Catalog Of Spiral Galaxies Shows Evidence Of Galactic Collisions

Written by Pam Frost, (614) 292-9475
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CATALOG OF SPIRAL GALAXIES SHOWS EVIDENCE OF GALACTIC COLLISIONS

COLUMBUS, Ohio -- Astronomers compiling a catalog of spiral galaxies have discovered that collisions between such galaxies, as well as near-collisions, are more common than had been thought.

"It means there were more close encounters," said Jay Frogel, professor of astronomy at Ohio State. Frogel and Paul Eskridge, a postdoctoral research fellow, compiled pictures of more than 200 nearby spiral galaxies over the last five years.

When viewed through traditional optical telescopes, about 30 percent of those 200 galaxies appear to contain a bar-shaped band of stars at the center. Astronomers call these galaxies "barred spirals."

Frogel and Eskridge discovered that the fraction of strongly barred galaxies was roughly twice as high when they viewed the same 200 galaxies through infrared telescopes.

The researchers presented their findings at a recent meeting of the American Astronomical Society in Austin, Texas.

For some spiral galaxies, pinwheel-shaped arms of gas, dust, and stars extend directly out from the spherical nucleus of stars at the center. For barred spirals, the arms fan out from the ends of a bar-like structure.

Astronomers think the bars form either when two galaxies collide or when they nearly miss each other as they drift through space.

According to visual observations of the past, only about one-third of the spiral galaxies near the Milky Way appeared to contain such a bar. However, computer simulations have predicted that many more barred spirals should exist.

"People who model the dynamics of galactic evolution were wondering why we weren't seeing more of them," said Eskridge.

Dust in the central regions of spiral galaxies can prevent a bar from being seen at optical wavelengths -- the wavelengths of visible light.

To see through the dust, the astronomers took advantage of a camera designed and built at Ohio State. The Ohio State InfraRed Imager and Spectrometer (OSIRIS) records infrared radiation the way normal cameras record visible light. Infrared energy penetrates dust much more readily than visible light does.

The astronomers' infrared observations revealed that approximately 30 percent of their 200-galaxy sample contains bars which are invisible at optical wavelengths.

The finding indicates that galaxies in our area of the universe have interacted with each other a good deal in the last 10 billion years.

Compared to other infrared cameras, OSIRIS takes a wider picture of the sky -- wide enough to encompass the diameter of most galaxies visible from earth. Frogel and Eskridge used OSIRIS to take pictures of galaxies through telescopes at Lowell Observatory in Flagstaff, AZ, and American Observatory in Cerro Tololo, Chile.

"Infrared observing gives us a chance to learn new things about galaxies, things that we just couldn't figure out before," said Frogel. "We get a much clearer picture of what galaxies really look like."

That's because most of the visible light from a galaxy emanates from young, hot stars that represent only a fraction of the total mass. Older, cooler stars don't shine as bright, but radiate most of their energy at infrared wavelengths. What's more, the bars of barred spirals contain mostly old stars.

Frogel explained that these older stars make up about 90 percent of the mass of a galaxy, so infrared observations give the best view of how much matter is in a galaxy and how the galaxy evolved.

For instance, by comparing the visible, or optical, view of a galaxy to the infrared view, astronomers can tell whether most stars in the galaxy formed long ago or more recently.

There have been other such surveys in the past, but Ohio State's survey is much more extensive. It also contains higher-quality images taken with long exposure times in both the optical and infrared wavelengths. Each of the 200-plus galaxies required six hours of observation -- two at optical wavelengths and four at infrared -- which adds up to more than 1200 hours of observation for the catalog as a whole.

Frogel and Eskridge want to place the photos on a CD-ROM and distribute them through NASA, which may be able to use the infrared data to plan future space-based observing missions.

This work was supported by the National Science Foundation. The funding for the astronomers to build OSIRIS came from an Ohio Academic Enrichment grant that Frogel received when he first came to the university 11 years ago.

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EDITOR'S NOTE: Select photographs of galaxies from the catalog are available for downloading in GIF format on the World Wide Web page for the Ohio State University Bright Spiral Galaxy Survey at http://www-astronomy.mps.ohio-state.edu/~survey/