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Galaxies stash mass in clouds of gas

Galactic haloes may have 10 to 100 times as much matter as previously thought

Spheres of gas, called haloes, surround galaxies like this one. Cool, unseen gas in these haloes may constitute most of a galaxy’s total mass, a new study suggests. 

JPL-CALTECH/NASA AND S. WILLNER/HARVARD-SMITHSONIAN CFA

For more than a decade, astronomers have been grappling with a mystery: Where do galaxies keep most of their visible matter? This is the weighty material we can see and that takes up space. Scientists have predicted there should be more of this matter than astronomers have seen. But where? At a science conference in January, astronomers offered a possible answer. This missing mass may be hiding in surrounding clouds of gas.

Matter is the stuff in the universe. Astronomers say most of the matter in any galaxy — and in the universe — is dark matter, which is mysterious and invisible and difficult to study. The new research focuses on ordinary matter, which is visible and known to consist of particles. Ordinary matter makes up the stuff scientists can see and measure. Mass is a measure of how much matter something has.

Galaxies are enormous collections of stars. Typically, a galaxy has millions or billions of them. Each star, in turn, may hold planets in tow. A galaxy also may host very dense black holes. Together, all of these celestial bodies contain a lot of mass. But not enough: Galaxies should have about three times as much ordinary matter as astronomers see.

That’s where haloes come in. These are giant spheres of gas and dark matter that surround galaxies. And glow. That shouldn’t be too surprising: Much of a halo’s gas has a scorching temperature of a million degrees Celsius (1.8 million degrees Fahrenheit) or more. But haloes also contain much cooler gas that’s only about 10,000 ºC (or about 18,000 ºF). That cooler gas barely emits any light. Which means ordinary telescopes can’t see it.

And that cooler gas has much more mass than scientists previously estimated, Jessica Werk reported Jan. 7. She presented her new findings at an astronomy meeting in Oxon Hill, Md. An astrophysicist at the University of California, Santa Cruz, Werk studies the physical nature of objects in space. She and her coworkers found that dark, cool gas haloes appear to be storing some of the galaxies’ missing mass.

The researchers analyzed observations made by the Hubble Space Telescope. Because they could not see the gas directly, they had to find a different way to gauge how much there was. So the astronomers studied light emitted by 38 faraway quasars. These objects look like bright stars and give off a lot of energy. (Some quasars may be billions — or even a trillion — times brighter than the sun, yet they are so far away they still look like stars.)

Quasars are among the most far-off objects in space. As their light zips through space, some of it will pass through the cold gas surrounding a galaxy. Certain atoms in the halo will absorb some of that light. By studying changes in the quasars’ light, Werk and her coworkers identified the types and amounts of atoms present in the haloes. They found evidence for carbon, silicon, magnesium and hydrogen.

They also calculated that a typical galaxy has 10 to 100 times more cold gas than astronomers had suspected. That could account for the missing mass.

“We were surprised” by how much cold gas there was, Werk told Science News.

Chris Churchill told Science News that the new data have almost convinced him that that a halo’s cool gas can make up a galaxy’s missing matter. An astronomer at New Mexico State University in Las Cruces, he did not work on the new study. Churchill says he wants to better understand how Werk knew the hydrogen gas she was looking at was cool, not hot. Only then, he says, will he truly be convinced.

But, he adds: “I think she’s probably right.”

Power Words

astronomy  The area of science that deals with celestial objects, space and the physical universe as a whole. People who work in this field are called astronomers.

astrophysics  An area of astronomy that deals with understanding the physical nature of stars and other objects in space.

atom  The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

mass  A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

matter  Something which occupies space and has mass. Anything with matter will weigh something on Earth.

quasar  (an acronym, short for quasi-stellar) A massive and extremely remote celestial object that emits exceptionally large amounts of energy. Although not a star, owing to its distance, a quasar typically has a starlike image, even in a telescope. 

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