NASA’s Hubble Space Telescope recently provided scientists with new insights into Andromeda galaxy’s massive halo. Researchers learned that the halo is six times larger and 1,000 times heavier than it was previously thought.
Taking into account the new finding, scientists speculate that Andromeda’s halo, which is invisible to the naked eye, may be nearly touching with Milky Way’s halo (if any). Yet, telling whether our galaxy holds a similar massive halo is impossible at this moment because we need to stand outside it to get the bigger picture.
Nevertheless, if Milky Way possesses a similar halo, researchers believe that the two galaxies may have already started to merge and will not finish the process until 4 billion years from now.
Moreover, the new finding looks very promising to astronomers because it can provide new details on the structure and current evolution of spiral galaxies such as the Milky Way and Andromeda.
Nicolas Lehner, the lead-author of the finding and senior researcher at the University of Notre Dame, Indiana, compared a galactic halo to a gaseous atmosphere of galaxies. Prof. Lehner explained that galactic halos usually provide crucial hints on the pace at which stars form within the galaxies that those halos envelop.
Scientists explain that Andromeda’s halo is so huge that it contains half the mass of all stars within the galaxy. And if it could be observed without help from a space telescope, we would see it on the night sky 100 times larger than the size of a full Moon.
Andromeda itself is about 6 times larger than the size of full Moon and the nearest largest galaxy to the Milky Way because it is located 2.5 million light-years away from our galaxy. For that reason, scientists usually consider the pair galactic “near-twins.”
But because the halo around Andromeda galaxy is extremely dark, scientists had to use a visual trick to be able to analyze it – they first looked at a bright background consisting in 18 cosmic lights, or quasars. Quasars are bright centers of galaxies that surround the massive black holes each of these galaxies have at their core.
Scientists studied the variations in the light emitted by those really distant quasars to take a grasp of the actual size and mass of Andromeda’s halo.
They published results of their research Sunday in The Astrophysical Journal.
Although, Hubble studied 44 galaxies and their halos, only Andromeda was close enough to our galaxy to allow the space telescope to render an accurate map of its halo. Moreover, because of its large area covered on the sky only Andromeda could provide a sufficient sampling of background cosmic lights. Because they looked very small on the sky, other galaxies had only one quasar located behind them. So mapping their halos was a daunting task.
Researchers explained that the light emitted by the 18 quasars is partially absorbed by the gaseous atmosphere around Andromeda. As a result, its halo seems darker than it really is, if observed under a “small wavelength range.”
“By measuring the dip in brightness in that range, we can tell how much halo gas from Andromeda there is between us and that quasar,”
one of the scientists added.
Image Source: Parsseh