The Andromeda galaxy, the nearest spiral galaxy to our Milky Way, is sporting a massive halo of hot gas spanning a million light-years into intergalactic space.
The NASA/ESA Hubble Space Telescope came across the finding after archival data of distant quasars was analysed.
“Halos are the gaseous atmospheres of galaxies. The properties of these gaseous halos control the rate at which stars form in galaxies according to models of galaxy formation,” said Nicholas Lehner of the University of Notre Dame.
This halo is a big deal; it is estimated to contain half the mass of all the stars in the galaxy itself and is 1,000 times more massive than previous estimates suggested. Galactic halos are thought to form the same time as their host galaxies and Lehner’s team have determined that Andromeda’s halo is enriched with heavy elements.
These heavy elements came from periods of intense supernova activity — when massive stars run out of fuel and explode — within the galactic disk and powerful stellar winds blew the heavy element-laced gases into intergalactic space, into Andromeda’s halo. It is thought that nearly half of all the heavy elements generated by supernovae over Andromeda’s lifetime can be found in this extended halo.
Interestingly, if our galaxy also possesses a halo comparable to Andromeda’s it is conceivable that both halos are currently in contact and mixing galactic material. Both galaxies are currently on a collision course and expected to merge in 4 billion years time. Therefore, understanding halo dynamics may reveal that, well before galactic mergers, the respective galaxies’ halos merge quiescently.
The discovery of the Andromeda halo’s immense scale and mass came from the study of distant quasars. Over the past 5 years, Hubble has amassed a wealth of quasar data and the researchers were able to focus on quasar data in the direction of Andromeda. As Andromeda’s halo is dark, it was only detectable by the dimming of quasar emissions. As several quasars have been studied beyond Andromeda, a picture of the halo’s landscape could be constructed (as described in the diagram below.
“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,” said co-investigator J. Christopher Howk, also of Notre Dame. “By measuring the dip in brightness in that range, we can tell how much halo gas from M31 (Andromeda) there is between us and that quasar.”
It is interesting to think that, far from being a vast intergalactic void, the 2.5 million light-year expanse between the Milky Way and Andromeda is in fact filled with hot stellar gases that are already mingling well in advance of the massive galactic merger that is predicted to occur in a few billion years’ time.