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NASA's Chandra Shows Milky Way is Surrounded by Halo of Hot Gas

For Release: September 24, 2012


Galactic Halo
Credit: Illustration: NASA/CXC/M.Weiss; NASA/CXC/Ohio State/A Gupta et al
Press Image and Caption

WASHINGTON -- Astronomers have used NASA's Chandra X-ray Observatory to find evidence our Milky Way Galaxy is embedded in an enormous halo of hot gas that extends for hundreds of thousands of light years. The estimated mass of the halo is comparable to the mass of all the stars in the galaxy.

If the size and mass of this gas halo is confirmed, it also could be an explanation for what is known as the "missing baryon" problem for the galaxy.

Baryons are particles, such as protons and neutrons, that make up more than 99.9 percent of the mass of atoms found in the cosmos. Measurements of extremely distant gas halos and galaxies indicate the baryonic matter present when the universe was only a few billion years old represented about one-sixth the mass and density of the existing unobservable, or dark, matter. In the current epoch, about 10 billion years later, a census of the baryons present in stars and gas in our galaxy and nearby galaxies shows at least half the baryons are unaccounted for.

In a recent study, a team of five astronomers used data from Chandra, the European Space Agency's XMM-Newton space observatory and Japan's Suzaku satellite to set limits on the temperature, extent and mass of the hot gas halo. Chandra observed eight bright X-ray sources located far beyond the galaxy at distances of hundreds of millions of light-years. The data revealed X-rays from these distant sources are absorbed selectively by oxygen ions in the vicinity of the galaxy. The scientists determined the temperature of the absorbing halo is between 1 million and 2.5 million kelvins, or a few hundred times hotter than the surface of the sun.

Other studies have shown that the Milky Way and other galaxies are embedded in warm gas with temperatures between 100,000 and 1 million kelvins. Studies have indicated the presence of a hotter gas with a temperature greater than 1 million kelvins. This new research provides evidence the hot gas halo enveloping the Milky Way is much more massive than the warm gas halo.

"We know the gas is around the galaxy, and we know how hot it is," said Anjali Gupta, lead author of The Astrophysical Journal Letters paper describing the research. "The big question is, how large is the halo, and how massive is it?"

To begin to answer this question, the authors supplemented Chandra data on the amount of absorption produced by the oxygen ions with XMM-Newton and Suzaku data on the X-rays emitted by the gas halo. They concluded that the mass of the gas is equivalent to the mass in more than 10 billion suns, perhaps as large as 60 billion suns.

"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 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."

The estimated mass depends on factors such as the amount of oxygen relative to hydrogen, which is the dominant element in the gas. Nevertheless, the estimation represents an important step in solving the case of the missing baryons, a mystery that has puzzled astronomers for more than a decade.

Although there are uncertainties, the work by Gupta and colleagues provides the best evidence yet that the galaxy's missing baryons have been hiding in a halo of million-kelvin gas that envelopes the galaxy. The estimated density of this halo is so low that similar halos around other galaxies would have escaped detection.

The paper describing these results was published in the Sept. 1 issue of The Astrophysical Journal. Other co-authors were Yair Krongold of Universidad Nacional Autonoma de Mexico in Mexico City; Fabrizio Nicastro of Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.; and Massimiliano Galeazzi of University of Miami in Coral Gables, Fla.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge.

For Chandra images, multimedia and related materials, visit:

For an additional interactive image, podcast, and video on the finding, visit:

Media contacts:
J.D. Harrington
Headquarters, Washington

Peter Edmonds
Chandra X-ray Center, Cambridge, Mass.

Visitor Comments (34)

Dear J.J.Madson,
Thanks for your questions. The total mass of the Milky Way is estimated to be somewhere between about a trillion and three trillion times the mass of the Sun. An average density can be found by using reasonable values for the dimensions of the galaxy.
-P.Edmonds, CXC

Posted by P. Edmonds on Friday, 11.9.12 @ 11:08am

Dear Boyd,
Thanks for your questions. I leave this as an exercise: the density of the halo gas can be approximated by using the mass estimates given in the press release and dividing by the volume in a sphere with a radius of 300,000 light years. Estimates of the density of the solar corona and the interstellar medium can be found with judicious Google searches.
-P.Edmonds, CXC

