Two New Mexico State University astronomers teamed up with colleagues in the Netherlands to discover a large galaxy in the immediate neighborhood of our own Milky Way galaxy.
Rene Walterbos, head of the astronomy department at NMSU, said the previously undetected galaxy is only about 20 million light years away -- a very close neighbor by galactic standards.
"It is surprising that we apparently have not found all the large nearby galaxies," Walterbos said. "Astronomers have been finding a lot of dwarf galaxies, but this is a fairly substantial galaxy."
Several large nearby galaxies lurking behind the dusty absorbing band of the Milky Way have also been discovered over the past decade, but this is the first large nearby galaxy found in the modern astronomical era that is only mildly obscured in this way.
Because it is the first nearby galaxy discovered in the constellation Cepheus, the newly discovered galaxy was named Cepheus 1. It belongs to a class known as Low Surface Brightness (LSB) galaxies, in which stars are spread further apart than in most galaxies.
Signs of Cepheus 1 were first noticed in observations made with the Dwingeloo 25-meter radio telescope in the Netherlands. Robert Braun of the Netherlands Foundation for Research in Astronomy and Butler Burton of Leiden University had been using that telescope to study compact clouds of hydrogen gas found swarming around the Milky Way. The motions of these gas clouds could be measured by their Doppler shifts -- changes in the wavelengths of the signals coming from the clouds -- and one was seen to move differently from the others.
Braun and Burton contacted Walterbos and Charles Hoopes, a doctoral student in astronomy at NMSU, who used the 3.5-meter optical telescope at Apache Point Observatory to verify that the hydrogen gas signature corresponded to a new galaxy. Apache Point, high in the Sacramento Mountains on one of the best observing sites in North America, is operated by NMSU for the Astrophysical Research Consortium, a group of seven universities and research institutions.
"This demonstrates very well the capabilities of Apache Point," Walterbos said. "It required a rapid response and it involved three different observational techniques."
The optical picture obtained by Apache Point showed what the astronomers described as a "rather anemic-looking galaxy" with only a few sites of recent star formation scattered across a large area. Further radio observations from the Dominion Radio Astrophysical Observatory in British Columbia revealed that the weak optical signal was embedded in a much larger and rapidly rotating disk of hydrogen gas, characteristic of a robust spiral galaxy.
Walterbos said Cepheus 1 is one of the dozen largest nearby spiral galaxies, "and one of only two large Low Surface Brightness spirals that we know of in the nearby universe."
LSB galaxies can be massive, with copious amounts of gas within them, but the gas is evolving to form stars very slowly compared with other galaxies, the astronomers said. Most galaxies occur in large clusters or groups and interact with each other gravitationally. The largest galaxies are believed to evolve by cannibalizing smaller ones. The Milky Way and its nearest large neighbor, the Andromeda galaxy, are on a collision course and probably will merge dramatically in a few billion years.
LSB galaxies, on the other hand, are commonly found in quite empty regions of space. With little external influence on their internal circumstances, the process of star formation is not triggered efficiently, leaving vast reservoirs of gas but only a few young, bright stars.
Discovery of Cepheus 1 gives astronomers a nearby example of LSB galaxies to study in detail. It also represents another step in completing the census of galaxies in the local neighborhood, which is important to determining the mass and luminosity characteristics of these fundamental building blocks of the universe.
A scientific article on the discovery by the four astronomers will appear in the January 1999 issue of the "Astronomical Journal," published by the American Astronomical Society.