Runaway stars may create the mysterious ultraviolet glow around some galaxies

Hot blue stars kicked out of their cradles may explain a mysterious ultraviolet glow that surrounds the disks of many spiral galaxies.

A new computer simulation demonstrates that these runaway stars can populate the vast expanses beyond a galaxy’s visible disk (SN: 3/23/20). These distant regions have gas that is too warm and tenuous to make new stars, yet young stars nevertheless exist there.

“It’s a big problem for classical star formation theory,” says Eric Andersson, an astrophysicist at Lund Observatory in Sweden.

The mystery of the far-flung young stars has persisted for some time. In 2003, NASA launched the Galaxy Evolution Explorer space telescope, which surprised astronomers by discovering diffuse far-ultraviolet light in the hinterlands of nearby spiral and irregular galaxies (SN: 2/15/05). Unlike ordinary ultraviolet radiation, far-ultraviolet light has such a short wavelength that most of it doesn’t penetrate the Earth’s atmosphere.

Stars that emit profuse amounts of this energetic radiation are hot, blue and usually much more massive than the sun. These stars don’t live long, so they must have formed recently. But the gas on the galactic outskirts isn’t cold and dense enough to collapse and create new stars.

Andersson and his colleagues propose a solution to the paradox: Many of these far-out far-ultraviolet-emitting stars weren’t born where they are now. Instead, they arose closer to the galaxy’s center and ran away from their homes.

The researchers conducted a computer simulation to model the motion of massive stars in a spiral galaxy. Some of the runaway stars in the simulation dart across thousands of light-years of space to take up residence beyond the visible edge of the galaxy’s disk, thereby explaining the far-ultraviolet light there, the researchers report online at arXiv.org on October 22.

The Milky Way has many of these runaway stars. A star can become a runaway when other massive stars fling it away through their gravity. Or, if the star orbits close to a massive star that explodes, the surviving star races away at the same speed it had been dashing around its companion. Most runaway stars are hot and blue, radiating just the type of far-ultraviolet light 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.” He also offers a way to test it: by exploiting the properties of different types of massive stars.

The rarest and most massive blue stars are so hot they ionize hydrogen gas, causing it to emit red light as electrons settle back into position around protons. But these very massive stars don’t live long, so any that reside on a galaxy’s outskirts must have been born there. After all, the stars didn’t have time to travel from elsewhere in the galaxy during their brief lives.

In contrast, less massive blue stars live longer and therefore could have reached the galactic periphery from elsewhere during their lifetimes. If the ratio of far-ultraviolet light to red light from ionized gas is much greater beyond the galaxy’s visible edge than in its disk, Krumholz says, that would suggest much of the far-ultraviolet glow in the exurbs does indeed come from runaway stars.