For the first time ever, astronomers have been able to observe and measure the orbital motion between two supermassive black holes — a discovery more than a decade in the making. A paper reporting this discovery is published in the Astrophysical Journal.
This artist’s conception shows two supermassive black holes, similar to those observed by Bansal et al, orbiting one another more than 750 million light years from Earth. Image credit: Josh Valenzuela, University of New Mexico.
The two supermassive black holes reside in 0402+379, an elliptical radio galaxy approximately 750 million light-years from Earth.
They have a combined mass of 15 billion solar masses and are separated by only about 24 light-years, extremely close for such a system.
“This is the first pair of black holes to be seen as separate objects that are moving with respect to each other, and thus makes this the first black-hole ‘visual binary’,” said co-author Greg Taylor, professor in the Department of Physics and Astronomy at the University of New Mexico.
“If you imagine a snail on Proxima b, the recently discovered Earth-like planet orbiting Proxima Centauri, moving at one centimeter a second, that’s the angular motion we’re resolving here,” said co-author Roger W. Romani, professor in the Department of Physics at Stanford University.
Supermassive black holes reside at the cores of most galaxies.
The presence of two such monsters at the center of a single galaxy means that the galaxy merged with another some time in the past.
In such cases, the two black holes themselves may eventually merge in an event that would produce gravitational waves that ripple across the Universe.
“We believe that the two supermassive black holes in this galaxy will merge,” said lead author Karishma Bansal, a graduate student at the University of New Mexico.
“The merger will come at least millions of years in the future.”
The galaxy 0402+379 was first observed in 1995. It was studied in 2003 and 2005 with NSF’s Very Long Baseline Array (VLBA).
Based on finding two cores in the galaxy, instead of one, Prof. Taylor and co-authors concluded in 2006 that it contained a pair of supermassive black holes.
Their latest research incorporates new VLBA observations from 2009 and 2015, along with re-analysis of the earlier VLBA data.
This work revealed motion of the two cores, confirming that the two black holes are orbiting each other.
“What we’ve been able to do is a true technical achievement over this 12-year period using the VLBA to achieve sufficient resolution and precision in the astrometry to actually see the orbit happening,” Prof. Taylor said.
“It’s a bit of triumph in technology to have been able to do this.”
One of the black holes in 0402+379 moved at a rate of just over one micro-arcsecond per year, an angle about 1 billion times smaller than the smallest thing visible with the naked eye.
Based on this movement, the team hypothesizes that one black hole may be orbiting around the other over a period of 30,000 years.
“We need to continue observing this galaxy to improve our understanding of the orbit, and of the masses of the black holes,” Prof. Taylor said.
“This pair of black holes offers us our first chance to study how such systems interact.”
The researchers also hope to discover other such systems.
“Now that we’ve been able to measure orbital motion in one such pair, we’re encouraged to seek other, similar pairs,” Bansal said.
“We may find others that are easier to study.”
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K. Bansal et al. 2017. Constraining the Orbit of the Supermassive Black Hole Binary 0402+379. ApJ 843, 14; doi: 10.3847/1538-4357/aa74e1
