Astronomers at the Space Telescope Science Institute have new evidence for explaining the mysterious X-ray background that permeates the universe. Astronomers Antonella Fruscione, Richard Griffiths and John Mackenty have found a number of "star-burst" galaxies which could help to account for the X- ray background. This at least rivals the contribution from quasars, which are known to account for about 30% of the background.
Their findings were presented at the meeting of the American Astronomical Society in Philadelphia, Pennsylvania on January 15, 1991.
Seen in X-rays from above the atmosphere, the sky glows brightly and uniformly. Though this X-ray glow was discovered in 1962, it still remains largely a mystery. Recently, Griffiths and his collaborators at the Space Telescope Science Institute have proposed that a large fraction of this X-ray background can be explained as the emission from "starburst" galaxies. Starburst galaxies are undergoing a period of intense star formation, perhaps triggered by close encounters with other galaxies.
As a result of rapid star formation starburst galaxies are very bright in both X-ray and infrared radiation. The infrared emission comes from dust which is heated by massive stars which recently formed in the galaxies. The X-ray emission is due to the fact that a large number of the massive stars pair up as binary star systems. Some of these binaries become very bright X-ray sources when one of the stars in the pair collapses to become a compact neutron star or black hole. X-rays are emitted when the collapsed star pulls in material from its normal companion and heats the infalling gas to millions of degrees.
In order to test this hypothesis, Fruscione and Griffiths used the data from two NASA space astronomy satellites - the infrared telescope IRAS (InfraRed Astronomy Satellite) and the Einstein X-ray Telescope (the data from these earlier space observatories are kept in archives by NASA for the use of all astronomers). By comparing the data from the two satellites, the STScI researchers found many galaxies which are bright at X-ray and infrared wavelengths.
Seeing a galaxy at both X-ray and infrared wavelengths, however, does not necessarily mean that it is a starburst galaxy. Quasars and other galaxies with massive black holes at their centers are also bright at both wavelengths.
Once the starburst candidates were identified, Fruscione and Griffiths conducted spectroscopic observations at the University of Arizona's Steward Observatory. The light from the candidate galaxies was split into thousands of component colors. In some cases, the researchers found the signature of massive young stars, showing that the galaxies are indeed starburst galaxies.
Mackenty and Griffiths next studied the shape and structure of the starburst regions inside these galaxies, using the 1.3-meter telescope at the National Optical Astronomy Observatories (NOAO) (operated by the Association of Universities for Research in Astronomy). Using a new infrared camera developed by Dr. Ian Galley and collaborators at NOAO, the astronomers were able to penetrate further into these galaxies than they could with visible light. The researchers found huge regions of bright infrared emission, powered by the massive stars in starbursts. These same starburst regions may contain the massive X-ray binaries which are, in turn, largely responsible for the sky's X-ray glow.