Galaxies’ missing mass may hide in gas clouds

OXON HILL, Md. — Vast reservoirs of previously undetected gas account for much of galaxies’ mass, according to research presented January 7 at a meeting of the American Astronomical Society. The finding could explain why earlier studies found far less mass in galaxies than cosmologists’ theories had predicted.

For more than a decade, astronomers have wondered about galaxies’ missing mass. Baryonic matter — the ordinary, visible stuff of the universe including the protons, neutrons and electrons that make up stars and planets — should account for about 17 percent of a galaxy’s matter; the rest is invisible dark matter. But when researchers try to estimate the  amount of baryonic matter in stars and in the 1 million degree Celsius and hotter gas that surrounds galaxies in giant halos, astronomers can account for only around a third of the baryonic matter they think galaxies should have.

Recent studies have revealed that galaxies’ halos also contain colder gas with temperatures around 10,000°. Telescopes cannot detect this gas directly because it is diffuse and emits little light. But a few halos are backlit by extremely bright, distant galactic nuclei called quasars that shine with a nearly uniform spectrum of light. When quasar light passes through a gas, the gas’s atoms and ions absorb certain wavelengths depending on the amount, temperature and makeup of the gas.

To weigh the cold gas in galactic halos, a team led by astrophysicist Jessica Werk of the University of California, Santa Cruz studied light from 38 quasars using the Hubble Space Telescope. By looking at the spectral fingerprints in this light, she found that galactic halos harbor far larger quantities of cold carbon, silicon and magnesium ions than researchers previously thought. Hydrogen ions do not show up in quasar fingerprints. But after analyzing her ion measurements, Werk estimated that cold hydrogen ions are also plentiful in galactic halos.

Adding up the gases, Werk found that at least 10 times, and possibly up to 100 times, as much cold gas surrounds galaxies as researchers had estimated. If her estimate is correct, it would account for the two-thirds of galaxies’ baryonic matter that astronomers have been looking for, Werk said. “We were surprised by how much” cold gas there is.

Chris Churchill, an astronomer at New Mexico State University in Las Cruces, would like to understand better how Werk and her team distinguished between hot and cold hydrogen gas. “If she could convince me she’s done that correctly,” he said, “I would be highly convinced” that much of galaxies’ missing matter is hiding in cold gas.

Still, Churchill added, “I think she’s probably right.”

Editor’s Note: This story was updated January 22, 2014, to correct the description of quasars and the temperature of the hot gas in galactic halos.