Astronomers Find Galaxies
Hidden Behind our Milky Way

About one-quarter of the distant Universe is hidden from photographic exploration by dust in our own Milky Way galaxy. Now astronomers are mapping unseen galaxies using radio telescopes, unveiling the so-called "Zone of Avoidance".

Reports are being presented by Dr. Patricia A. Henning and Mr. Andrew J. Rivers, of the University of New Mexico, in Albuquerque, NM, to the American Astronomical Society meeting in San Diego, California. Henning and Rivers represent two international teams of astronomers from the University of New Mexico and Australia, Mexico, the Netherlands, and Great Britain.

[The teams: - for the Dwingeloo Obscured Galaxies Survey: - Andrew J. Rivers, Patricia A. Henning (University of New Mexico, Albuquerque, NM), Renee C. Kraan-Korteweg (University of Guanajuato, Mexico), Ofer Lahav (University of Cambridge, England), and W. Butler Burton (University of Leiden, the Netherlands).
- for the HI Parkes Southern Zone of Avoidance Survey: - Patricia Henning (UNM), Lister Staveley-Smith, Ronald D. Ekers (Australia Telescope National Facility), Anne J. Green (University of Sydney), Raymond F. Haynes (Australia Telescope National Facility), Sebastian Juraszek (University of Sydney), Michael J. Kesteven, Baerbel Koribalski (Australia Telescope National Facility), Renee C. Kraan-Korteweg (University of Guanajuato, Mexico), R. Marcus Price (Australia Telescope National Facility), and Elaine M. Sadler (University of Sydney).]

Two surveys for optically-unseen galaxies are underway, one with the 25-m (82-ft) diameter Dwingeloo radio telescope in the Netherlands, and the other using the 64-m (210-ft) diameter Parkes radio telescope in Australia. Both telescopes use a similar technique, searching for the signature of atomic hydrogen, which is commonly found in many galaxies. The emission from atomic hydrogen, which has a much longer wavelength than that of light, passes easily through the dust which obscures the visible light of galaxies behind the Milky Way. This technique was pioneered by Frank J. Kerr (University of Maryland, College Park, MD) and Henning in 1987, with the discovery of 18 previously- unknown galaxies, using the late National Radio Astronomy Observatory's 300-ft telescope in Green Bank, WV.

The Dwingeloo survey has discovered about 40 hidden galaxies, including several clumped together in a previously-unknown nearby group, only a few million light years away, nearby neighbours by cosmological standards. The Parkes survey has uncovered 103 galaxies, only 28 of which were mapped before. "For the first time we can trace long chains of galaxies across the sky, behind our Milky Way galaxy, and see them pop out the other side" says Henning, Assistant Professor of Physics and Astronomy at the University of New Mexico. The southern survey has uncovered a huge S-shaped string of galaxies, which crosses and re-crosses the Milky Way's obscuring band. This filament is about 300 million light years long, its true length unappreciated until the missing galaxies could be filled in. Also, the portion of the Universe being probed by the Parkes telescope contains the predicted location of the centre of the "Great Attractor", a conglomeration of mass first inferred from galaxies' motions by Donald Lynden-Bell and collaborators in 1988. Mr. Sebastian Juraszek, a student at the University of Sydney, is searching this area and has found 82 galaxies, with a peak in the velocity distribution at the predicted redshift of the Great Attractor. As the survey with the Parkes telescope continues and more galaxies are found, the galaxy distribution in this critical region of space will finally become clear.

All of the newly-discovered galaxies are the subject of follow-up observations for confirmation and further study. More detailed radio images are being made by the National Science Foundation's Very Large Array, near Socorro, NM, the Westerbork Synthesis Radio Telescope of the Netherlands Foundation for Scientific Research (which also supports the Dwingeloo telescope) and the Australia Telescope, funded by the Commonwealth of Australia. The research of Patricia Henning is supported by the National Science Foundation.

For more information contact: Dr. Patricia A. Henning (UNM)