The nearest large galaxy to our own has a halo that stretches out for a million light-years around it and almost half way to the Milky Way, The Hubble Telescope has observed. The halo is six times as large as previously thought, and more surprisingly, has a thousand times the anticipated mass.

As the delicate gas surrounding galaxies, halos might not seem that important. But Dr Nicolas Lehner of the University of Notre Dame said, “The properties of these gaseous halos control the rate at which stars form in galaxies.” The discovery that Andromeda's halo contains half the mass of that galaxy's stars, announced in The Astrophysical Journal, suggests they are more important than we knew.

The Andromeda Galaxy, also known as M31, is the most similar member of the Local Group of galaxies to the Milky Way in size, although it contains 2-5 times as many stars. As such it is frequently used as a model for our own galaxy, which can be hard to study from the inside.

Galactic halos are difficult to scrutinize in depth, since the gas is dark in the absence of stars to light it up. Lehner and his colleagues looked for quasars which were potentially partially obscured by the halo. "As the light from the quasars travels toward Hubble, the halo's gas will absorb some of that light and make the quasar appear a little darker in just a very small wavelength range," says co-author J Christopher Howk. The reduction in brightness seen at particular ultraviolet wavelengths is attributed to the halo gas.

At such great distances, it's not precisely clear whether interfering gas comes from Andromeda's halo or the surrounds of a smaller galaxy, or perhaps the Milky Way. However, the authors say, “We present several arguments that gas...observed in these directions originates from M31 circumgalactic medium (CGM).” In particular, they show that the gas is moving at velocities similar to dwarf galaxies that circle Andromeda, but very different from those expected if it was around other objects. The halo also appears to end quite suddenly around a million light-years from Andromeda.

The absorption characteristics of the halo suggest that it is not only primeval hydrogen and helium, but has been enriched with heavier elements. The authors propose that these elements are the products of supernovae, expelled beyond the galaxy by the force of the explosion of the star that formed them.

The authors demonstrate that the further one gets from Andromeda, the more heavily ionized the hydrogen component of the gas becomes.

Andromeda is visible under dark skies and is around six times the diameter of the moon. However, if we could see the halo it would be capable of stretching from the horizon halfway to overhead.

If the Milky Way's halo is of similar size the two would almost touch, a preview of the day when the two galaxies will combine. However, measuring the boundaries of our own halo is much harder.