New Stars Born at Milky Way’s Edge

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Gas from two satellite galaxies have collided with the Milky Way’s own gaseous features, triggering the birth of bright new stars at the edge of our own galaxy. The newborn stars paint a portrait of what happens when small, gas-rich galaxies smash together to give rise to giant ones.
Discovered in the 1970s, the Magellanic Stream is a long ribbon of gas that stretches nearly halfway around our galaxy. At the head of the stream are two dwarf galaxies that orbit the Milky Way, the Large and Small Magellanic Clouds. Specifically, there is also a gaseous bridge between the two galaxies, as well as the Leading Arm, the shorter gaseous feature that leads the galaxies in orbit. 
Just last year, astronomers using NASA’s Hubble Space Telescope discovered the origin of the stream. Turns out, most of the gas came from the Small Magellanic Cloud about 2 billion years ago, and a second region of the stream originated more recently from the Large Magellanic Cloud. Unlike our other satellite galaxies, the Clouds have retained their gas and are still forming stars. And as they approach the Milky Way, its gravity and halo of hot gas push the Magellanic gas out. That, combined with the gravitational tug-of-war between the Clouds, leads to the production of the stream. 
And now, for the first time ever, researchers have spotted stars in the Leading Arm of the Magellanic Stream. The young, luminous stars must have formed recently when Magellanic gas collided with the gaseous hot halo and disk of the Milky Way
“This is the one and only galaxy interaction we can model in very much detail,” Yale’s Dana Casetti-Dinescu tells Scientific American. Other collisions of gas clouds between galaxies are farther away and harder to observe. 
Using the 6.5-meter Walter Baade Telescope at Las Campanas Observatory in Chile, Casetti-Dinescu and colleagues uncovered six blue stars in the Leading Arm. Five of the six are about 60,000 light-years from the Milky Way’s center. The sixth one is twice as far, located about 130,000 light-years from the center of our galaxy.
“They are formed in situ,” Casetti-Dinescu explains to SciAm. “They have to be, because they’re too young — they don’t have enough time to travel from the Clouds to their current location in their lifetime.”
At least five of the newborn stars share the velocity of gas in the Leading Arm, suggesting they formed when its gas crashed into the Milky Way’s outer disk; this compressed the Magellanic gas, triggering star formation. The sixth and farther star is extremely hot (about 44,000 Kelvins) and could have been born in the vast outer halo, beyond the galaxy’s stellar disk. 
The Milky Way likely has a long history of enhancing its luster by grabbing gas from its satellites to form stars on its outskirts, which could explain how it grew into such a giant galaxy.
The work was published in this month’s Astrophysical Journal Letters
Image: D. Nidever et al., NRAO/AUI/NSF and A. Mellinger, Leiden-Argentine-Bonn (LAB) Survey, Parkes Observatory, Westerbork Observatory, and Arecibo Observatory via HubbleSite

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