Chuck Bednar for redOrbit.com – @BednarChuck
Using NASA’s Hubble Space Telescope, a team of astronomers have discovered that a massive halo of gas surrounding the neighboring Andromeda galaxy is approximately six times larger and 1,000 times more massive than previous measurements had indicated.
Credit: NASA/STScI
According to the US space agency, the dark and nearly invisible halo stretches about one million light years from its host galaxy (about halfway to our own Milky Way galaxy). The discovery is expected to provide new insight about the evolution and structure of giant spiral galaxies, which are one of the most common types of galaxies in the universe.
“Halos are the gaseous atmospheres of galaxies. The properties of these gaseous halos control the rate at which stars form in galaxies according to models of galaxy formation,” said Nicolas Lehner, an astrophysicist at the University of Notre Dame and lead author of a study published online Monday in The Astrophysical Journal.
Using quasars and Hubble’s UV capabilities to investigate
Lehner and his colleagues report that the halo is believed to contain half of the mass of the stars in the entire Andromeda galaxy in the form of a hot, diffuse gas. If the feature was visible to the naked eye, they added, it would be 100 times the diameter of the full moon.
Since the gas cannot be seen, however, the researchers had to look at bright background objects whose light is obscured by the gas in the halo and observe changes in its brightness. They used quasars, distant star-like objects that are the cores of active galaxies powered by black holes, to investigate how material is distributed behind Andromeda’s visible disk.
“As the light from the quasars travels toward Hubble, the halo’s gas will absorb some of that light and make the quasar appear a little darker in just a very small wavelength range,” said co-author J. Christopher Howk, an associate professor of physics at Notre Dame. “By measuring the dip in brightness, we can tell how much halo gas from M31 there is between us and that quasar.”
Andromeda, which the most massive galaxy in the Local Group (which includes the Milky Way and about 45 other known galaxies), is home to one trillion stars and is located 2.5 million light-years from the Milky Way. The researchers were able to examine Andromeda’s halo thanks to Hubble’s unique ability to study ultraviolet light at high spectroscopic resolution.
These capabilities allow spectral features to be seen and accurately modeled, giving scientists a glimpse at fundamental information about the nature and extent of the galaxy’s halo gas. In their research, the Notre Dame-led team looked at five years worth of Hubble data, finding that over the course of the galaxy’s lifetime, nearly half of all heavy elements made by its stars have been expelled far beyond its 200,000 light-year-diameter stellar disk.
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