Hot blue stars ejected from their cradle may explain a mysterious ultraviolet glow that surrounds the disks of many spiral galaxies.
A new computer simulation shows that these shooting stars can populate the extensions beyond the visible disk of a galaxy (SN: 23/3/20). These distant regions have too hot and faint gas to make new stars, but there are new stars.
“It’s a big problem for the classical theory of star formation,” says Eric Andersson, an astrophysicist at the Lund Observatory in Sweden.
The mystery of the distant new stars has lingered for a long time. In 2003, NASA launched the Galaxy Evolution Explorer space telescope, which surprised astronomers by discovering diffuse ultraviolet light inside nearby spiral and irregular galaxies (SN: 15/02/05). Unlike common ultraviolet radiation, distant ultraviolet light has such a short wavelength that most do not penetrate the Earth's atmosphere.
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Stars that emit abundant amounts of this energy 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 very distant ultraviolet-emitting stars were not born where they are now. Instead, they emerged closer to the center of the galaxy and fled their homes.
The researchers performed a computer simulation to model the motion of massive stars in a spiral galaxy. Some of the simulation's fleeing stars travel thousands of light-years into space to reside beyond the visible edge of the galaxy's disk, thus explaining ultraviolet light there, researchers reported online at arXiv.org on October 22nd. .
The Milky Way has many of these runaway stars. A star can become a fugitive when other massive stars launch it through its gravity. Or, if the star orbits near a massive exploding star, the surviving star runs away at the same speed as it ran around its companion. Most shooting stars are hot and blue, radiating only the type of ultraviolet light that is seen beyond the visible edges of galactic disks.
Mark Krumholz, an astronomer at the Australian National University in Canberra, describes the idea as "a plausible explanation". It also offers a way to prove it: by exploiting the properties of different types of massive stars.
The rarest and most massive blue stars are so hot that they ionize hydrogen gas, causing it to emit red light when electrons are placed back around the protons. But these very massive stars do not live long, so anyone residing in the vicinity of a galaxy must have been born there. After all, the stars did not have time to travel from other places in the galaxy during their short lives.
On the contrary, less massive blue stars live longer and could therefore reach the galactic periphery of other places during their lifetime. If the ratio of distant ultraviolet light and red light from ionized gas is much higher beyond the visible edge of the galaxy than on its disk, Krumholz says, that would suggest that much of the distant ultraviolet glow in the suburbs comes from fleeing stars.