BLACK STACK: The black holes within galaxies identified from this Hubble image do not glow brightly enough to be individually identified in x-rays. But when their emission is summed by stacking hundreds of galaxies on top of one another [inset], the collective black hole activity becomes apparent. Image: X-ray: NASA/CXC/U.Hawaii/E.Treister et al; Infrared: NASA/STScI/UC Santa Cruz/G.Illingworth et al; Optical: NASA/STScI/S.Beckwith et al
It's the source of a long-standing cosmological quandary. Galaxies or black holes: Which came first? Today, they exist as neatly matched pairs, a black hole nested in the heart of a swirling galaxy, but it seems possible that the growth of one drove the growth of the other. By peering deep into the early universe, astronomers have edged ever closer to an answer. But they did not identify a leader of this cosmic tango, as galaxies and black holes appear to have matched each other step for step as early as a billion years after the universe began. Perhaps, then, they simply developed in tandem.
"This chicken-and-egg problem of what was there first, the galaxy or the black hole, has been pushed all the way to the edge of the universe," Yale University astrophysicist Kevin Schawinski said in a June 15 press conference at NASA Headquarters in Washington, D.C.
Schawinski was part of a team of researchers that used two renowned orbiting observatories, the Hubble Space Telescope and the Chandra X-Ray Observatory, to identify a population of black holes in galaxies at redshift 6, which corresponds to a time about 950 million years after the big bang. (Redshift is a measure of cosmological distance; higher redshifts indicate greater distances and hence earlier epochs in the universe's development.) Even at that early time, the density of black holes in space indicates that galaxies and black holes were regulating each other's development. The researchers reported their findings in the June 16 issue of Nature. (Scientific American is part of Nature Publishing Group.)
"What our observations of galaxies in the early universe tells us is these very early young galaxies at the dawn of the universe and their growing baby black holes already had some deep fundamental connection between them," Schawinski said. "They were already growing together."
Peering so deep into the universe requires extremely long-duration exposures, even on sensitive spaceborne telescopes. The researchers drew on deep x-ray imagery from Chandra, built up over four million seconds (46 days) of telescope exposure time, to identify the ancient black holes. (For although black holes are dark, the regions around them glow brightly in x-rays as infalling matter compresses and heats up.) But even with the benefit of that prolonged look, the individual black holes were not visible as points in the Chandra images.
So the researchers tried a different tack. They located galaxies in ultra-deep field Hubble photographs—in visible light and infrared—of the same region of sky and pinpointed where those galaxies ought to be in the Chandra image. By stacking all of those points on top of one another, the researchers combined the faint x-ray glow from the heart of hundreds of galaxies, which were undetectable individually, into a brighter aggregate (see photo inset). Even if the properties of individual black holes remained opaque, perhaps their collective properties would tell the story of black hole activity at that epoch.
In the stacked images of galaxies that existed 950 million years after the big bang, the x-ray emissions from black holes indeed became readily apparent, especially in higher-energy x-rays. (For older, even more distant galaxies, the researchers were not able to see black hole activity as clearly, but they did set upper limits on x-ray luminosity.) The skew toward high-energy x-rays indicates that the black holes were heavily shrouded by dust and gas, through which only the most energetic radiation could pass.
That obscuration means that black holes could have grown more rapidly early on than had been thought possible, because their volatile behavior would have been kept in check by the surrounding gas and dust. Turbulent, unchecked black hole growth in the ancient universe would have left marks on the intergalactic medium that astronomers have not observed. "But what these new results show is that black holes were protected, they were enshrouded in a cocoon of dust that dampened the effect that they had on their surroundings," said Mitchell Begelman of the University of Colorado at Boulder, who did not contribute to the new study. That dampening, he said, means that "these black holes could have grown quite early without having a dramatic and unobserved effect on the universe."
Just how those early black holes appeared in the first place remains unresolved. "It is pretty clear that you first make small seed black holes in the early universe, and over cosmic time, by swallowing gas in their vicinity, they grow," said study co-author Priya Natarajan, a Yale astrophysicist. "How precisely you form seed black holes is an open-ended question." One possibility is that seed black holes grew out of the demise of the earliest stars; another explanation is that gaseous pre-galactic disks gravitationally collapsed to create nascent black holes. Both possibilities are broadly consistent with the new black hole observations. "Of course a lot more data is needed before we can adjudicate between these two models," Natarajan said.