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Spring 2009, Issue 10

What Came First—the Galaxy or the Black Hole?

Marianne VestergaardMarianne Vestergaard, PhD, joined the Department of Physics and Astronomy in 2007. Vestergaard earned her MSc and PhD in astrophysics from the astronomy department of the Niels Bohr Institute at the University of Copenhagen. She did dissertation research at the Harvard-Smithsonian Center for Astrophysics and postdoctoral research at Ohio State University and the University of Arizona. Her research interests include the physics of the nuclear engines of quasars, the mass of black holes, and the relation of an active black hole to its galaxy.

When you look deep into the night sky, you are looking back in time. The light from distant galaxies takes time to travel the vast distances across the universe to Earth. So we see distant galaxies as they were millions or billions of years ago when light left them. Vestergaard analyzes data from large telescopes that detect visible light and other electromagnetic radiation. Looking at these data, she can see the effects of events that occurred soon after the universe came into being, when galaxies were forming. “The universe we believe right now is 13.8 billion years old, so we know and study quasars where the light has been traveling for up to 13 billion years,” says Vestergaard.

Most of the data Vestergaard is currently working with are from the Sloan Digital Sky Survey, a collaborative project that maps part of the universe from Apache Point Observatory in New Mexico. “The idea was to survey about a quarter of the sky and go very deep—deeper than has ever been done before—and do both imaging and spectroscopy,” says Vestergaard. “Spectroscopy gives us information about the intensity at different frequencies and different energies, and that’s really a fingerprint of what that object contains. What elements are in there? How fast are things moving? What is the temperature of the object? It’s a bit like a genetic code.”

A black hole is a region of space with gravity so strong that not even light can escape. Scientists believe all galaxies have black holes. Our own Milky Way Galaxy has a relatively small, quiet black hole. Some black holes are supermassive and extremely active. An active galactic nucleus, which is composed of a supermassive black hole and its surrounding accretion disk of matter, emits vast amounts of energy (as visible light and other electromagnetic radiation) as matter is pulled into the black hole.

Galaxies are thought to have formed from matter and energy that originated in the Big Bang, the theoretical explosion that started the expansion of the universe. How supermassive black holes formed at the center of active galaxies is under lively debate. The dominant school of thought is that matter clumped together to form stars during the initial phases of the first galaxies, and some stars were so large and dense that they could not withstand their own gravity and imploded to create black holes. Another school of thought, to which Vestergaard’s research has contributed, is that black holes came into being first and stars formed around them to create galaxies.

Eight to ten years ago, astronomers had data from only 4,000 or so very distant and very bright active galactic nuclei called quasars. Thanks to the Sloan Digital Sky Survey, they now have data from nearly 100,000 quasars, and these data were obtained using the same telescope and instruments, and with the same observing and selection criteria. “That means now we can do statistics!” Vestergaard says with a broad smile. “I’ve been taking the spectra, modeling them, and using the measurements to determine the mass of black holes. Then I’ve been looking at what the distribution is across the age of the universe, and we’re finding that even early on black holes are very massive.” She has also found evidence that the galaxies around these early supermassive black holes were very young, with intense star formation. Other astronomers have established that the mass of a black hole and the mass of its galaxy are strictly correlated. These data support the theory that early black holes formed first and galaxies formed around them. “But we need a lot more data on this to know for sure if this hypothesis is correct,” says Vestergaard.

The James Webb Space Telescope is a large, infrared-optimized instrument scheduled for launch in 2013 that should provide data that will help astronomers answer some of these questions. Until then, Vestergaard is hard at work optimizing the use of currently available data. She would like to collaborate with a computer scientist or statistician interested in astronomy, or an astronomer interested in specializing in astrophysical statistics. Vestergaard believes the paucity of such specialists is holding the field of astronomy back. “There are a few statisticians that we rely on,” says Vestergaard. “There are only probably a handful of people in the entire field, compared to how many thousands of astronomers there are.” Vestergaard welcomes anyone interested in collaboration or more information on her research to contact her at m.vestergaard@tufts.edu.

For more information on Vestergaard’s research, please go to http://cosmos.phy.tufts.edu/~mveste01.

 

 

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