Strange Trail Suggests Presence Of Galactic Interloper

As reported in the September issue of The Astronomical Journal, astronomer Peter R. McCullough at the University of Illinois at Urbana-Champaign and research scientist Robert Benjamin at the University of Wisconsin at Madison found a straight and narrow filament of ionized gas stretching 2.5 degrees across the sky near the Big Dipper in the constellation Ursa Major.

“We believe the gas trail was produced by the radiation from a white dwarf or some other

low-luminosity source zipping through the local interstellar medium and leaving behind an ionized wake,” McCullough said. “The problem is that we have not yet identified the source.”

While other possible explanations were considered – such as a jet of low-density stellar radiation or a linear wisp of gas associated with some nearby nebula – they are not favored because the filament’s properties are so different from other examples of those types of objects, McCullough said.

The filament is roughly Y-shaped. The vertical segment of the Y is about 1.2 degrees long and about 20 arcseconds wide. The full width of the two diagonal segments is about 5 arcminutes. The distance to the gas trail is not known, but it is suspected to be approximately 300 light-years from Earth.

“We know that white dwarfs – hot, dense stars not much bigger than a large planet – can leave these kinds of trails, but they will be very faint,” Benjamin said. Such trails had been predicted to exist by two Harvard astronomers in the early 1980s, but had never been seen. “This could be the brightest trail visible from Earth and therefore the first one found.”

If that turns out to be the case, astronomers might locate other such trails by photographing candidate white dwarfs whose distance and direction of motion are accurately known.

The object was first photographed in January 1997 with a small camera equipped with a hydrogen-alpha filter. Additional observations were made in April and May 1999 with a different filter mounted on the UI’s 40-inch reflecting telescope at Mount Laguna Observatory in southern California. The researchers also detected the object with the Wisconsin Hydrogen-Alpha Mapper (WHAM), confirming that the source was not from beyond our galaxy. The research was funded in part by the National Science Foundation.

“The filament’s large angular size also suggests it is nearby, and therefore we should be able to identify what created it,” McCullough said. If the source can be identified and studied, astronomers could use its properties to probe interesting parameters of the local interstellar medium – such as the density of the ambient gas and the level of turbulence in interstellar space.

“The culprit could be sitting right under our noses and we don’t recognize it,” McCullough said. Additional observations with other telescopes may solve this cosmic whodunit.