An international team of astronomers has uncovered a galaxy in our own cosmic back yard. Though only ten million light-years away (or five times the distance of the Andromeda galaxy - closest assemblage of stars to our Milky Way Galaxy), this newly discovered city of more than 100 billion stars has gone undetected previously because it is hidden from view behind our Milky Way galaxy.
Called "Dwingeloo 1," after the radio telescope that first detected it, the galaxy probably belongs to a nearby group of galaxies that include two named Maffei 1 and 2. The astronomers report that its gravitational pull is likely to affect the motion of the Milky Way and the other galaxies comprising our Local Group.
Located in the circumpolar constellation Cassiopeia, which lies along the Milky Way, "Dwingeloo 1" is behind a region of the sky containing a great deal of gas and dust that cut off light from objects beyond our galaxy (at an apparent brightness, or magnitude, of +14.8 , the galaxy is more than 100-times fainter than it would be if the galaxy were unobscured).
The new galaxy was initially detected in radio light that penetrates this obscuring dust, and then ground-based telescopes were used to observe directly the galaxy with its distinctive barred-spiral shape, as seen through a crowded field of foreground stars belonging to the Milky Way.
"For a long time, astronomers did not realize the importance of interstellar dust in shaping our view of the universe, and simply ignored parts of the sky close to the plane of the Milky Way when it came to studying extragalactic objects," says Dr. Harry Ferguson of the Space Telescope Science Institute, Baltimore, MD. "It is sobering to realize that a galaxy as close as Dwingeloo 1 could have remained hidden until now, and could have a real impact on our understanding of the motion of the Milky Way through the cosmos."
The discovery team included Ferguson, Dr. Renee Kraan-Korteweg (University of Groningen), Andy Loan (a research student), Dr. Ofer Lahav and Professor Donald Lynden-Bell of the University of Cambridge; Professor Butler Burton (Leiden University); and Dr. Patricia Henning (University of New Mexico, USA).
The galaxy's identification is a result of a long term project called the Dwingeloo Obscured Galaxy Survey (DOGS), which uses the 25-meter radio telescope at Dwingeloo in the Netherlands to look for radio emissions from galaxies that might be hidden from the view of optical telescopes by the Milky Way.
On August 4, 1994, this "blind search" identified a radio "signature," that is characteristic of a spiral galaxy (the radio "image" is about half the apparent size of the full Moon). Within days, the discovery team contacted colleagues who observed the galaxy at visible and infrared wavelengths with telescopes on La Palma (Canary Islands), in Hawaii, and in Israel. These observations confirmed that the galaxy is a barred-spiral.
The astronomers expect to find more galaxies behind the Milky Way as work on the survey continues.
The results are being reported in the November 3 issue of Nature Magazine.
Ray Villard, STScI
Dr. Harry Ferguson, STScI