Astronomers Discover Milky Way’s Hot Halo

Artist’s illustration of a hot gas halo enveloping the Milky Way and Magellanic Clouds (NASA/CXC/M.Weiss; NASA/CXC/Ohio State/A.Gupta et al.)

Our galaxy — and the nearby Large and Small Magellanic Clouds as well — appears to be surrounded by an enormous halo of hot gas, several hundred times hotter than the surface of the Sun and with an equivalent mass of up to 60 billion Suns, suggesting that other galaxies may be similarly encompassed and providing a clue to the mystery of the galaxy’s missing baryons.

The findings were reported today by a research team using data from NASA’s Chandra X-ray Observatory.

In the artist’s rendering above our Milky Way galaxy is seen at the center of a cloud of hot gas. This cloud has been detected in measurements made with Chandra as well as with the European Space Agency’s XMM-Newton space observatory and Japan’s Suzaku satellite. The illustration shows it to extend outward over 300,000 light-years — and it may actually be even bigger than that.

While observing bright x-ray sources hundreds of millions of light-years distant, the researchers found that oxygen ions in the immediate vicinity of our galaxy were “selectively absorbing” some of the x-rays. They were then able to measure the temperature of the halo of gas responsible for the absorption.

The scientists determined the temperature of the halo is between 1 million and 2.5 million kelvins — a few hundred times hotter than the surface of the Sun.

But even with an estimated mass anywhere between 10 billion and 60 billion Suns, the density of the halo at that scale is still so low that any similar structure around other galaxies would escape detection. Still, the presence of such a large halo of hot gas, if confirmed, could reveal where the missing baryonic matter in our galaxy has been hiding — a mystery that’s been plaguing astronomers for over a decade.

Unrelated to dark matter or dark energy, the missing baryons issue was discovered when astronomers estimated the number of atoms and ions that would have been present in the Universe 10 billion years ago. But current measurements yield only about half as many as were present 10 billion years ago, meaning somehow nearly half the baryonic matter in the Universe has since disappeared.

Recent studies have proposed that the missing matter is tied up in the comic web — vast clouds and strands of gas and dust that surround and connect galaxies and galactic clusters. The findings announced today from Chandra support this, and suggest that the missing ions could be gathered around other galaxies in similarly hot halos.

Even though previous studies have indicated halos of warm gas existing around our galaxy as well as others, this new research shows a much hotter, much more massive halo than ever detected.

“Our work shows that, for reasonable values of parameters and with reasonable assumptions, the Chandra observations imply a huge reservoir of hot gas around the Milky Way,” said study co-author Smita Mathur of Ohio State University in Columbus. “It may extend for a few hundred thousand light-years around the Milky Way or it may extend farther into the surrounding local group of galaxies. Either way, its mass appears to be very large.”

Read the full news release from NASA here, and learn more about the Chandra mission here. (The team’s paper can be found on

Inset image: NASA’s Chandra spacecraft (NASA/CXC/NGST)

NOTE: the initial posting of this story mentioned that this halo could be dark matter. That was incorrect and not implied by the actual research, as dark matter is non-baryonic matter while the hot gas in the halo is baryonic — i.e., “normal” —  matter. Edited. – JM

26 Replies to “Astronomers Discover Milky Way’s Hot Halo”

  1. what is this, minimalist reporting??? We’ve figured out a substantial portion, if not all of dark matter, shown our mass to continue possibly to our neighbours, shown incredible heat in the coldness of intergalactic space, proposed clouds of matter that would influence every phenomenon seen through them and all in a seven-paragraph article?

    1. I thought they were perfectly clear in the article that this was not “dark” matter. This matter is baryonic, and it has been pretty conclusively shown that baryons can only account for a small portion of observed “dark matter.” Besides, the matter we are talking about here was already known to exist; it had just not been detected.

      Instead, this discovery accounts for the discrepancy between the previously observed ratio of baryonic mass to gravitational mass in our galaxy and the ratio predicted by the WMAP. It turns out that many of our galaxy’s baryons are in a large cloud around it.

  2. And why do they always call them HALOS? A halo is a skinny donut, torus, ring, what have you. A spherical cloud is just that, not a halo at all.

  3. Am I understanding this article correctly? Are they saying that this finding obviates the need for dark matter?

  4. So, hot gas…oxygen, check. Hydrogen? Helium? Any way of detecting the other elements?
    How difuse? How did Oxygen form so far away from a source of fusion? Primordial elements cooked in the BB? What percentage of the necessary mass for galactic orbital dynamics? Really, more questions here than answers!!

  5. I recall articles about space ships that had huge hydrogen collecting cones on the front so that they could fuel themselves as they went. Endless supply! Is this looking more promising?? Hot hydrogen could power the ship as well!

  6. I presume that this hydrogen became so hot during the reionization period. Something had to kick these protons and electrons with a fair amount of energy.


    1. What about the time when the Milky Way galaxy had an active galactic nucleus (AGN)?

      BTW, here’s the (PDF) paper:
      A huge reservoir of ionized gas around the Milky Way: Accounting for the Missing Mass?

    2. > lcrowell

      Talking of which – seem to recall you ‘kicking’ me for suggesting there might yet be large pools of as yet undiscovered baryonic matter out there.


      In all seriousness though – it’ll be interesting to see what else can be deduced from this cloud and how it’s temperature varies where it intersects with inter-galactic filaments and perhaps other large-scale structures like satellite galaxies.

      With many Planets, Stars and Galaxies having enveloping gaseous ‘atmospheres’, leaves me wondering whether we might yet discover something similar at the scale of the solar system. Perhaps the heliosheath itself might be warmer, denser and more significant than anyone is expecting?

      Interesting times.

      1. The discovered matter tests standard cosmology and especially its understanding of CMB thoroughly, the amount is about what was predicted I believe.

      2. This 60 billion solar mass halo is a tiny fraction of the dark matter halo that weighs in at around a trillion solar masses. This discovery does little to change that overall picture of the universe. What this does is to account for the massing baryonic matter needed to find a good fit for the end of the radiation dominated period seen in the CMB.


  7. Hotter than the surface of the sun??? WTF does that mean? How can gas that is so sparse be hotter than the surface of the sun?

      1. Anyone want to speculate that this ‘halo’ is a protectant for the galaxy, from an even harsher intergalactic space environment that we have yet to learn about?

  8. if this is true than we got the missing mass of the Universe…so much for the dark matter theory…exactly this way any theory can evaporate one day for a much simpler statement…

    1. The discovered baryonic matter tests standard cosmology and especially its understanding of CMB thoroughly, the amount is about what was predicted I believe.

      And that has already happened with DM as well, which is predicted and observed at the required amount.

      This means standard cosmology and especially its dark matter is a very fruitful and healthy theory.

    2. Dark Matter is just a placeholder for an observed phenomenon that doesnt have a satisfactory explanation yet. If this is the explanation, then it is a success for science because that is what it is doing – looking for an actual explanation.
      But this doesnt seem to provide a full explanation, only a significant portion of the explanation. And the confirmation of similar halos around other galaxies still needs to be done before we can actually include this in detail as part of the Dark Matter problem.

  9. Am wondering if the recently discovered anti matter fountains at either pole of our galaxy tie into this?

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