Active galactic nuclei reveal the presence of enormous amounts of matter interacting with a supermassive black hole at the center of a galaxy. Because galaxies merge over the history of the Universe, we should see the signatures of supermassive black holes merging in some places. An analysis of X-ray observations may have shown just that.
X-ray image of SDSS J171544.05+600835.7 from the Chandra X-ray observatory, showing two locations of high intensity. X-rays are signatures of AGN processes, where matter interacts with a supermassive black hole.
Two fundamental tenets of modern galaxy formation theory are that big galaxies form through mergers of smaller galaxies, and that nearly all galaxies host supermassive black holes (SMBHs) at their cores. Putting these facts together implies that some galaxies, having recently undergone a merger, will host two SMBHs before they spiral into the center of the new galaxy and combine to form a single bigger black hole. During some of this time, both SMBHs could consume large amounts of gas, causing them both to shine brightly in X-rays as active galactic nuclei (AGN).
In addition to galaxy formation and growth, there is the crucial question of SMBH growth in the Universe, which suggests that black holes of the size present in the Universe could have only come about through mergers. Studying such galaxies in the real Universe, where SMBHs are seen to be in the process of merging, provides a crucial piece in the twin puzzles of galaxy and SMBH formation, but thus far, finding them has been difficult. The most famous example is a galaxy named NGC 6240, a well-known cosmic train wreck, whose highly disturbed, disorganized appearance strongly indicates that it has undergone a significant recent merger event. Observations with the Chandra Space telescope show strong X-ray emission from two different objects near the center of this galaxy, both of which clearly indicate the presence of an AGN.
Such visually obvious mergers are relatively rare, but theory suggests that the dual SMBHs should remain present well after the most conspicuous signs of the merger have faded. At that point the SMBHs would be spiralling around each other closer and closer until they finally get to a common center in about 100 million years. Finding supermassive black holes in a later stage of merger is an important test of the theory. A recently popular approach to searching for subtler dual-SMBH galaxies involves looking at particular emission lines in galaxy spectra that are associated with AGN. Detection of two such lines at slightly different wavelengths, caused by the different motions relative to us as SMBHs orbit each other, can signal the presence of two AGN. The difficulty with this approach is that similar double lines can also be caused by hot gas being expelled in opposite directions from the neighborhood of a single SMBH.
Recently, KIPAC researchers Brian Gerke and Greg Madejski, along with their collaborators Julie Comerford and Dave Pooley at the University of Texas, have been using Chandra to observe some promising double-line galaxies to look for X-ray emission from two AGN, in the manner of NGC 6240. In a recent paper, they reported the presence of two X-ray sources in one of their target galaxies, which goes by the glamorous name of SDSS J171544.05+600835.7. The most likely explanation for their data is indeed the presence of two orbiting SMBHs shining as AGN and in the process of merging, although they caution that the X-ray emission could also come from hot, outflowing gas from a single AGN. In the later case, however, one would also expect bright X-ray emission from the location of the supposed single central AGN itself, which is not seen, though such emission could potentially be obscured by dust.
The observations strongly point toward the two black holes interpretation. Meanwhile, the team is awaiting Chandra data from two other promising candidate dual-AGN systems. The smoking gun for this particular object would be a high resolution radio image from an interferometer, where the jet structure of the AGNs could be seen in greater detail. The upcoming data will help to clarify the nature of this system and other double-line AGN, and with that may be another step toward understanding galaxy - and black hole - formation and evolution in the history of the Universe.
This highlight is based on a paper submitted to the Astrophysical Journal Letters and available from astro-ph at arXiv:1106.0746.
Tidbit Authors: Brian Gerke and Jack Singal