Dr. Marc Seigar, assistant professor of Physics and Astronomy at the University of Arkansas at Little Rock, and his research team have concluded that the larger the black hole at the center of a spiral galaxy, the tighter the galaxy's arms wrap around itself. If correct, the simple relationship would give researchers an easy way to learn about black holes.

"This is a really easy way to determine the masses of these super-massive black holes at the centers of galaxies that are very far away," Seigar said. "This gives us a way to measure the size of these black holes out to larger distances than ever before, up to 8 billion light-years away."

Since supermassive black holes were discovered in nearby galaxies, researchers have been determining their masses by looking at how fast the stars were moving in the very central regions of those galaxies. But that method only works for relatively nearby galaxies, Seigar said. "For galaxies out to the distances we're talking about, you have to develop alternative methods," Seigar said. "And we have developed such a method."

Seigar's team studied photographs of 27 spiral galaxies, including our galaxy, the Milky Way, and its nearest neighbor, the Andromeda Galaxy. Galaxies with the smallest black holes had spiral arms with angles of up to 43 degrees between the arms and the central bulge. Those with the biggest black holes had spiral arms at angles of only 7 degrees between their arms and central bulges.

"One of the important reasons to learn about the very distant black holes is, when you are looking at galaxies very far away, you are looking at them as they were in the past, so you can learn about how masses of black holes grow over time," Seigar said.

The black holes he is studying are supermassive black holes that are millions or billions of times more massive than our sun. Since they seem to be found at the centers of all galaxies, they could be a key element of how galaxies form in the first place. The growth in the mass of these black holes should be related to the growth in the sizes of the galaxies in which they are found.

His studies also indicate that the mass of a black hole may depend on how centrally concentrated the dark matter is in a galaxy. "But that is a hypothesis that has yet to be proven," he said. "We're going to work on that."

Seigar's team will next study more distant active black holes, which are in the process of devouring their surroundings. They are easier to measure than their tranquil cousins, but they are also rare and can be weighed within about 5 billion light-years from Earth.

The researchers want to make sure that the simple relationship they found between spiral arm and black hole mass does not change as the galaxies evolve over time. Says Seigar: "We have to make sure the correlation itself doesn't evolve."

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