The closest galaxy to our own Milky Way is Messier 31, or M31, better known as Andromeda. Andromeda is 2.5 million light years away much larger than the Milky Way, and has about twice the number of stars. According to new research is may have once been even larger still. Nearly half of the heavy elements made by the Stars in Andromeda have made their way out, forming a massive halo which stretches about a million light years beyond the visible perimeter of M31.
The gas is invisible, but researchers at the University of Notre Dame used NASA’s Hubble Space Telescope to measure it by observing the filtering effect it has on the light of distant quasars.
Andromeda’s halo is estimated to contain half as much mass as all of the stars in that galaxy combined.
According to the researchers, studying this and other halos will provide insight into the evolution and structure of spiral galaxies.
“Halos are the gaseous atmospheres of galaxies. The properties of these gaseous halos control the rate at which stars form in galaxies,” said Nicolas Lehner, the lead investigator, in a statement.
The gas of galactic halos, like many other objects that are transparent or too small to see, is studied by looking at bright background objects whose light is filtered or distorted by them. Ideally astronomers use quasars for this, because they are very bright.
A quasar is created when material falls into a black hole. Gas, and other material form an accretion disk around the black hole, as it rotates, this material heats up to temperatures of million of degrees and emits a stream of radiation.
By observing the quasars, astronomers can see the gas or, more accurately, the effect of the gas on the quasar’s light.
“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. By measuring the dip in brightness, we can tell how much halo gas from M31 there is between us and that quasar,” said J. Christopher Howk, associate professor of physics at Notre Dame, and co-investigator on the project.
The same method has been used to observe other, more distant, galaxies but none have ever been observed at such close range. Because of the proximity and the size of the halo, the researchers were able to use 18 different quasars at varying distances to determine the presence and size of Andromeda’s halo.
“This is a new milestone because typically only one quasar is used to probe the halos of galaxies beyond the Local Group [of galaxies]. Here we have assembled a large sample of quasars that directly demonstrate the true extent of the halo of a single massive galaxy,” said Lehner.
The team drew from five years of Hubble data to make their observations and hope to gather more data still in order to better understand the relationship between galaxies and their halos.
In about 4 billion years, Andromeda and the Milky Way will collide to create a new much larger galaxy. Stars are so far apart that when galaxies collide there is little chance of collisions between them. However, the increase in material and gravitational forces can lead to an increase in new star formation and supernovae.
The new research on Andromeda has been published in the Astrophysical Journal.
