Even Thin Galaxies Pack Hefty Black Holes
An artist's illustration showing two types of spiral galaxies that populate our universe: those with plump middles, or central bulges (upper left), and those lacking the bulge (foreground). New observations show slender, bulgeless galaxies can harbor supermassive black holes at their cores like fatter galaxies.
Credit: NASA/JPL-Caltech

AUSTIN, Texas — A half-dozen hefty black holes hide out where they are least expected, in relatively skinny galaxies. The discovery implies galaxies don't need bulging bellies to harbor monstrous black holes.

Until now, astronomers had thought central concentrations of stars called galactic bulges were required for black holes to grow. Our own Milky Way, like most spiral galaxies, has such a bulge. In many large galaxies, the bulge feeds binge eating by the black hole, creating a chaotic scene of high-speed consumption and copious radiation.

Some smaller and thinner galaxies — those without bulges — lack evidence for supermassive black holes.

New observations from the Spitzer Space Telescope, however, show galactic obesity isn't the only path to black hole generation.

"This finding challenges the current paradigm," said Shobita Satyapal of the George Mason University in Fairfax, Va. "The fact that galaxies without bulges have black holes means that the bulges cannot be the determining factor." Satyapal presented the research here at a meeting of the American Astronomical Society.

Instead, they think dark matter, an invisible substance thought to account for about 85 percent of all matter in the universe, might play a role in the early development of supermassive black holes.

Bulge or no bulge

Past research has shown the more massive the bulge, the more massive the associated black hole. That led astronomers to think a galaxy's bulge and its supermassive black hole were intimately related.

For instance, our Milky Way is a massive disk of stars as seen from above. But if viewed edge-on it would appear like the shape of an airplane viewed head-on with its wings extended. The wings represent the galaxy's disk, and the bulbous fuselage would be the bulge.

The mostly dormant black hole at the center of our galaxy weighs several million times the mass of the sun. In more active galaxies, black holes can surpass a billion solar masses. (The record-setter, at 18 billion solar masses, was announced this week.)

"Scientists reasoned that somehow the formation and growth of galaxy bulges and their central black holes are intimately connected," Satyapal said.

A few recent studies have caused astronomers to question the conventional thinking. In 2003, scientists discovered a relatively "lightweight" supermassive black hole in a bulge-less galaxy. And even more recently, Satyapal and her team spotted another supermassive black hole lurking in a similarly svelte galaxy.

Now, Satyapal and her colleagues have turned up six more monstrous black holes in thin galaxies with minimal bulges.

Veiled by dust

Satyapal suggests these black holes were only recently detected because they have been shrouded by their dusty abodes. Galaxies with such minimal midsections tend to be extremely dusty. Infrared light can penetrate the dust, meaning Spitzer could unmask the black holes with its infrared capabilities.

"A feeding black hole spits out high-energy light that ionizes much of the gas in the core of the galaxy," Satyapal said.

Perhaps, Satyapal speculates, the missing piece of the black-hole puzzle is dark matter. Early on in the galaxy's life, this invisible matter might somehow set the mass of the black hole. Other theorists have figured in recent years that dark matter was integral to galaxy formation.

"Maybe the bulge was just serving as a proxy for the dark matter mass," Satyapal said. So the amount of dark matter is "the real determining factor behind the existence and mass of a black hole in a galaxy's center."

The study will be published in the April 10 issue of the Astrophysical Journal.

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