A hefty black hole in an otherwise spindly galaxy has astronomers wondering if there is any limit to the range of configurations for galaxies and the gravity wells they sometimes harbor.
Astronomers examined the center of a pancake-thin spiral galaxy, which ought not have a significant central black hole, theory says. Yet they found one that packs the mass of about 66,000 suns.
That's a far cry from the heft of our own Milky Way's black hole, said to be about 2.6 million solar masses. But the Milky Way is pudgy. Like most large spiral galaxies, it has a bulging stellar belly, an obvious spherical region around the black hole that is loaded with stars, most of them very old.
Other supermassive black holes are suspected of weighing as much as a billion suns, and they too are found in galaxies with central bulges.
The latest study, of a relatively nearby galaxy called NGC 4395, calls into question a fairly well established assumption that black holes and bulges go hand-in-hand.
able --> three years ago of the black hole in NGC 4395. In fact, this galaxy has intrigued astronomers for a long time. In 1993, while a graduate student, Ho looked into the galaxy with Alex Filippenko of the University of California, Berkeley. They used the Hubble Space Telescope to confirm even older observations that the luminous center of NGC 4395 was likely powered by a black hole.The long road to firmer evidence -- still not proof -- illustrates just how elusive black holes can be.
Black holes can't be seen, because they trap all matter and light that falls into them. But scientists have long theorized that they come in two main flavors.
The supermassive variety develops along with a galaxy and resides at its center, most astronomers figure. Despite incredible mass, each supermassive black hole makes up less than 1 percent of its host galaxy's mass. The other type, stellar black holes, are no more massive than a few hundred suns. Most are less than a dozen times as massive as the Sun. Each is thought to form via the collapse of a dying star.
Theorists can't say for sure how supermassive black holes come to be.
In recent years, evidence has turned up for middleweight black holes that can be thousands of solar masses. They might be the building blocks of supermassive black holes, some astronomers think. Their existence remains controversial, however.
The black hole in NGC 4395 is none of the above. Or, rather, it might be called a featherweight in the supermassive category, suggest Ho and Filippenko, who teamed up again for this latest study.
"The supermassive black hole in NGC 4395 is the smallest one yet found in the center of a galaxy," Filippenko said.
"On the other hand," Filippenko told SPACE.com, "there is a central star cluster." Measuring the speeds of stars in the cluster allowed the calculation of the black hole's mass. "So in a sense, the star cluster is like a little tiny bulge, though not a classic bulge."
NGC 4395 is 11 million light-years away. The first strong evidence for its black hole came from X-ray observations, made by the Japanese-U.S. ASCA X-ray telescope. As matter spirals toward a black hole, it is accelerated to a significant fraction of the speed of light, gets superheated, and gives off X-rays.
The latest observations, with the Keck I telescope in Hawaii, determined the black hole's mass by measuring the average velocity of stars near the galactic center, in the cluster. Because astronomers can't see black holes, they use the speed of stars to tell them how much mass must be packed into the heart of a galaxy, driving the stars that race around the center
The thin galaxy with the surprising black hole may be at an evolutionary endpoint, however. And it is perhaps an example of what happens to loners. Our Milky Way has thrived and grown and fed its black hole through the consumption of countless other galaxies, astronomers believe.
"NGC 4395 doesn't have enough material close to the center to ever develop a big bulge and a big black hole," Filippenko said, "unless it undergoes a merger with another galaxy later."
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