Black Holes, Galaxy Mergers, Quasars: A Quest to Understand the Ordinary

There are more than 100 billion galaxies in the universe, according to most estimates. Some estimates go as high as 2 trillion (a “2” followed by 12 zeros). Whether hundred billion or trillion, the fact is that there are a lot of galaxies. Most of us, on this tiny planet orbiting a random star in an average-sized galaxy, imagine black holes and galaxy collisions to be rare and exotic. They’re not. The closest major galaxy to us is Andromeda. It takes light about 2.5 million years to reach us from Andromeda, but that’s pretty close on the scale of the universe. Astronomers predict that about 4.5 billion years from now, our Milky Way will collide and merge with Andromeda to form one giant galaxy. Depending on how things go, life forms in distant galaxies could see evidence of this merger in the form of quasars—bright, powerful objects that form at the center of galaxies. Generally speaking, quasars are the result of black holes devouring a huge supply of dust and gas. These “supermassive” black holes eat everything within the boundary of their event horizons. However, some of the particles on the outskirts of an event horizon have a different fate—they are sent shooting into space at nearly the speed of light, forming jets that we observe as quasars. As far as astronomers can tell, all galaxies have a black hole at their center, and whether the black hole produces a quasar depends on the amount of "food" available for consumption. For this reason, some galaxies may have a quasar early in their formation when gas and dust is plentiful that dies out over time. Galaxies can also form quasars during mergers, when the force of the merger pushes gas and dust toward the black holes. Exactly how galaxy mergers power quasars is an open question that’s difficult to study with observations. Galaxy mergers can be hard to identify—from a distance it’s hard to tell the difference between a single galaxy and two that are very close together. But last week at the 232nd meeting of the American Astronomical Society, Scott Barrows from the University of Colorado, Boulder (UC) presented a method for identifying quasars in merging galaxies that has led to new insights. The team, which includes Barrows, Julie Comerford from CU, Jenny Greene from Princeton University, and David Pooley from Trinity University, identified merging galaxies by comparing two types of data—images of galaxies from surveys like the Sloan Digital Sky Survey and images by NASA’s x-ray observatory, Chandra. Quasars produce a strong x-ray signal, so by overlaying x-ray data on visible images of the same region of sky, the team can pinpoint the location of quasars in galaxies. Here’s the key—supermassive black holes live at the center of galaxies unless the galaxy is in the midst of a merger. In that case, forces between the galaxies throw off the symmetry. Put simply, off-center quasars indicate off-center black holes, which indicate a galaxy merger. This color-composite image from the Sloan Digital Sky Survey illustrates the concept of an off-center quasar. The image shows the merging galaxy system known as NGC 3341. The nucleus indicates the center of the system from which two bright objects are offset. Object B was determined to a quasar. Image Credit: Barth, A. J., Bentz, M. C. Greene J. E., & Ho, L. C. 2008, ApJ, 683, L119–L122. Reproduced by permission of the AAS. Using this method, the team identified mergers via galaxies with off-center quasars. After identifying a merger, the researchers examined high resolution images of the region taken by the Hubble Space Telescope. From the images, they classified each merger as between galaxies with similar masses or between galaxies with very different masses. Some mergers had one quasar, others had two. In an interesting new result, says Barrows, mergers with just one quasar tend to be between two galaxies with very different masses. This suggests that mergers between galaxies with similar masses are more violent—causing more structural disturbances—than mergers between galaxies with very different masses. Most likely there is only one quasar in these cases because only one of the black holes has sufficient fuel. On the other hand, the researchers found that mergers with two quasars, one in each galaxy, can result from galaxies with similar masses and galaxies with very different masses. The reasons why aren’t clear yet, but more complicated factors seem to be at play. The ultimate goal of this project is to learn more about where and when quasars form during mergers, and then compare observational data to theoretical predictions in order to get at the how. In this galaxy-filled universe, mergers happen all the time. But even things that are ordinary can be challenging to study from our vantage point on this tiny planet.