FOR RELEASE: 9:20 a.m. CST, January 16, 1996


A trio of astronomers displayed images of overlapping pairs of galaxies today which may resolve a contentious astronomical debate: are spiral galaxies largely transparent or opaque to visible light? Christopher Conselice, a University of Chicago undergraduate, in collaboration with Drs. Raymond White and William Keel at the University of Alabama in Tuscaloosa, presented the results today at the semi-annual meeting of the American Astronomical Society in San Antonio, Texas. The researchers, working together in an NSF-sponsored Research Experience for Undergraduates program, showed that typical spiral galaxies are nearly transparent in the regions between their arms, but the spiral arms themselves are opaque, due to the absorption of light by dust grains in interstellar space. Even when a spiral is in front of another galaxy, one can look between the arms and see the background galaxy. The astronomers selected partially overlapping pairs of galaxies for study so that the non-overlapping parts of the galaxy pairs could be used to reconstruct the light contribution of each galaxy in the overlapping regions. This way, they could directly determine how much light from the background galaxy is lost in passing through the foreground galaxy.
Whether spiral galaxies are largely transparent or opaque has ramifications for the nature of dark matter, the star formation rates in galaxies, and the observability of quasars, which are the most distant objects in the Universe. Spiral galaxies are flattened, rotating disks of stars and gas, with bright arms radiating out from the center in a spiral pattern. Astronomers estimate the visible masses of spiral galaxies by adding up the light from their stars and by knowing the typical masses of such stars from measurements in our own Milky Way galaxy. But spiral galaxies contain much more mass than is observed in visible stars and gas. This additional mass, detected only through its gravitational effects on galaxies, is attributed to so-called "dark matter," the nature of which is one of the most important unsolved problems in astronomy. If spiral galaxies are opaque to their own visible light, then astronomers may be underestimating the amount of normal stellar matter they contain and attributing too much mass to the "dark matter."
Infrared telescopes, such as the Infrared Astronomical Satellite, have found many galaxies which are strong emitters of far-infrared radiation, which is thought to be a sign of intense bursts of star formation: since most stars are formed deep in dusty clouds of interstellar gas, their starlight is not directly visible in the optical, but light from the young stars heats the surrounding dust, which in turn radiates at infrared wavelengths which can escape the dusty clouds. However, if dust is very widespread and thick throughout galaxies, the stars which heat the surrounding dust may not be just the young stars, so the observed infrared radiation would not be the good measure of star formation rates that it is usually assumed to be. Thus, clarifying the dust content of typical galaxies is important to understanding how they evolve over time, as interstellar gas is transformed into stars.
The transparency or opacity of spiral galaxies may also limit how deeply we can see into space. The most distant known objects are quasars, very energetic galactic nuclei, which are seen in greater and greater numbers the further out astronomers probe. But beyond a certain distance, about 85% of the way to the edge of the observable universe, quasars are no longer detected. Some astronomers suggest that this cutoff is due to looking through too many intervening galaxies on the way to distant quasars, with these galaxies blocking the light from further out. However, if spiral galaxies are transparent, they could not be the cause of the quasar cutoff.
The classical test of whether spiral galaxies are largely transparent ("optically thin") or opaque ("optically thick") is to look at a large sample of galaxies at various inclinations on the sky, ranging from face-on to edge-on. If spiral galaxies are transparent, then their surface brightness (how bright they appear in a given angular area on the sky) will be greater when they are edge-on than face-on: since spiral disks are very flattened, the light reaching us from such a galaxy comes from a smaller area of the sky when edge-on than when face-on. If spiral galaxies are instead opaque, then one can only see a little way into the outer "skin" of the galaxy, so the galaxy would have the same surface brightness whether it is edge-on or face-on. Some recent statistical studies have used this classical test to conclude that spiral galaxies are largely opaque, despite the fact that we can look out of our own spiral Milky Way galaxy when we look away from our disk. Nonetheless, when we look at the band of light we call the Milky Way, we are looking into the disk of our galaxy, and the obvious dark bands and cut-outs in the Milky Way show the effects of absorption by dust grains in interstellar space.
This ambiguity is what inspired the team to electronically search large catalogs of galaxies for partially overlapping pairs, so that any light blockage by a foreground spiral could be directly, rather than statistically, determined from the non-overlapping parts. The results presented today, that opacity is confined to spiral arms, confirm and extend earlier results by White and Keel, who were the first astronomers to use this surprisingly simple technique to try to resolve this hotly debated issue. "Basically, we've shown that spiral galaxies are both transparent and opaque: their spiral arms are opaque, but they are transparent between their arms," explains Dr. Raymond White, an Associate Professor at the University of Alabama. "Furthermore, since the bulk of the disk light comes from these same spiral arms, this association of absorbing material with the light emitting regions may explain why statistical studies erroneously concluded that spiral galaxies are largely opaque." The spiral arms, where the bulk of the disk light comes from, do indeed tend to be opaque, so their surface brightness would be the same at every inclination. But between the arms, spiral galaxies are nearly transparent, which is consistent with our ability to look out of our own galaxy (the Sun lies between spiral arms) and anecdotal examples of other spiral galaxies with more distant objects visible through them.
The astronomers used several telescopes in the course of their investigation: the 1.5-meter telescope at the Cerro Tololo Interamerican Observatory, near La Serena, Chile; the 2.1-meter telescope at Kitt Peak National Observatory, near Tucson, Arizona; and the 1.1-meter telescope at the Lowell Observatory in Flagstaff, Arizona. They are now looking forward to analyzing their upcoming observations of some of these overlapping galaxies with the Hubble Space Telescope and the Infrared Space Observatory. Conselice, who will be starting graduate school in astronomy next year, said that "the REU program allowed me to participate in a really exciting research project and made it possible for me to join the observing trip to Chile, which was fun." Complete details of this investigation are about to be submitted to the Astrophysical Journal. This work was supported by the National Science Foundation Research Experience for Undergraduates program as well as the joint NSF-State of Alabama Experimental Program for the Stimulation of Competitive Research (EPSCoR).

For More Information:
Dr. Raymond E. White (205)348-1640
. . . . WWWebsite:
Dr. William C. Keel (205)348-1641
. . . . WWWebsite:
Mr. Christopher J. Conselice (312)834-2942