GOODS uncovers hidden black holes in the distant universe

Images from NASA’s new Spitzer Space Telescope have allowed researchers to detect the long sought population of “missing” supermassive black holes that powered the bright cores of the earliest active galaxies in the young universe. The discovery completes a full accounting of all the X-ray sources seen in one of the deepest surveys of the universe ever taken. The results were presented at the meeting of the American Astronomical Society in Denver, Colorado.

Mark Dickinson, of the National Optical Astronomy Observatory in Tucson, Ariz., and Principal Investigator for the new observations, says, “With these ultra-deep Spitzer images, we are easily seeing objects throughout time and space, out to redshifts of 6 or more, where the most distant known galaxies lie. Moreover, we see some objects that are completely invisible to optical telescopes, but whose existence was hinted at by previous observations from the Chandra and Hubble Observatories.”

The project combined the power of NASA’s three Great Observatories in space – the Hubble Space Telescope (HST), the Chandra X-ray Observatory, and the Spitzer Space Telescope (SST). All three telescopes peered across 13 billion light-years of space into a small region of dark sky (called the Great Observatories Origins Deep Survey, GOODS) that is ideal for perusing thousands of galaxies.

Each observatory works with different wavelengths of electromagnetic radiation; Chandra detects high-energy X-rays, Hubble discriminates visible light, and Spitzer recognizes the infrared. Their combined data gave images that were not possible with data from any one observatory alone.

“The great sensitivity of the new Spitzer infrared cameras, and with the superb spatial resolution of Chandra and Hubble, means that finding all of the black holes in distant galaxies is now possible,” says GOODS astronomer Meg Urry, professor of physics and astronomy, and Director of the Yale Center for Astronomy & Astrophysics at Yale University.

Chandra detected X-rays from over two hundred X-ray sources believed to be supermassive black holes lying in the centers of young galaxies. The X-rays are produced by interstellar gas that is attracted by the gravity of the black holes and is heated to very high temperatures just before it falls in.

Combining data from the three Great Observatories, Urry’s team took a census of the supermassive black holes that formed 2-5 billion years after the big bang. Theoretical arguments had suggested that most of these young black holes are shrouded by dust but few had previously been found. Now the GOODS data have verified that “most, perhaps three-quarters, of the active galactic nuclei in the early Universe are shrouded,” says Urry. They were missed because their visible radiation is so dim they look like faint, ordinary galaxies. “With the new Spitzer data,” says Urry, “these very luminous, distant objects are easily visible.”

“The longer-wavelength Spitzer data still to come will reveal even more shrouded AGNs,” she adds, “including some, missed even by X-ray observations that look like ultraluminous infrared galaxies.”

Seven of the objects detected in the Spitzer images may be part of the long-sought population of “missing” supermassive black holes that powered the bright cores of the very earliest active galaxies. Hubble’s Advanced Camera for surveys revealed optical galaxies around almost all the X-ray black holes. However, there remained seven mysterious X-ray sources for which there was no optical galaxy in the Hubble images.

Astronomer Anton Koekemoer of the Space Telescope Science Institute in Baltimore, Md., who discovered these sources, presented three intriguing possibilities for their origin: “The galaxies around these black holes may be completely hidden by thick clouds of dust absorbing all their light, or may contain very old, red stars. Or some of them could be the most distant black holes ever observed – perhaps as far as 13 billion light-years.” In this case all their optical light would be shifted to very long infrared wavelengths by expansion of the Universe.

Because Spitzer observes in infrared light, at wavelengths up to 100 times longer than those probed by Hubble, it was able to detect the telltale infrared glow from the host galaxies around these optically invisible X-ray black holes. Additional Spitzer observations later this year will help confirm what kind of objects these are.