The merger of the Milky Way and Andromeda galaxy won’t
happen for another 4 billion years, but the recent discovery of a massive halo
of hot gas around Andromeda may mean our galaxies are already touching. The Andromeda
Galaxy is the largest member of a ragtag collection of some 54
galaxies, including the Milky Way, called the Local Group.
University of Notre Dame astrophysicist Nicholas
Lehner led a team of scientists using the Hubble
Space Telescope to identify an enormous halo of hot, ionized gas at
least 2 million light years in diameter surrounding the galaxy. With a trillion
stars — twice as many as the Milky Way — it shines 25% brighter and can easily
be seen with the naked eye from suburban and rural skies.
Think about this for a moment. If the halo extends
at least a million light years in our direction, our two galaxies are MUCH
closer to touching that previously thought. Granted, we’re only talking halo
interactions at first, but the two may be mingling molecules even now if our galaxy is similarly
cocooned. Lehner describes halos as the “gaseous atmospheres of galaxies”.
Despite its enormous size, Andromeda’s nimbus is virtually invisible.
To find and study the halo, the team sought out
quasars, distant star-like objects that radiate tremendous amounts of energy as
matter funnels into the supermassive black holes in their cores. The brightest
quasar, 3C273 in Virgo, can be seen in a 6-inch telescope! Their brilliant,
pinpoint nature make them perfect probes.
To detect Andromeda’s halo, Lehner and team studied how the light of 18 quasars (five shown here) was absorbed by the galaxy’s gas. Credit: NASA
“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 J. Christopher Howk , associate professor of physics at Notre Dame and co-investigator. “By measuring the dip in brightness, we can tell how much halo gas from M31 there is between us and that quasar.”
Astronomers have observed halos around 44 other
galaxies but never one as massive as Andromeda where so many quasars are
available to clearly define its extent. The previous 44 were all extremely
distant galaxies, with only a single quasar or data point to determine halo
size and structure. Andromeda’s close and huge with lots of quasars peppering
its periphery.
Six examples of quasars photographed with the Hubble. Quasars are distant, brilliant sources of light, believed to occur when a massive black hole in the center of a galaxy feeds on gas and stars. As the black hole consumes the material, it emits intense radiation, which is then detected as a quasar. Lehner and team measured Andromeda’s halo by studying how its gas affected the light from 18 different quasars. Credit: NASA/ESA
The team drew from about five years’ worth of
observations of archived Hubble data to find many of the 18 objects needed for
a good sample. The halo is estimated to contain half the mass of the stars in
the Andromeda galaxy itself, in the form of a hot, diffuse gas. Simulations
suggest that it formed at the same time as the rest of the galaxy.
Although mostly composed of ionized hydrogen — naked
protons and electrons — Andromeda’s aura is also rich in heavier elements,
probably supplied by supernovae. They erupt within the visible galaxy and
violently blow good stuff like iron, silicon, oxygen and other familiar
elements far into space. Over Andromeda’s lifetime, nearly half of all the
heavy elements made by its stars have been expelled far beyond the galaxy’s
200,000-light-year-diameter stellar disk.
You might wonder if galactic halos might account for
some or much of the still-mysterious dark
matter. Probably not. While dark matter still makes up the bulk of the
solid material in the universe, astronomers have been trying to account for the
lack of visible matter in galaxies as well. Halos now seem a likely
contributor.
The next clear night you look up to spy Andromeda,
know this: It’s closer than you think!
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