Friday, July 10, 2015

An Over-Massive Black Hole in the Young Universe

An artist's impression of the Chandra X-ray Telescope in Earth orbit. Astronomers have used Chandra to identify an X-ray bright supermassive black hole from an era only two billion years after the big bang. The finding appears to challenge conventional wisdom about how quickly such supermassive black holes can form in galaxies. Credit: NASA/Chandra

Astronomers generally accept the notion that black holes at the centers of galaxies co-evolve with their host galaxies, and that they have done did so during all cosmic epochs, from the early period after the big band until today. This means that as a galaxy grows in mass, which it does by accreting material (and perhaps also consuming other galaxies) from the intergalactic medium, its black hole also accretes matter and grows. Indeed, the black hole is so massive -- perhaps as much as ten percent (!) of the entire stellar mass of the galaxy – that these two growth processes may be related, for example, because the black hole influences accretion onto the galaxy. The black hole accretion process may have other consequences too, like suppressing star formation by heating and/or disrupting nearby molecular clouds.

Testing these ideas is difficult because it requires measuring black hole properties in cosmic epochs when the universe was only a few billion years old and at the correspondingly cosmic distances even ultraluminous galaxies appear faint to us. A few recent studies have indicated that some supermassive black holes actually grew faster than their galaxies in epochs about three billion years after the big bang, but these measurements were made only on exceptionally X-ray luminous objects which are perhaps not representative of most systems and whose galaxy masses are very uncertain. 

CfA astronomers Francesca Civano, Martin Elvis, and Hyewon Suh joined their colleagues in using the Chandra X-Ray Observatory and another X-ray mission, XMM-Newton, to select an X-ray bright black-hole nucleus only two billion years after the big bang, and then to observe it in the infrared with the Keck telescope to study its ionized hydrogen gas, a tracer of black hole growth. The astronomers find that the black hole in this galaxy has apparently grown much more efficiently than the galaxy itself, contrary to conventional models. In fact, it is as much fifty times more massive compared to its galaxy than all but the most extreme local examples – it is a whopping seven billion solar masses. The important implication is that it grew to this size in a very much shorter time than local galaxies, which had thirteen billion years to grow. The galaxy also appears to be making stars without any suppression. The new paper both challenges the conventional theoretical paradigm and steers future research toward examining these distant X-ray monsters.


 "An Over-Massive Black Hole in a Typical Star-Forming Galaxy, 2 Billion Years After the Big Bang," Benny Trakhtenbrot, C. Megan Urry, Francesca Civano, David J. Rosario, Martin Elvis, Kevin Schawinski, Hyewon Suh, Angela Bongiorno, Brooke D. Simmons, Science, 2015 (in press).