Hot blue stars thrown from their cradles may explain a mysterious ultraviolet glow that surrounds the disks of many spiral galaxies.
A new computer simulation shows that these runaway stars can populate the vast expanses beyond the visible disk of a galaxy (SN: 03/23/20). These distant regions have gas that is too warm and too thin to form new stars, but young stars still exist there.
“It’s a big problem for classical star formation theory,” says Eric Andersson, an astrophysicist at Lund Observatory in Sweden.
The secret of the distant young stars has been around for some time. In 2003, NASA launched the Galaxy Evolution Explorer space telescope, which surprised astronomers by discovering diffuse far-ultraviolet light in the hinterland of nearby spiral and irregular galaxies (SN: 02.15.05). Unlike ordinary ultraviolet radiation, distant ultraviolet light is so short in wavelength that most of it does not penetrate the Earth’s atmosphere.
Stars that emit copious amounts of this energetic radiation are hot, blue, and usually much more massive than the sun. These stars do not live long, so they must have formed recently. But the gas on the galactic outskirts is not cold and dense enough to collapse and create new stars.
Andersson and his colleagues propose a solution to the paradox: Many of these distant, far-ultraviolet-emitting stars were not born where they are now. Instead, they got up closer to the center of the galaxy and ran away from their homes.
The researchers performed a computer simulation to model the motion of massive stars in a spiral galaxy. Some of the runaway stars in the simulation flit thousands of light years into space to settle beyond the visible edge of the galaxy’s disk, thus explaining the far ultraviolet light there, the researchers reported online on October 22 at arXiv.org.
The Milky Way has many of these runaway stars. A star can become an outlier when other massive stars throw it away by their gravity. Or if the star orbits near an exploding massive star, the surviving star will race away at the same speed as it orbited its companion. Most runaway stars are hot and blue, and emit only the kind of distant ultraviolet light that can be seen beyond the visible edges of galactic disks.
Mark Krumholz, an astronomer at the Australian National University in Canberra, calls the idea “a plausible explanation”. It also offers a way to test this: by taking advantage of the properties of different types of massive stars.
The rarest and most massive blue stars are so hot that they ionize hydrogen gas and emit red light when the electrons reposition themselves around the protons. But these very massive stars don’t live long, so anyone living on the edge of a galaxy must have been born there. After all, the stars did not have time to travel from anywhere else in the galaxy in their short life.
In contrast, less massive blue stars live longer and therefore could have reached the galactic periphery from elsewhere during their lifetime. If the ratio of far-ultraviolet light to red light from ionized gas beyond the galaxy’s visible edge is much greater than in its disk, Krumholz said, it would suggest that much of the far-ultraviolet light in the outskirts is actually out of control guessed stars.