The giant halo of Andromeda

Thanks to the data collected by Hubble a team of scientists was able to accurately estimate the size of the halo of Andromeda gas: about half of the contents of the stars in the same galaxy. This discovery allows us to learn more on the evolution and the structure of one of the most common types of galaxies in the universe.
A team of scientists using data from NASA’s Hubble Space Telescope has found that the vast halo of gas that surrounds the Andromeda galaxy is about six times the size and 1,000 times more massive than was previously estimated. The dark halo and nearly invisible surrounding Andromeda spans about one million light years from its host galaxy, halfway than our Milky Way. This discovery provides valuable information to learn more about the evolution and structure of the majestic giant spirals, one of the most common types of galaxies in the universe.


Credit: NASA, ESA, and A. Feild (STScI)

“The halos are the gas atmospheres of galaxies. According to models of galaxy formation, the properties of these halos have repercussions on the rate at which stars form in galaxies, “explained Nicolas Lehner, head of the study and researcher at the University of Notre Dame in Indiana. It is estimated that the vast mass of the moon alone contains half of the stars in the Andromeda galaxy in the form of a hot gas. If you could see with the naked eye, the halo would occupy a large portion of the sky 100 times the diameter of the full moon, which is equivalent to the size of two basketballs kept at a distance of an outstretched arm.
The Andromeda galaxy is 2.5 million light years away and appears as a faint cloud tapered great about 6 times the diameter of the full Moon. It is considered a galaxy near-twin of the Milky Way.
Since the gas halo of Andromeda does not emit light, the team studied the bright objects that were prospectively in the background and it looked like you changed their light passing through the gas. It is a bit like watching a glowing light on the bottom of a pool at night. The ideal sources of light for a study like this are quasars, extremely bright nuclei of active galaxies powered by black holes. The team used 18 quasars that are from our point of view behind Andromeda, and thanks to these observations could fathom how it distributed the gaseous material beyond the visible disk of the galaxy. Their findings will be published next May 10 in the journal The Astrophysical Journal.
Previous research, called Hubble Cosmic Origins Spectrograph (COS) Halos program, had studied 44 galaxies finding halos such as Andromeda, but never before had seen a halo so massive in a nearby galaxy. Because galaxies studied previously they were much more distant, appeared much smaller in the sky. Thus, at each galaxy it was possible to observe a single quasar, providing a single point of anchorage to map their size and structure of the halo. With its proximity to the Earth and its resulting amplitude in the sky, Andromeda offers a chance to sample much larger.
“As the light of the quasar travels to the Hubble telescope, the gas absorbs some of the halo ‘and make the quasar a bit’ less bright in a short range of wavelengths,” says J. Christopher Howk, who He collaborated on the study and who works at the University of Notre Dame. “By measuring the decrease in brightness in the end we can estimate how much gas halo of the galaxy between us and the quasar.”
Scientists have taken advantage of the unique ability of Hubble to study the ultraviolet light coming from the quasar. The light in the ultraviolet band is absorbed by the atmosphere, making it difficult to observe with a telescope from the ground. To conduct this research, the team has used about 5 years of observations in the data collected by Hubble. Several previous observational campaigns carried out with Hubble have used quasars to study halos of gas more distant Andromeda but always in his direction, so there was already a rich set of data.
But what is the origin of this giant halo? Large-scale simulations suggest that the halo is formed simultaneously with the rest of Andromeda. The team also determined that the halo has been enriched by elements much heavier than hydrogen and helium, and the only way to obtain the presence of these elements is assumed the explosion of supernovae. Supernovae explode in the disk of the galaxy and blowing violently heavy elements into space away. During the life of Andromeda, almost half of all heavy elements were expelled from its stars well over 200,000 light years of his stellar diameter.
What does this mean for our galaxy? Since we live inside the Milky Way, scientists are not able to determine if our aura is as massive and extended. If the Milky Way was surrounded by a halo as large, the halos of the two galaxies could almost touch and start to blend into each other long before they meet the galaxies. Observations made with the Hubble telescope show in fact that the Andromeda galaxy and the Milky Way will merge to form a giant elliptical galaxy between about 4 billion years.

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