Mariska Kriek, a Ph.D. 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," says Kriek.

The astronomers used the powerful Gemini Near Infrared Spectrograph to analyze the light of distant galaxies simultaneously over many different wavelengths. They studied 20 galaxies so distant that their light had been traveling for nearly 11 billion years, or 80% of the age of the universe.

"The unexpected finding is what was not found -- we expected to see a prominent signal from ionized Hydrogen, the tell-tale signature of star birth. Remarkably, for nine of the 20 galaxies that we observed, this signature is not seen at all," says Pieter van Dokkum, associate professor of astronomy and physics at Yale. "It gives a firm limit on the stellar birth rate in these objects."

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.

"Evidence for the presence of these black holes is seen in several of the galaxies studied, lending support to the idea that black holes serve as cosmic contraceptives in the young universe," says van Dokkum.

The research was funded by the Netherlands Foundation for Research, the Leids Kerkhoven-Bosscha Fonds, the National Science Foundation and NASA. Other authors on the paper were Ryan Quadri, Eric Gawiser, David Herrera, Danilo Marchesini and C. Megan Urry, Yale; Marijn Franx, Edward N. Taylor and Stijn Wuyts, Leiden; Garth D. Illingworth, University of California, Santa Cruz; Ivo Labbe, Carnegie Observatories, Pasadena, California; Paulina Lira, Universidad de Chile; Hans-Walter Rix, Max-Planck-Institute für Astronomie; Gregory Rudnick, National Optical Astronomy Observatory, Tucson, Arizona; and Sune Toft, European Southern Observatory, Munchen, Germany.

-- By Janet Rettig Emanuel

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Conference celebrates 60th year of Yale's Directed Studies Program

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Campus Notes

Correction