Andromeda galaxy's 'halo' is nudging the Milky Way

A halo is the large envelope of gas that surrounds a galaxy. Scientists used Hubble to comprehensively study and map Andromeda's halo and were surprised to discover its unique structure, as well as its massive size.

The halo extends out 1.3 million light-years from the galaxy, almost halfway to our galaxy, and as much as 2 million light-years in other directions.

These galaxies are actually on a collision course that will cause them to merge 4 billion years from now. This merger will form one giant elliptical galaxy. And given the extension of Andromeda's halo in our direction, it's already nudging the Milky Way's galactic halo.

"Understanding the huge halos of gas surrounding galaxies is immensely important," said Samantha Berek, co-investigator of the new study published Thursday in The Astrophysical Journal and undergraduate student at Yale University, in a statement.

"This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. It's full of clues regarding the past and future evolution of the galaxy, and we're finally able to study it in great detail in our closest galactic neighbor."

Andromeda's halo also has two nested layers of gas that form distinct shells.

"We find the inner shell that extends to about a half million light-years is far more complex and dynamic" than the outer shell, said Nicolas Lehner, lead study author and research professor of astrophysics at the University of Notre Dame, in a statement.

"The outer shell is smoother and hotter. This difference is a likely result from the impact of supernova activity in the galaxy's disk more directly affecting the inner halo."

The research team found a large amount of heavy elements in Andromeda's halo that were likely released from the interiors of stars when the stars exploded, called supernova explosions.

Given how similar the two galaxies are, understanding Andromeda's halo could shed light on the Milky Way's halo as well -- which is harder to study because we live inside the Milky Way.

Understanding our galactic neighbor

Andromeda, which likely contains as many as 1 trillion stars, is similar in size to our large galaxy, and it's only 2.5 million light-years away. That may sound incredibly distant but on an astronomical scale, that makes Andromeda so close that it's visible in our autumn sky. You can see it as a fuzzy cigar-shaped bit of light high in the sky during the fall.

And if we could see its massive halo, which is invisible to the naked eye, it would be three times the width of the Big Dipper constellation, dwarfing anything else in our sky.

While the gaseous halo may be invisible to our eyes, it's clear to Hubble's ultraviolet light capabilities.

Hubble orbits Earth from 340 miles away, and it's been operating for 30 years. Ground-based telescopes can't observe ultraviolet light because it's absorbed by our atmosphere.

A previous study of Andromeda's halo, conducted in 2015, was only able to use the light from six quasars, or distant active galactic cores that use black holes as their engines. While scientists have been able to study the halos of much more distant galaxies, they appear so small that the researchers can only use about one quasar.

How did they do it?

For the study, researchers used the Project AMIGA program, or Absorption Map of Ionized Gas in Andromeda, collected by Hubble. This included an analysis of light from 43 quasars that were scattered behind Andromeda's halo.

The scattered light from these quasars allowed the researchers to analyze multiple parts of the halo and determine how it absorbed the light of the quasars. The scientists were able to detect how parts of the halo absorbed light differently.

This light absorption was the best way to study the halo and its composition because it doesn't emit easily detectable radiation. However, Hubble was used to study the ultraviolet light emitted by the quasars, allowing the research team to detect signatures of carbon, silicon and oxygen in the gas.

"This is truly a unique experiment because only with Andromeda do we have information on its halo along not only one or two sightlines, but over 40," Lehner said. "This is groundbreaking for capturing the complexity of a galaxy halo beyond our own Milky Way."