What weighs thousands of times as much as the sun, emits powerful bursts of X-rays and is so dense that even light isn’t fast enough to escape its gravitational pull? The answer may be a new class of black holes.A group of University astronomers recently discovered strong evidence for a new type of mid-sized black hole in a nearby galaxy called M74. Traditionally, there are two known black hole sizes. First are the stellar mass black holes that form when massive stars die in spectacular explosions called supernovae. Stellar mass black holes are around a dozen solar masses. The second type is found deep in the cores of galaxies. These super-massive black holes are millions to billions of times as massive as the sun.The newly discovered intermediate-mass black hole, however, appears to be several thousand times larger than stellar mass black holes, but still far too small to be classified as a super-massive black hole. Astronomers have yet to understand the origins of this new breed of black holes, but the puzzling objects may give clues to the process of galaxy formation.Despite their name, black holes are among the most luminous objects in the night sky. When matter falls into a black hole, it doesn’t go without a fight. Matter falling into the black hole tends to have some amount of rotation. This rotation resists gravity and causes the falling matter to form a disk of gas around the black hole. “We expect a disk around (the black hole), and we expect a very high temperature in the disk,” said Rackham student Jifeng Liu, who is the lead author on the recent paper in the March 1 issue of the Astrophysical Journal Letters that gives evidence for the presence of an intermediate mass black hole. In its dying gasp, the matter falling into the black hole heats up and emits X-rays that can be detected with space telescopes“We think we know (the intermediate mass black holes are) there. The question is how to find them,” said Liu. Liu and his colleagues used the Chandra and XMM Newton space telescopes to study what is known as an ultra-luminous X-ray source or ULX in the galaxy M74 . ULXs are rare objects that are thousands of times brighter than stellar mass black holes. “From the luminosity itself, you’d imagine it’s an intermediate mass black hole,” Liu said. Other astronomers believe that ULXs are actually stellar mass black holes, whose X-ray emissions are directed at the Earth like the beam from a lighthouse. Astronomy Prof. Joel Bregman, the second author on the paper, said he originally believed the same theory. “For a long time I thought beaming had to be the answer,” he said.But the ULX in galaxy M74 was more than just bright. It also showed a quasi-periodic oscillation or QPO, meaning its intensity changes in an almost regular pattern. “You don’t really expect (oscillations) from a jet, but you do from a disk,” Bregman said. Since intensity of light from the black hole exhibited a periodic pattern, this led the team to conclude that the object was not simply a stellar mass black hole beaming its X-rays directly to the Earth. Moreover, the astronomers could deduce the size of the black hole from the oscillations of the black hole. “This type of quasi-periodic behavior seems to depend on mass,” he added. Similar observations of small as well as ultra-massive black holes have been made. The period of oscillations increases with black hole mass. The observed period led the team of astronomers to estimate the black hole in M74.The existence of intermediate sized black holes has implications in understanding galaxy formation. “If you want to form a galaxy, you need a core,” Liu said. One possibility that astronomers have considered is that intermediate mass black holes formed from the first stars in the universe, which were more massive than present stars. Those black holes merged to form intermediate mass black holes which in turn formed the present supermassive black holes that are the anchors for galaxies. “You need these intermediate black holes as a seed,” Liu said. If the theory is correct, “there should be leftovers,” he added.This isn’t the only explanation for mid-sized black holes. One theory proposes intermediate mass black holes are the result of hundreds of stellar mass black holes merging at the center of a star cluster. Another suggests that they could be the remnant cores of small galaxies that have collided with, and are being incorporated into, larger galaxies. The situation of multiple explanations for a new phenomenon “often happens in astronomy,” Bregman said. “In the end, you just have to let nature lead you.”Other authors on the paper are Ed Lloyd-Davies, Jimmy Irwin, Catherine Espaillat and Patrick Seitzer, all from the University astronomy department.

Chelsea Trull
Spiral galaxy M74, which may contain a new class of black hole. (Photo Credit: NASA/CXC/U. Michigan/J. Liu Photo)

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