Astronomers have detected a massive yet elusive nimbus of hot gas surrounding Andromeda Galaxy.

Andromeda's halo
Andromeda's Halo dwarfs the galaxy itself, extending out to a million light-years from the galaxy's center.

Our sister galaxy, astronomers have discovered, lounges in a gargantuan cushion of hot gas that extends out to at least a million light-years, almost halfway to the Milky Way.

Andromeda Galaxy is in Milky Way’s weight class, 220,000 light-years across and containing hundreds of billions of stars. And it turns out its halo is almost as massive as the galaxy itself — Nicolas Lehner (University of Notre Dame) and colleagues calculate in the May 10th Astrophysical Journal that at least 10 billion Suns’ worth of gas floats outside Andromeda.

Despite its mass and extent, this halo has proven difficult to study. Its gas is hot (around 10,000 or 100,000 degrees), mostly made of ionized hydrogen (bare protons and electrons floating in the intergalactic breeze), and so sparse that hydrogen’s signal can’t be detected.

Yet astronomers have long suspected that this kind of halo must surround most galaxies. Of the universe’s mass, 17% is normal matter (not the mysterious, dark variety). Based on a galaxy’s dark mass, which can be measured for example by the galaxy's rotation curve, you expect a certain amount of accompanying normal matter. But if you add up a galaxy’s normal matter in the form of stars, cold interstellar gas, and X-ray-emitting (read: very hot) halo gas, you come up short. Only about 40% of normal matter is accounted for.

The missing matter must be hiding somewhere, undetected, and the most likely spot is galactic halos. Simulations agree: hot gas, both inflowing and outflowing, ought to envelop growing galaxies.

To reveal this gas, Lehner’s team pinointed 18 distant quasars whose light streams through the space where Andromeda’s halo ought to be. Heavier (and rarer) elements stripped of their outer electrons, such as doubly ionized silicon, will cast a shadow on this background light, and the Hubble Space Telescope recorded the specific ultraviolet wavelengths that the halo gas absorbed.

Measuring Andromeda's Halo
To detect Andromeda's gargantuan halo, Lehner's team looked through the halo to 18 distant quasars (not all of them shown here), measuring where and how the halo absorbed their light.

The measurements show that Andromeda’s circumgalactic medium is massive, containing 3 billion Suns’ worth of gas within 200,000 light-years and probably 10 times that out to a million light-years.

Almost all of this gas is ionized, and in fact, Lehner’s team finds that the gas gets even more ionized further from the galaxy, confirming theoretical predictions. Because the hotter gas is more difficult to detect (and can’t be detected within these Hubble observations), it’s possible the halo continues even further than what the team was able to record.

In short, Lehner says, this study and an accompanying study of halos in more distant galaxies have “essentially solved” the missing matter problem.

To read more about the study (and what it means for the pending collision between Andromeda and Milky Way), see Hubble’s press release.

Intrigued by galaxies? Subscribe to Sky & Telescope and receive a special feature on how galaxies grow in our September issue.


Image of Faye_Kane_girl_brain


May 13, 2015 at 7:11 pm

First if all, another excellent Monica story, thank you!

> the gas gets even more ionized further from the galaxy.

Why would that be? If the ionization energy comes from Andromeda, it would be stronger near the center. And if it's extragalactic, it would be isotropic.

Is this the galaxy equivalent of the Oort cloud?

> this seems to have “essentially solved the missing matter problem.

Dark matter was conjured up as an explanation for the anomalous orbits of the outermost stars in a galaxy. If this is this the the so-called "dark matter" and If it's spherically symmetric, how could it possibly affect the shape of arms and bars, and the galactic year length in the host galaxy?

—Faye Kane ♀ girl brain

You must be logged in to post a comment.

Image of Peter Wilson

Peter Wilson

May 13, 2015 at 7:43 pm

Missing matter problem is not the dark matter problem.

You must be logged in to post a comment.

Image of Monica Young

Monica Young

May 14, 2015 at 9:38 am

Hi Faye, great questions! To the first question, the hot gas is expected as part of galaxy evolution, it comes from inflows/outflows to/from the galaxy. The gas further out is hotter than the inner gas simply because hotter gas will tend to be more spread out. To the second question, Peter is correct, the missing matter problem described in this article isn't dark matter, it's just ordinary matter. It hadn't been detected yet because this hot and diffuse gas is very difficult to see.

You must be logged in to post a comment.

Image of jaime


May 14, 2015 at 1:00 pm

This halos on both galaxies makes then closser together 🙂 Which makes them more romantic and excitement to watch Andromeda ; our only galaxie seen on a dark clean night with a good view of our eyes.

You must be logged in to post a comment.

Image of Mike


May 14, 2015 at 1:31 pm

The Hubble press release linked in this article states "Because we live inside the Milky Way, scientists cannot determine whether or not such an equally massive and extended halo exists around our galaxy." Yet we can observe these wavelengths of light from the distant Quasars, which are blocked by Andromeda's halo. Doesn't that prove that our galaxy does NOT have a similar halo?

You must be logged in to post a comment.

Image of StarChaser55


May 14, 2015 at 8:39 pm

Mike, perhaps it is because there is no "clean" line of sight - you would see no dip (shadow) at the wavelength of doubly ionized silicon no matter which way you looked.

You must be logged in to post a comment.

Image of Monica Young

Monica Young

May 15, 2015 at 1:36 pm

Hi Mike: Yes, you're right on the mark that several sightlines to quasars ought to traverse the Milky Way's halo as well. The problem is, we're in the Milky Way and that limits what we can see - quasars along the galactic plane are often obscured by our galaxy's dust and gas. This problem is confounded by the fact that the halo gas is probably denser near the galactic plane. So the further from the plane you go, the clearer your view, but also the sparser the gas. Ultimately, Milky Way's halo becomes difficult to detect, and especially difficult to detect it reliably at different distances from our galaxy's center.

You must be logged in to post a comment.

Image of

May 15, 2015 at 12:42 pm


Thanks for the link to Hubble's press release.

Is there any estimate on the number of supernovae during Andromeda's lifetime?

You must be logged in to post a comment.

Image of Monica Young

Monica Young

May 15, 2015 at 1:43 pm

Good question. The common number quoted for the Milky Way is one supernova per century. I would assume Andromeda Galaxy's rate would be roughly the same (to within an order of magnitude), seeing as it's also a disk galaxy, but it's star formation rate is lower. One source ( says that Andromeda's supernova rate is about half that of the Milky Way.

You must be logged in to post a comment.

You must be logged in to post a comment.