Posted by P. Edmonds on Friday, 11.9.12 @ 11:07am

Dear Vlad,
Thanks for your question. The density of the gas in the halo can be approximated by using the mass estimates given in the press release and dividing by the volume in a sphere with a radius of 300,000 light years.
-P.Edmonds, CXC

Posted by P. Edmonds on Friday, 11.9.12 @ 11:06am

Dear Martian,
Thanks for your question. As we explain in more detail in this blog post, the matter in the halo does not have anything to do with dark matter:

The gas is very hot, but it's also extremely diffuse so the radiation it directly produces is weak. This method of looking for absorption by the hot gas seems to be effective.
-P.Edmonds, CXC

Posted by P. Edmonds on Friday, 11.9.12 @ 11:03am

Dear Isaac,
Thanks for your comment. It's important because it may be a significant part of the "normal" matter near our galaxy, and by analogy for other galaxies. But, more work is needed to make better estimates of the mass of the halo.
-P.Edmonds, CXC

Posted by P. Edmonds on Friday, 11.9.12 @ 10:58am

I wonder if the mass of this halo is contemplated in the total mass of the visible matter in the universe.

Posted by Isaac on Tuesday, 10.30.12 @ 13:20pm

Am I the only one that, at first glance, thought ''hmmm, nice Oort cloud you got there, Milky Way.''?

Posted by guillaumeodinduval on Sunday, 10.28.12 @ 21:31pm

Lyman alpha "blobs" at >10G ly are clear indications of "early" Super Massive Black Holes [SMBH} seen blasting & illuminating their galactic hosts to radii ~350,000 ly. Lyman alpha blobs vacuum UV wavelengths reddened to IR allowing their visibility is from Hydrogen. Our MW halo absorption credited to Oxygen supports Supernova origin and genesis as blasted into the halo. A halo warmed by a feeding SMBH [quasar, AGN power-plant] could irradiate a MW halo episodically. It would not be surprising if a small SMBH [i.e., Milky Way's heart] provided much energy to the halo while eating.

Posted by lsporter on Sunday, 10.28.12 @ 19:09pm

I'm surprised there's not been more theoretical speculation about what this means for current theories of the standard model? Surely this undermines one of the needs for supersymmetry, dark matter, but if it's also this hot surely it would have been radiating em quanta and that would have shown up in our astronomical observations? I'm confused by this!

Posted by Martian on Friday, 10.26.12 @ 16:56pm

If this halo so to speak is dark matter then what if it all comes together and Incases us in matter?

Posted by Night hawk on Thursday, 10.25.12 @ 12:04pm

Has anyone estimated the density of the hot gas?

Posted by vlad on Monday, 10.22.12 @ 18:02pm

This gas and dust(?) is at what expected density? Please compare it to the density of the gas/dust in our Sun'd corona which is also stated to be millions of degrees Kelvin but of such a scarce density as you would not feel the heat. At least I have read such statements. I know not of their validity. Is this gas 300,00 LY from the MW center close to the same density as the interstellar gas inside the MW? Appreciate your answers.

Posted by Boyd Corcoran on Tuesday, 10.9.12 @ 23:11pm

Howdy.....Hello.....I like tonnage, therefore with this observation I am better able to understand the tonnage of the total G for our galaxy. 1 Your best estimate of the TOTAL tonnage of the Milky Way Galaxy is what? Please. Density (total gas) per volume area, as a weight(tonnage) plus observables (galatic disk, etc.)2 like an onion, temp differences between layers and within layers.(next observations)

Posted by J.J.Madson on Saturday, 10.6.12 @ 18:23pm

Many of your questions in the comments are being answered in this blog article

Posted by CXC on Thursday, 10.4.12 @ 10:47am

After about 35 yrs, mans inventions have only reached the outer part of our solar system, which doesn't even feature in the scale of the image. The vastness of this bubble is mind-blowing. I am wondering if this gas is perhaps leftover from when our galaxy formed. If the M.W. has this halo - bubble, it stands to reason that other galaxies should have it also. Perhaps this is where a lot of the missing mass is placed. I wonder if Captain Kirk knew about this ?

