Courtesy Chandra X-ray Center
and World Science staff

Astronomers have reported finding a huge blob of hot gas around a spiral galaxy, evidence that galaxies like our Milky Way are drawing in material from a gradual inflow of intergalactic gas.

“What we are likely witnessing here is the ongoing galaxy formation process,” said Kristian Pedersen of the University of Copenhagen, Denmark, and lead author of a paper on the finding.

Images from NASA’s orbiting Chandra X-Ray Observatory show the glob, the researchers said: it extends more than 60,000 light years on either side of the starry disk of a galaxy known as NGC 5746. A light year is the distance light travels in a year.


(A composite image of the massive spiral galaxy NGC 5746 shows a large halo of hot gas, blue, surrounding the optical disk of the galaxy, white. The hot gas would not normally be visible to the eye, because it emits X-ray light, which is not visible. Its form is revealed through the X-ray detecting telescope. (Credit: X-ray: NASA/CXC/U. Copenhagen/K.Pedersen et al; Optical: Palomar DSS.)

The discovery eases a long-standing problem for the theory of galaxy formation, the astronomers added.

Spiral galaxies are thought to form from enormous clouds of intergalactic gas that collapse to form great, spinning disks of stars and gas. The theory predicts large spiral galaxies should be immersed in blobs or “halos” of hot gas left over from the formation process.

Hot gas has been found around spiral galaxies before, but usually blowing outward rather than falling in, inconsistent with the theory, the researchers explained.

“Our observations solve the mystery of the missing hot halos around spiral galaxies,” said Pedersen. “The halos exist, but are so faint that an extremely sensitive telescope such as Chandra is needed to detect them.”

NGC 5746 is a massive spiral galaxy about 100 million light years from Earth. Its starry disk is visible almost edge-on. The astronomers believe the hot gas their instruments detected isn’t blowing outward, because there seem to be no ongoing processes that would typically cause this. These would include bursts of star formation or violent activity in the galaxy’s core.

If the gas isn’t going outward, it is probably moving inward, attracted by the galaxy’s gravity, the researchers reasoned.

“What we found is in good agreement with computer simulations in which galaxies are built up gradually from the merger of smaller clouds of hot gas and dark matter,” said Jesper Rasmussen of the University of Birmingham, U.K., and a co-author of the report.

Dark matter is a substance that astronomers think makes up more than 90 percent of the universe by weight, though they don’t know what it is because it’s detectable only through its gravitational pull on nearby objects.

The simulations were carried out by Jesper Sommer-Larsen, also a co-author, and colleagues at the University of Copenhagen. The paper is to appear in the April issue of the research journal New Astronomy.

ABOUT THE AUTHOR

Science writer Jack Lucentini is founder and editor of the World Science science news webzine. He has worked as a staff writer at three daily newspapers, and as a freelance science writer for a range of publications including The Washington Post, Discover magazine and The Scientist magazine. He earned his bachelor's degree at Oberlin College in Oberlin, Ohio in 1993.