Andromeda and the Milky Way might collide sooner than expected
© NASA
Andromeda’s halo is gargantuan. Extending for at least 2 million light years, if we could see in our night sky it would be 100 times the diameter of the Moon or 50 degrees across!
The Andromeda Galaxy is the largest member of a ragtag collection of some 54 galaxies, including the Milky Way, called the Local Group. 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.
© NASA/ESA
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.
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.
© NASA
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.
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. 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.
© NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger
This illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. In this image, representing Earth’s night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull.
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!
For more on the topic, here are links to Lehner's paper in and the Hubble release.
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