FOR RELEASE: 9:20 a.m. CST, January 16, 1996
ASTRONOMERS SEE GALAXIES THROUGH THICK AND THIN
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