Scientists Study the Ultimate Cookie Monster

Posted: June 8, 2009 at 1:00 am, Last Updated: June 5, 2009 at 4:34 pm

By Leah Kerkman Fogarty

Since the term “black hole” was coined in the 1960s, it has been used to describe everything from an insatiable appetite to a messy kid’s room. One physicist called black holes “the ultimate cookie monsters.”

So just what are these fascinating celestial phenomena? In short, stellar black holes are collapsed stars whose gravitational pull is so strong, not even light can escape their grasp.

Because they are indeed black, it is a challenge for astronomers to determine their presence in a galaxy. Usually, they can be found by observing the area surrounding them. Stars will rotate around a black hole’s gravitational field, and unlucky matter, such as gas, will spiral toward the black hole, eventually getting sucked into its abyss.

How Big? How Small?

Lev Titarchuk

Lev Titarchuk

Astrophysicists are still discovering new characteristics about black holes. For example, take Lev Titarchuk. He recently developed an innovative technique that allows scientists to gauge how large or small a black hole may be.

Titarchuk is a research professor in computational and data sciences in Mason’s College of Science who also works at the Naval Research Laboratory in Washington, D.C. He first suggested the technique a decade ago and, with his former graduate student and Mason alumnus Nikolai Shaposhnikov, has since been working on finding evidence to support it.

Titarchuk’s technique weighs a black hole by looking at its accretion disk, which is the area surrounding the collapsed star that hosts spiraling matter before it falls into the hole’s center.

Titarchuk has found that the distance between where matter piles up on the accretion disk and the black hole’s center is directly related to its mass.

“Our method can measure a black hole’s mass when the optical observation method fails,” Titarchuk explains.

He and Shaposhnikov have tested their technique on several black holes near our galaxy. When compared with results from other methods of measurement, their method determines the masses of the black holes with only a minor margin of error.

At a meeting of the American Astronomical Society’s High Energy Astrophysics Division last year, they presented evidence of a black hole with a solar mass of 3.8, or 3.8 times the mass of our sun. When compared with the Milky Way’s black hole, which boasts a solar mass of 3.7 million, Titarchuk and Shaposhnikov’s black hole is downright tiny.

Titarchuk emphasizes that his method can be used to determine the mass of any black hole, large or small. The technique can also be used to establish the distance between a black hole and the sun.

If You Can’t See It, How Do You Know It’s There?

Shobita Satyapal

Shobita Satyapal

Shobita Satyapal is also making strides in the world of black holes.

An associate professor in Mason’s Physics and Astronomy Department, Satyapal focuses on extragalactic astronomy, that is, the study of galaxies outside the Milky Way.

To understand how a particular extragalactic galaxy forms and evolves, Satyapal looks at what lies in the center of most galaxies: a black hole.

Galaxies fall into two main categories: elliptical and spiral, or skinny. Elliptical galaxies are a collection of stars in a spherical shape or bulge, while skinny galaxies are shaped like a flat disk.

Astronomers once believed that black holes reside only in the middle of elliptical galaxies since that was the only kind of galaxy with an observable black hole. Furthermore, a correlation seemed to exist between the size of a galaxy’s bulge and its black hole.

Satyapal explains, “One of the questions we wanted to answer in our research was, is a bulge somehow necessary for a black hole to form? Can you find black holes in galaxies that have no bulges?”

NASA photo

NASA photo

Because galaxies without bulges have a lot of dust and gas in their center, astronomers have not been able to infer the presence of a black hole because visible light can’t be seen through the dust.

So Satyapal used a tool that no one had used before to investigate these low- to no-bulge galaxies—NASA’s Spitzer Space Telescope. This infrared telescope has a longer wavelength than optical light has and is able to penetrate to the center of these dusty, skinny galaxies.

By looking for the high-energy radiation that’s only known to occur with active black holes, Satyapal and her research team set out to discover whether these skinny galaxies hosted black holes.

Of the 33 skinny galaxies Satyapal surveyed, eight were home to black holes. These black holes are significant, too, she says, since all are at least 100,000 times the size of our sun. Using optical light, none of these black holes was visible and therefore wouldn’t have been detected without the use of infrared.

“Now, we know that black holes actually do reside in galaxies with no bulge,” says Satyapal. “That’s an important result because it implies that galaxies don’t require a bulge to form and grow a black hole. That will have a critical impact on theories of galaxy formation and evolution.”

This article originally appeared in Mason Research 2009.

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