A galactic satellite reveals where some of our galaxy's elusive material is hiding
ByAs the Large and Small Magellanic Clouds orbit the Milky Way—and each other—they shed their gas (shown in pink) into our galaxy's halo. .
The trouble is, all of our galaxy’s known stars and interstellar matter add up to only about 60 billion solar masses: 50 billion in stars and 10 billion in interstellar gas and dust. (The Milky Way has more than 100 billion stars, but most are smaller than the sun.) That leaves a whopping 110 billion solar masses of ordinary material unaccounted for. If than currently estimated, this missing baryon problem gets worse—and the same conundrum afflicts other giant galaxies as well.
Fortunately for astronomers, the Milky Way is so mighty that it rules a retinue of smaller galaxies that revolve around it just as moons orbit a planet. The most splendiferous satellite galaxy is the , shining 160,000 light-years from Earth. Like all the other galactic satellites, this one moves around the Milky Way, but unlike most of its peers, it abounds with gas, which gets stripped as it rams into the halo's own gas. The amount of gas lost depends on the speed at which our neighbor moves and how dense the halo gas is. And that density can yield a mass estimate for the halo's gas.
Recently, . This allowed astronomers Munier Salem of Columbia University, Gurtina Besla of the University of Arizona and their colleagues to study the stripped gas and estimate that the gas density in the Milky Way's halo near the Large Magellanic Cloud is 0.0001 atoms per cubic centimeter. That's not much—only about 10,000 times more tenuous than the interstellar gas in the Milky Way's disk—but the halo covers a lot of real estate. In research submitted for publication in , the astronomers assume that the gas density declines with distance from the Milky Way's center, and calculate that , or close to half the amount in all of the Milky Way's stars. Matthew Miller, a graduate student at the University of Michigan who is completing his dissertation on the circumgalactic medium, says this number corresponds with previous estimates but is based on a more direct measurement of the density.
Besla predicts that future work may yield a better measurement. Another gas-rich galaxy—the Small Magellanic Cloud, 200,000 light-years from Earth—orbits the Large Magellanic Cloud. Their dance has spilled gas into . Most of this Magellanic Stream extends beyond the Large Magellanic Cloud and thus should probe the halo's gas density elsewhere, Besla says, further constraining the mass of the circumgalactic medium.
Indeed, astronomers on Earth are lucky: They inhabit . "It's amazing how much information this system provides us," Besla says. In contrast, all satellites orbiting a more typical giant galaxy have run out of gas, and any astronomers there may look upon their peers in the Milky Way with quiet envy.
No comments:
Post a Comment