Galaxies are not isolated systems, they are actually very open. Stars come in and fly out, gas flows in and is pushed out, and sometimes other galaxies just smash into them. This is happening everywhere in the universe and it got researchers wondering, is the Milky Way gaining or losing mass or is it keeping its books balanced?
As reported in a paper accepted by The Astrophysical Journal, researchers studied the flow of gas around our galaxies to find out. They estimate that our galaxy loses about 53,000 Earth's worth of gas each year, but gains over three times as much: 176,000 Earths, or 53 percent of the mass of the Sun. This was quite a surprise for the researchers.
"We expected to find the Milky Way's books balanced, with an equilibrium of gas inflow and outflow, but 10 years of Hubble ultraviolet data has shown there is more coming in than going out," lead author Andrew Fox of the Space Telescope Science Institute, Baltimore, said in a statement.
Most of the gas being siphoned by our galaxy is from the intergalactic medium, but the team suspects that some could be coming from the small satellite galaxies that surround the Milky Way. Our galaxy has already cannibalized some of its companions, so stealing from them is really not a big deal.
The observations were possible thanks to Hubble’s Cosmic Origins Spectrograph (COS), which was installed in the last servicing mission in 2009. The gas is very diffuse, making it invisible, so the team used the light of distant quasars to work out the properties of these gas clouds. The 200 observations of the diffused gas halo provided them with enough data to paint this peculiar picture of the Milky Way.
"Studying our own galaxy in detail provides the basis for understanding galaxies across the universe, and we have realized that our galaxy is more complicated than we imagined," co-author Philipp Richter of the University of Potsdam in Germany, added.
The team has several ideas on how they are going to follow up this research. They are interested in understanding precisely what the sources of the gas inflow are, and studying how other big galaxies behave. There should be enough observations of Andromeda, our nearest large galaxy, to do a similar study.