Washington: Scientists using NASA`s Swift satellite and the Chandra X-ray Observatory has found a second supersized black hole at the heart of an unusual nearby galaxy already known to be sporting one.
The galaxy, which is known as Markarian 739 or NGC 3758, lies 425 million light-years away toward the constellation Leo. Only about 11,000 light-years separate the two cores, each of which contains a black hole gorging on in-falling gas.
"At the hearts of most large galaxies, including our own Milky Way, lies a supermassive black hole weighing millions of times the Sun`s mass," Michael Koss, the study`s lead author at NASA`s Goddard Space Flight Center in Greenbelt, Md., and the University of Maryland in College Park (UMCP), said.
"Some of them radiate billions of times as much energy as the Sun," Koss revealed.
Astronomers refer to galaxy centres exhibiting such intense emission as active galactic nuclei (AGN). Yet as common as monster black holes are, only about one percent of them are currently powerful AGN.
Binary AGN are rarer still: Markarian 739 is only the second identified within half a billion light-years.
Since 2004, the Burst Alert Telescope (BAT) aboard Swift has been mapping high-energy X-ray sources all around the sky. The survey is sensitive to AGN up to 650 million light-years away and has uncovered dozens of previously unrecognised systems.
Follow-up studies by Koss and colleagues published in 2010 reveal that about a quarter of the Swift BAT AGN were either interacting or in close pairs, with perhaps 60 percent of them poised to merge in another billion years.
"If two galaxies collide and each possesses a supermassive black hole, there should be times when both black holes switch on as AGN," co-author Richard Mushotzky, professor of astronomy at UMCP, said.
"We weren`t seeing many double AGN, so we turned to Chandra for help," he stated.
For decades, astronomers have known that the eastern nucleus of Markarian 739 contains a black hole that is actively accreting matter and generating prodigious energy. The Chandra study shows that its western neighbour is too.
This makes the galaxy one of the nearest and clearest cases of a binary AGN.
The distance separating the two black holes is about a third of the distance separating the solar system from the centre of our own galaxy.
The dual AGN of Markarian 739 is the second-closest known, both in terms of distance from one another and distance from Earth. However, another galaxy known as NGC 6240 holds both records.
"Markarian 739 West shows no evidence of being an AGN in visible, ultraviolet and radio observations," co-author Sylvain Veilleux, a professor of astronomy at UMCP, said.
"This highlights the critical importance of high-resolution observations at high X-ray energies in locating binary AGN," Veilleux added.
The study will appear in a forthcoming issue of The Astrophysical Journal Letters.