Why don't spiral galaxies run out of gas? Look to the (extended) halo.
New research suggests that galaxies hold more 'normal matter' than scientists could image. This matter in the form of gas resides in halos that turn out to be about twice as vast as previously estimated.
University of Utah/AP
Spiral galaxies like the Milky Way appear to host far more ordinary matter than previously estimated, according to a recent study. That may help explain why spiral galaxies in the nearby universe are still producing stars at respectable rates when, by all rights, star formation in these galaxies should have run out of gas billions of years ago.
This matter resides as gas in an extended halo nearly 1 million light-years across. For a galaxy such as the Milky Way, whose disk is roughly 100,000 light-years wide, the halo holds as much ordinary, or baryonic matter as the Milky Way's disk, with all of its stars, gas, and dust.
Moreover, this gas, much of which is expelled from the galaxy itself, appears to get recycled – returning to the galaxy to serve as the raw material for new stars and planets.
The evidence for this baryonic bonanza was gathered by an international team of astrophysicists and astronomers initially interested in some basic accounting, as well as in galaxy evolution.
Over the past decade, researchers have uncovered the recipe for the universe: 70 percent dark energy, a kind of anti-gravity that is accelerating the expansion of the universe; 25 percent dark matter, which astronomers can't see but infer because of its gravitational influence on matter astronomers can see; and ordinary, or baryonic, matter, which astronomers can see directly and exerts gravitational influence as well.
Theory suggests that the proportions of dark and baryonic matter in the universe should hold for galaxies as well. But the amount of baryonic matter astronomers detect in large spirals is only about 20 percent of what theory says should be there.
"Most of the matter is still out between the galaxies," either in a region around a galaxy where its gravity still holds sway, or beyond that in deep intergalactic space, says Michael Shull, an astrophysicist at the University of Colorado at Boulder and a member of the team, which was led by colleague John Stocke, also at the University of Colorado.
The gas is so tenuous, however,. that astronomers haven't been able to capture images of it. Instead, the team used the Hubble Space Telescope's exquisitely sensitive Cosmic Origins Spectrometer to detect the gas, using bright objects called quasars as backlights.
The quasars' light passes through the gas, producing spectra that reveal the gas's abundance, composition, and its motion and velocity.