The Andromeda Galaxy is the closest large spiral to our own Milky Way, and the only major Galaxy moving toward us. Turns out its on a direct collision course, but we still have 3.5 Billion years to prepare, so its not exactly pressing news. On the plus side, studying Andromeda allows us to infer properties of more distant galaxies, and it gives us a map of what our own Milky Way Galaxy may look like. Astronomers using the Hubble Space Telescope have discovered a giant halo of gas around Andromeda, and the Milky Way may have a similar one.
By looking at a population of 18 quasars in the rough direction of Andromeda in a specific wavelength of light, astronomers noticed that some of the quasars appeared slightly dimmer in this wavelength than they should. By measuring this tiny dip, the astronomers could calculate how much gas was present between the quasar and Earth. By using this technique for each quasar, they found an estimate for the size of the cloud and its distribution of gas.
Previous studies have looked at 44 distant galaxies and found gaseous haloes similar to Andromeda’s, though they could not be mapped in such an extensive way. Their distance is much greater so they appear smaller on the sky, and only one quasar was found along the line of sight of each galaxy, giving a less precise measure of the amount of gas in each halo.
Based on their calculations, the astronomers estimate that over andormeda’s lifetime, nearly half of the heavy elements forged in the heart of its stars have been expelled into the halo surrounding the galaxy. This is a huge amount of gas to lose.