ESA Science & Technology - Observations Indicate Nearby Hydrogen Clouds May Be Associated With Galaxies

"This is a revolutionary finding, if supported by future observations," says Dr. John Bahcall of the Institute for Advanced Study in Princeton, NJ., "We would have never thought of looking for this explanation if it hadn't kicked us in the face."

Drs. Bahcall, Buell Jannuzi and Donald Schneider, all of the Institute for Advanced Study and George Hartig of the Space Telescope Science Institute, Baltimore, MD, announced their results today to a press conference at the 179th meeting of the American Astronomical Society in Atlanta.

For the past two decades, observations with ground-based telescopes have shown that the spectra of high-redshift quasars contain complex "thickets" of absorption features. These absorption lines are unrelated to the quasars themselves, but are attributed to invisible, intervening clouds of hydrogen gas which absorb certain frequencies of a quasar's light.

Ground-based observations have shown that the number of these clouds rapidly rises as one looks back in time, so it was thought, says Hartig "that the number of nearby hydrogen clouds was relatively small."

Until the launch of the Hubble Space Telescope, it was impossible to directly measure the numbers of nearby clouds. This unusual situation, understanding the distant clouds more than the nearby ones, arose because, as Jannuzi explains: "The recession velocity of the nearby clouds is so low that the hydrogen absorption features occur in a part of the spectrum, the far ultraviolet, that is inaccessible with ground-based telescopes."

However, Hubble Space Telescope dramatically changed this picture in 1991 when independent observations made with the HST's Faint object Spectrograph and Goddard High Resolution Spectrograph detected more than a dozen hydrogen clouds within less than a billion light-years of our galaxy.

Astronomers have assumed that the clouds are strung along the line-of-sight to a quasar, like beads on a string. The new HST observations show that some absorption lines are more clumped together than expected for randomly distributed material. In addition, a few of the lines can be directly associated with galaxies that lie along the line of sight to the quasar. This suggests that the material producing the nearby hydrogen absorption may be associated with individual galaxies or clusters of galaxies.

The new results are based upon Faint object Spectrograph observations of a bright and relatively nearby (approximately four billion light-years distant) quasar H1821+643. "We are very pleased with the quality of the data," says Hartig. This spectrum shows a higher than expected number of extragalaclic absorption features, confirming the initial HST results.

One grouping of lines in the complex spectrum has the same redshift, and hence the same distance, as the quasar. These types of lines, called self-absorption systems, have been found in high-redshift quasars and were thought to be produced by material in the quasar itself. Schneider notes that: "Recent ground-based studies have shown that H1821+643 is surrounded by a large number of galaxies, so it is possible that the self-absorption system is caused by material belonging to the cluster or to an individual cluster galaxy."

The HST observations provide additional evidence that the absorption is associated with galaxies. A foreground galaxy that lies along the line of sight to the quasar has the same distance from us as does one of the cloud groupings. The cloud may be located within 300 000 light-years from the center of the galaxy.

HST spectra of other nearby quasars are being obtained to see if there is additional evidence of clumping of the hydrogen clouds or if any other absorption systems can be identified as belonging to galaxies. If so, then one of astronomy's many current mysteries may at last be solved.