This tenuous, nearly invisible halo of diffuse plasma extends 1.3 million light-years from the galaxy -- about halfway to our Milky Way -- and as far as two million light-years in some directions, said the study published in The Astrophysical Journal.
This means that Andromeda's halo is already bumping into the halo of our own galaxy.
The scientists also found that the halo has a layered structure, with two main nested and distinct shells of gas.
"Understanding the huge halos of gas surrounding galaxies is immensely important," explained co-investigator Samantha Berek of Yale University in New Haven, Connecticut, US.
"This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae," Berek said, adding that it is full of clues regarding the past and future evolution of the galaxy.
A signature of this activity is the team's discovery of a large amount of heavy elements in the gaseous halo of Andromeda.
Heavier elements are cooked up in the interiors of stars and then ejected into space -- sometimes violently as a star dies.
The halo is then contaminated with this material from stellar explosions.
The Andromeda galaxy, also known as M31, is a majestic spiral of perhaps as many as one trillion stars and comparable in size to our Milky Way.
At a distance of 2.5 million light-years, it is so close to us that the galaxy appears as a cigar-shaped smudge of light high in the autumn sky.
If its gaseous halo could be viewed with the naked eye, this would easily be the biggest feature on the night-time sky.