How To Weigh a Black Hole: Just Look At Its Galaxy

My father was a rancher, who had the uncanny ability to accurately estimate the weight of each animal in his herd of cattle simply by looking at them. Today, at the American Astronomical Society meeting in St. Louis, astronomers announced a new, simple way of determining masses of super-massive black holes by just looking at images of galaxies. Dr. Marc Seigar from the University of Arkansas at Little Rock has been studying images available at the Hubble Space Telescope archive site, and looking at the how tightly the galaxy’s arms wrap around itself in relation to the size of the galaxy’s super-massive black hole. “This provides a much simpler method of determining black hole mass,” said Seigar. “You just need an image of a galaxy and you can measure the tightness of the spiral structure. This can easily be applied to distant galaxies, up to 8 billion light-years away.”

Usually astronomers determine masses of super-massive black holes by looking at how fast the stars are moving in the central regions of the galaxies. But that method only works for nearby galaxies. Astronomers have been looking for a new method for galaxies that are father away. This new inexpensive method can use already-existing images, as Seigar has used from the Hubble Site.

Seigar and his team looked at photographs of 27 spiral galaxies including the Milky Way and the Andromeda Galaxy. They observed galaxies with the smallest black holes had spiral arms with angles 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 the arms and the bulge.

Seigar said its also possible that the main factor in determining the mass of a super massive black hole is the amount of central concentration of dark matter in a galaxy. “We think dark matter is driving most of the relationships between black holes and their galaxies,” he said. “The masses of these black holes can be determined indirectly from the characteristics of the light emitted from in falling material.”

Seigar will continue to use this method to verify his findings, as well as looking at other aspects. “We have to determine if the relationship between spiral arms and black hole mass evolves over time.”

Source: AAS press conference