Summer 2000 -- Vol. 17, No. 1

Research in Hubble Deep Field Affirms Galaxies as Sites for Early Star Birth

Astronomers using NASA's Chandra X-ray Observatory have made the first long-duration X-ray survey of
the Hubble Deep Field North, discovering X-ray emission from six galaxies and also revealing the surprising
lack of X-rays from galaxies known to emit radiation at submillimeter wavelengths, in the range between radio
and infrared.

"For the first time, we have been able to determine the X-ray characteristics of the submillimeter sources in the
Hubble Deep Field North and its vicinity," said Niel Brandt, assistant professor of astronomy and astrophysics at Penn State and one of the leaders of the research team that conducted the survey.

The Hubble Deep Field North is an area in the northern sky that was observed with the Hubble Space Telescope in December 1995 "as deeply as reasonably possible," according to the National Aeronautics and Space Administration (NASA).

Astronomers make "deep," or unusually long, observations to detect emission from objects that appear faint because they are very far away from Earth.  The Hubble Space Telescope picture of the Hubble Deep Field North shows about three thousand galaxies emitting visible electromagnetic radiation, or ordinary light.

The Chandra image of the same area, which is the deepest X-ray observation ever reported, reveals that six of the objects are emitting X-rays.

"An object has to be really hot or it has to have something really violent going on to emit X-rays," said Ann E. Hornschemeier, a Penn State graduate student in astronomy and astrophysics and the first author of the paper describing the discovery, which will be published in the September 2000 issue of the Astrophysical Journal.  For example, supermassive black holes a million to a billion times the mass of our Sun, known as active galactic nuclei (AGN), create X-rays as they gobble up, or accrete, material from a disk-like area around their massive centers, heating the surrounding gas to temperatures of millions of degrees or more.  Among the X-ray-emitting objects the research team discovered are a distant AGN galaxy, some elliptical galaxies containing hot gas, an extremely red galaxy, and a spiral galaxy in which the X-rays may come from a super-luminous X-ray-binary-star system or the remnant of an exploded star.

The astronomers also carefully looked at the sources emitting submillimeter radiation in this region.  "We see
submillimeter emission when we look at an object that is glowing brightly in the infrared but is located at a great distance from Earth in the expanding universe," Hornschemeier explains.

Such objects are moving away from Earth very fast because the universe is expanding, so the wavelength of the infrared light they emit gets lengthened or "redshifted"--moved farther toward the red end of the spectrum into the submillimeter range.

Star formation in dusty galaxies creates a lot of infrared emission but not much visible light, so astronomers on Earth who want to study star formation in the very distant universe look for submillimeter radiation--the redshifted result of its infrared signature.  "Starburst galaxies are thought to have an exceptionally high rate of star formation and can be among the most luminous galaxies known, but in the very distant universe we typically detect a large percentage of their total energy output in the submillimeter range," Hornschemeier said.

In contrast, luminous X-rays are the characteristic signature of extremely violent AGN galaxies that harbor a massive central black hole.  Although the black hole can 'kick up' a lot of gas and dust that can block visible and other wave-lengths, X-rays are very energetic and usually can penetrate through, eventually reaching Earth.  "X-ray surveys provide the most  direct and unbiased probe of massive-black-hole accretion activity throughout the universe," Brandt said.

"We did not see any X-rays at all coming from the submillimeter sources in the Hubble Deep Field North," Brandt said.  "This discovery means either there is a heck of a lot of star formation out there in submillimeter galaxies or these objects contain the best-hidden black holes in the universe."

Brandt explains the lack of X-rays could mean either the submillimeter sources are "starburst" galaxies in the early universe whose characteristically weak X-ray signature is too faint for even Chandra to detect or, less likely, they could be AGN galaxies whose massive central black holes are enshrouded by gas clouds so thick they completely obscure the galaxy's X-ray emission.

"The amount of X-rays an AGN emits is known to be much greater than that of a star-forming galaxy," he said.  "With Chandra, we have been able to place the best X-ray constraints ever on this submillimeter population, and we are the first to study enough sources in detail with X-rays to reasonably characterize their high-energy properties." The results indicate less than 15 percent of submillimeter sources also have luminous X-rays emissions.

"It was very surprising to find no X-rays coming from the submillimeter sources," Brandt said.  "People knew of about five submillimeter sources in the Hubble Deep Field North, and prior to Chandra we were expecting about five X-ray sources in this field as well, and so we thought it would be nice if the five X-ray sources happened to line up with the five submillimeter sources . . . but it didn't turn out that way."

The best previous X-ray observatories did not have Chandra's sharpness of focus, so astronomers were not able to unambiguously pinpoint an X-ray source as coming from any particular optical counterpart in the Hubble Deep Field North.  "Chandra is unprecedented in that it is the first X-ray observatory ever to be able to resolve point sources at the arc-second level," Hornschemeier said.  "Many of a galaxy's characteristics are detectable only in optical wavelengths, so it its very important to make a definitive optical match for an X-ray source."  A definitive optical match also allows an object to be studied with large ground-based telescopes such as the Hobby-Eberly Telescope.

Chandra's increased spatial resolution--the capability to concentrate X-rays into a smaller area--also increases its sensitivity for detecting fainter X-ray sources than ever before.  "Many of our sources are fainter than any source that was detected in the deepest observations by all previous X-ray observatories," Hornschemeier said.  The research team used Chandra's Advanced CCD Imaging Spectrometer (ACIS) to make the unusually deep observation of the Hubble Deep Field in X-rays for about 46 hours in November 1999.

The ACIS X-ray camera was conceived and developed for NASA by Penn State and the Massachusetts Institute of Technology under the leadership of Gordon Garmire, Evan Pugh Professor of Astronomy and Astrophysics at Penn State.  The deep observation is part of the guaranteed observing time allocated to Garmire.

This research was supported by NASA, the Alfred P. Sloan Foundation, and the National Science Foundation.  The authors of the research paper, titled "X-Ray Sources in the Hubble Deep Field Detected by Chandra," include collaborators from California Institute of Technology, Carnegie Mellon University, the European Space Research and Technology Center, and Massachusetts Institute of Technology.  Along with those collaborators, the Penn State contingent comprise: Brandt, Garmire, Donald Schneider, Patrick Broos, Leisa Townsley, David Burrows, Eric Feigelson, and John Nousek.

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