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Black Holes Were Abundant among Earliest Stars

Early black holes

The Cosmic Microwave Background (shown on left), the earliest light we can see, is from the hot, dense Universe after the Big Bang. This was followed by the Dark Ages, when there were no sources to produce light. Later, the first stars and black holes formed. While we can’t see these individually, their collective light is seen in the cosmic infrared and X-ray backgrounds.

By comparing infrared and X-ray background signals across the same stretch of sky, an international team of astronomers, including IfA Director G√ľnther Hasinger, has discovered evidence of a significant number of black holes that accompanied the first stars in the Universe. Using data from NASA’s Chandra X-ray Observatory and NASA’s Spitzer Space Telescope, which observes in the infrared, the team concluded one of every five sources contributing to the infrared signal is a black hole.

“Our results indicate black holes are responsible for at least 20 percent of the cosmic infrared background, which indicates intense activity from black holes feeding on gas during the epoch of the first stars,” said Alexander Kashlinsky, an astrophysicist at NASA’s Goddard Space Flight Center. The cosmic infrared background (CIB) is the collective light from an epoch when structure first emerged in the Universe. Astronomers think it arose from clusters of massive suns in the Universe’s first stellar generations, as well as black holes, which produce vast amounts of energy as they accumulate gas.

Even the most powerful telescopes cannot see the most distant stars and black holes as individual sources. But their combined glow, traveling across billions of light-years, allows astronomers to begin deciphering the relative contributions of the first generation of stars and black holes in the young cosmos. This was at a time when dwarf galaxies assembled, merged, and grew into majestic objects like our own Milky Way galaxy. “We wanted to understand the nature of the sources in this era in more detail, so I suggested examining Chandra data to explore the possibility of X-ray emission associated with the lumpy glow of the CIB,” said Hasinger.

The work began in 2005, when Kashlinsky and his colleagues studying Spitzer observations first saw hints of a remnant glow. The glow became more obvious in further Spitzer studies by the same team in 2007 and 2012. The 2012 investigation examined a region known as the Extended Groth Strip, a single well-studied slice of sky in the constellation Bootes. In all cases, when the scientists carefully subtracted all known stars and galaxies from the data, what remained was a faint, irregular glow. There is no direct evidence this glow is extremely distant, but telltale characteristics lead researchers to conclude it represents the CIB.

In 2007, Chandra took especially deep exposures of the Extended Groth Strip as part of a multiwavelength survey. Along a strip of sky slightly larger than the full Moon, the deepest Chandra observations overlap with the deepest Spitzer observations. Using Chandra observations, lead researcher Nico Cappelluti (National Institute of Astrophysics, Bologna, Italy, and University of Maryland, Baltimore County), produced X-ray maps with all of the known sources removed in three wavelength bands. The result, paralleling the Spitzer studies, was a faint, diffuse X-ray glow that constitutes the cosmic X-ray background (CXB).

Comparing these maps allowed the team to determine whether the irregularities of both backgrounds fluctuated independently or in concert. Their detailed study indicates fluctuations at the lowest X-ray energies are consistent with those in the infrared maps. “This measurement took us some five years to complete and the results came as a great surprise to us,” said Cappelluti.

The process is similar to standing in Los Angeles while looking for signs of fireworks in New York. The individual pyrotechnics would be too faint to see, but removing all intervening light sources would allow the detection of some unresolved light. Detecting smoke would strengthen the conclusion that at least part of this signal came from fireworks. In the case of the CIB and CXB maps, portions of both infrared and X-ray light seem to come from the same regions of the sky. The team reports black holes are the only plausible sources that can produce both energies at the intensities required. Regular star-forming galaxies, even those that vigorously form stars, cannot do this.