A vast halo of cosmic gas surrounding our nearest neighbor galaxy, the Andromeda Galaxy, is much larger and much more massive than previous observations have suggested, astronomers say.
Measures conducted using the Hubble Space Telescope show the nearly invisible halo is six times as large and a thousand times as massive than was previously thought, they say.
It stretches outward from Andromeda for almost a million light years, they add, almost halfway toward our own home Milky Way galaxy, and the hot, diffuse gas is estimated to contain half the mass of all the stars in the galaxy itself.
If our Milky Way has a similar halo, and scientists see no reason why it wouldn't, then our galaxy and our nearest neighbor might be mixing material in the interstellar space between them.
Studying Andromeda's halo will yield insights into the formation, structure and evolution of giant spiral galaxies, one of the more common types in the universe.
"Halos are the gaseous atmospheres of galaxies," says study lead investigator Nicolas Lehner from the Notre Dame University. "The properties of these gaseous halos control the rate at which stars form in galaxies according to models of galaxy formation."
Such halos are so tenuous as to be invisible, requiring some innovative observational techniques to detect them.
For the new study, the astronomers used observations in ultraviolet wavelengths of 18 distant quasars to map the halo of Andromeda, also known as the M31 galaxy.
"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," Notre Dame astronomer J. Christopher Howk explains. "By measuring the dip in brightness in that range, we can tell how much halo gas from M31 there is between us and that quasar."
The halo, consisting of hydrogen and helium, probably formed at the same time as the rest of Andromeda, and has since been enriched with heavier elements created within the galaxy by exploding supernovae.
Over the galaxy's lifetime, almost half of those heavy elements have likely been expelled outward far beyond the 200,000-light year diameter of its visible stellar disk, the astronomers say.
Our Milky Way galaxy and Andromeda, already close neighbors, are moving even closer and will eventually collide and merge, forming one giant elliptical galaxy — in about 4 billion years, the astronomers say.
