Tight Arms Linked to Bigger Black Holes in Galaxies

Anne Minard
for National Geographic News
June 2, 2008
Images from the Hubble Space Telescope have revealed an important relationship between supermassive black holes and spiral galaxies: The more massive the black hole, the tighter the host galaxy's arms.

"This means that to determine the mass of a supermassive black hole, you only need an image of a galaxy," said Marc Seigar of the University of Arkansas at Little Rock.

Within the past decade astronomers have determined that almost every galaxy has a black hole lurking at its center that can range from ten thousand times to a billion times the mass of the sun.

Astronomers usually measure a supermassive black hole based on how fast stars are swirling around it—the faster the stars' orbits, the more massive an object must be present.

But that method only works for relatively nearby galaxies where individual stars can be observed, said Seigar, who announced the find today at the 212th meeting of the American Astronomical Society in St. Louis, Missouri.

For more distant galaxies, astronomers have been searching for a different technique—and looking at the positions of spiral arms could be a solution, based on comparisons of 27 spiral galaxies near enough to size up their black holes.

Using Hubble pictures, Seigar and colleagues found that galaxies with smaller black holes had angles of up to 43 degrees between the arms and the central bulge, while galaxies that harbored enormous black holes had arms at angles of about 7 degrees to their centers.

"The real significance of this is that it is relatively easy to detect spiral structure in galaxies, even out to distances of eight billion light-years," Seigar said. "So we can estimate the masses of very distant black holes now and determine how black holes grow over time."

Messy Structures

New Hubble images are also helping astronomers study the tumultuous events that give rise to starburst and quasar galaxies.

Mike Brotherton at the University of Wyoming said the pictures provide support for a leading theory of galaxy evolution: "that galaxy interactions and mergers are a key step."

Galactic mergers are believed to trigger "starburst" periods of stellar birth, although dust and debris can shroud the new stars from view.

And if the interaction fuels the combined galaxy's supermassive black hole, it can produce a halo of matter called a quasar.

The quasar will blow out debris, clearing a view to the newborn stars. But often by that time the stars have become too faint to outshine the powerful quasar.

Brotherton and his team found an object in the late 1990s that possessed the spectral signatures of both a quasar and an older starburst.

Hubble captured images of the rare object and showed that it was the remnant of a galaxy merger.

The researchers have since used Hubble to follow up on 29 candidate quasars identified by the Sloan Digital Sky Survey.

"The images started coming in, and we were blown away," Brotherton said.

"We see not only merger remnants … but also post-starburst quasars with interacting companion galaxies, double nuclei, starbursting rings, and all sorts of messy structures."

(Related pictures: "'Toothbrush,' 'Firefly' Among Galactic Smashups" [April 24, 2008].)

Star Lab

Conference presenter Médéric Boquien, an astronomer at the University of Massachusetts, added that debris from galaxy collisions can be used as a laboratory to study the process of star formation.

Boquien's team has been using the Spitzer Space Telescope, Galaxy Evolution Explorer, and eight ground-based telescope to peer at six interacting galaxy systems.

All of the systems are within 375 million light-years of Earth.

In each of the interacting galaxies, up to 85 percent of star formation takes place in the collision debris between galaxies instead of inside the parent galaxies themselves.

"The best regions to study star formation would be those completely devoid of old stars, and we were able to find some regions which satisfy this criteria," Boquien said.

Such areas are generally quite isolated, unlike the star-forming regions inside galaxies, which can be surrounded by many bright astronomical objects.

"As star formation apparently occurs in a similar way in galaxies, results we obtain studying intergalactic star-forming regions can be confidently extended to galaxies."

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