Astronomers discover eight buried dual AGN candidates
Astronomers discovered eight buried dual AGN candidates, the largest sample of hidden accreting supermassive black holes in late stage galaxy mergers, selected using NASA's Wide-Field Infrared Survey Explorer (WISE) space telescope. This result will be presented by graduate student Ryan Pfeifle of George Mason University (Fairfax, Virginia, U.S.) at the annual meeting of the European Astronomical Society (EWASS2019) in Lyon, France, on Friday, 28 June.
Observational campaigns and theoretical studies have shown both that supermassive black holes (SMBHs) reside at the centers of most galaxies and that galaxy interactions are ubiquitous in the Universe. As a result, galaxies grow and evolve hierarchically through collisions, which also fuel and consequently grow the SMBHs at their centers.
Late-stage mergers should in fact host dual accreting SMBHs, or active galactic nuclei (AGN)—unambiguous evidence for an ongoing merger—with pair separations smaller than 10 kpc and are predicted to facilitate the most rapid growth of the black holes. Decades of observational campaigns have demonstrated, however, the exceeding rarity of dual accreting SMBHs.
Not only do the existence, frequency, and accretion rates of such dual AGN systems have important astrophysical implications on the formation and growth of SMBHs and their connection to the host galaxies in which they reside, they are the precursors of SMBH mergers, which will be the most titanic gravitational wave events in the Universe—a topic that is of current interest.
In a follow-up study the international team of scientists also reports—using high-spatial resolution X-ray, near-IR, and optical spectroscopic diagnostics—a case of a triplet of accreting supermassive black holes with mutual separations < 10 kiloparsec (= 32615.63797 light year) in the advanced merger SDSS J084905.51+111447.2.
The dynamics of a triple massive black hole system can greatly shorten the merger timescale for two black holes.
Ryan Pfeifle explains: "The discovery suggests that this may be an important mechanism for driving the binary black hole inspiral and producing sources of gravitational waves."
In addition, triple interactions can result in slingshot ejections of one of the black holes with velocities high enough to escape the host resulting in either ejected or wandering supermassive black holes.
Pfeifle concludes: "The results of our study demonstrates the true power of mid-infrared selection: it can select cases of dual AGN candidates that would be missed by optical selection techniques, and it now offers a method through which we can preselect triple mergers to try and find more triple AGN."