Posted by Ray Murray on Thursday, 10.4.12 @ 05:42am

I think this shows how a narrow view of the cosmos can lead to poor assumptions. 20 yrs ago I-galactic space had1 atom/cu.m, now what else will we find !

Posted by Mike on Wednesday, 10.3.12 @ 17:59pm

Interesting finding. To what extent may the halo correlate to magnetic fields and to plasmas between galaxies?

Posted by David Harrison on Tuesday, 10.2.12 @ 11:05am

How exciting this news is! A great tribute to all the astronomers from Galileo to S. Chandrasekhar.

Posted by G N Viswanath on Saturday, 09.29.12 @ 11:50am

The Halo of gas of 300,000 light years radius, I wonder could it be the missing dark matter?

Posted by sunderajan on Saturday, 09.29.12 @ 10:52am

Wow, A1

Posted by Anton Doppler on Saturday, 09.29.12 @ 10:24am

This is quite a revelation. That our milky way is engulf in hot gas. Please, keep it up.

Posted by Godwin Idemudia on Saturday, 09.29.12 @ 02:35am

A fascinating, and in some ways comforting finding, possibly solving the baryonic issue.

Posted by Mitchell Dormont on Friday, 09.28.12 @ 12:53pm

What is heating up all this interstellar matter to such high temperatures? Considering its vicinity, shouldn't it actually be only a few degrees above absolute zero?

Also, maybe it's not really a halo but rather us looking from inside a pocket of empty space formed in the nebula from which our galaxy once coalesced.

Posted by Saud on Thursday, 09.27.12 @ 11:28am

Are there any implications of this new data that could change how we look at Dark Matter or Dark Energy?

Posted by Fernando Sanchez on Wednesday, 09.26.12 @ 21:15pm

So, Here are some dumb questions from a lay person... Is this the mysterious "dark matter" we have been looking for? Why is it so hot? Is "dark energy" simply Boyle's law on a cosmic scale? This is an outstanding discovery and a much nicer solution to the missing mass in the universe than the "dark matter" voodoo.

Posted by Jason on Wednesday, 09.26.12 @ 16:42pm

Is the heating mechnism of that hot gas halo are the compact X-ray source objects or something else? I belive without heating it should have lost already since long time its energy via dissipative radiation.

Posted by Attila on Wednesday, 09.26.12 @ 08:25am

Awesome, so exciting information... thanks for your information. Go ahead, we are with you.

Posted by Sanjit Kumar Bandyopadhyay on Wednesday, 09.26.12 @ 06:51am

This is very interesting, I am curious about how this cloud of gas is producing so much heat?

Posted by Stephen on Tuesday, 09.25.12 @ 21:02pm

How and where does the heat come from?

Posted by Jim Fisher on Tuesday, 09.25.12 @ 07:48am

Very interesting indeed! One question though, if we (our galaxy) are within a Halo of hot gas of very high temperature, why are the temperature beyond Earth's atmosphere, so low. Kindly explain.

Posted by Souma Datta on Tuesday, 09.25.12 @ 06:05am

I have read somewhere on a NASA web page that our the Milky Way Galaxy is about 100,000 LY in diameter. If that is so, then the drawing above is not to scale. I count between 6 and 7 Milky Way diameters for the cloud's radius.

Posted by Odo Siahaya on Monday, 09.24.12 @ 19:22pm

Awsome! But how much does the estimates of the halo take up the 'lost mass' of the cosmos? If it has the equavelent of one galaxy, that isn't much...

Posted by Stark on Monday, 09.24.12 @ 18:46pm

Very exciting! Galactic rotation curves (velocity of rotation versus the distance from the galactic center) can not be explained by only the visible matter. The current best theory is that galaxies are composed largely of a roughly spherically symmetric halo of dark matter with the visible matter concentrated in a disc at the center. Can the observed rotation curves be fully accounted for by such a baryonic (not dark matter) halo of similar magnitude in other galaxies?

Posted by Publius on Monday, 09.24.12 @ 15:50pm

This is a magnificent discovery. What I would like to ask is how this massive gas halo will effect the planet earth and sun during the winter soltice and when the planets are aligned with the Milky Way galaxy?

Posted by Pamela Greenfield on Monday, 09.24.12 @ 14:23pm