Astronomers have discovered what may be the most compact binary supermassive black hole ever known in a galaxy about 400 million light years from Earth.
An artistic illustration of a binary black hole. Image credit: NASA / ESA / G. Bacon, STScI.
The binary supermassive black hole is located inside the gas-rich interacting spiral galaxy NGC 7674 (Markarian 533) in the constellation Pegasus.
The apparent separation of the two supermassive black holes in this system is less than one light-year.
“The dual black hole we found has the smallest separation of any so far detected through direct imaging,” said Professor David Merritt, from Rochester Institute of Technology.
“The combined mass of the two black holes is roughly 40 million times the mass of the Sun, and the orbital period of the binary is about 100,000 years.”
“A class of smaller black holes form when massive stars explode as supernovae. A collision of stellar mass black holes led to the landmark discovery of gravitational waves in 2015 using the Laser Interferometer Gravitational-wave Observatory (LIGO),” he said.
“A supermassive binary generates gravitational waves with much lower frequency than the characteristic frequency of stellar-mass binaries and its signal is undetectable by LIGO.”
NGC 7674 (seen just above the center) is the brightest and largest member of the so-called Hickson 96 compact group of galaxies, consisting of four galaxies. This Hubble image shows NGC 7674 nearly face-on. The central bar-shaped structure is made up of stars. The shape of NGC 7674, including the long narrow streamers seen to the left of and below the galaxy, can be accounted for by tidal interactions with its companions. Image credit: NASA / ESA / Hubble Heritage Team / STScI / AURA / A. Evans, University of Virginia, Charlottesville / NRAO / Stony Brook University.
To simulate a highly sensitive detector, Professor Merritt and colleagues used a method to make radio telescopes around the world work together as a single large telescope and achieve a resolution roughly 10 million times the angular resolution of the human eye.
“Using very long baseline interferometry (VLBI) techniques, two compact sources of radio emission were detected at the center of NGC 7674,” Professor Merritt explained.
“The two radio sources have properties that are known to be associated with massive black holes that are accreting gas, implying the presence of two black holes.”
NGC 7674 loudly emits radio waves. The detection confirms a theory predicting the presence of a compact binary in radio galaxies bearing a ‘Z’ shape.
“The name ‘Z-shaped’ refers to the twisted morphology of the galaxy’s radio emission on much larger scales,” Professor Merritt said.
“This morphology is thought to result from the combined effects of the galaxy merger followed by the formation of the massive binary.”
A paper reporting the discovery is published in the journal Nature Astronomy.
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P. Kharb et al. A candidate sub-parsec binary black hole in the Seyfert galaxy NGC 7674. Nature Astronomy, published online September 18, 2017; doi: 10.1038/s41550-017-0256-4
