SOUTH BEND, Ind., May 8 (UPI) -- The massive halo of gas that surrounds the Andromeda Galaxy, the Milky Way's nearest neighbor, is much larger than previously thought.
New data collected by NASA's Hubble Space Telescope suggest the feature is six times larger and 1,000 times more massive than the last time scientists measured. Hubble's probe of the gaseous halo is offering a chance for researchers to better understand the evolution of spiral galaxies.
"Halos are the gaseous atmospheres of galaxies," study leader Nicolas Lehner, a researcher at the University of Notre Dame, explained in a press release. "The properties of these gaseous halos control the rate at which stars form in galaxies according to models of galaxy formation."
But faraway gas is hard to see, even with telescopes -- hence the difficulty in measuring the size of Andromeda's halo. To get a more accurate sense of the halo, researchers measured how light from stars behind Andromeda changes as it passes through the halo's gas -- like measuring the water's depth using the lights at the bottom of a pool.
Scientists zeroed in on light emitted by quasars, some of the most luminous bodies in space. They found 18 quasars behind Andromeda's large footprint. Quasars are the compact areas of gas and matter that glow with energy as they're sucked into the accretion disc around a black hole.
"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," explained researcher J. Christopher Howk, also a scientist at Notre Dame. "By measuring the dip in brightness in that range, we can tell how much gas there is between us and that quasar."
Further analysis, including computer modeling, showed Andromeda's halo to contain a plethora of heavy elements -- elements expelled by supernovae over the galaxy's lifetime.
Researchers don't know whether the Milky Way also has a halo, but in a recent survey, scientists found halos around 44 distant galaxies. None of them, however, were as massive as Andromeda's.
If the Milky Way does have a halo, it's likely nearly touching the halo of Andromeda. The two halos will merge well before the two galaxies collide four billion years from now.