Hubble spots pair of wild galaxies joining cosmic civilization

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Baltimore (UPI) Aug 11, 2016

Scientists discover how disk galaxies form spiral arms Fayetteville, Ark. (UPI) Aug 11, 2016 – The Milky Way is one of many disk galaxies with spiral arms, elongated and curved collections of gas and stars that snake outward from a dense galactic center. New research by a pair of astrophysicists from the University of Arkansas offers support for a long-debated theory for how spiral arms form. “Spiral galaxies are fascinating structures in astronomy, and the exact mechanism of the formation of spiral arms is still a mystery in astrophysics,” scientist Hamed Pour-Imani said in a news release. The evidence, detailed this week in the Astrophysical Journal Letters, lends strong support to the density wave theory of spiral galaxies. The density wave theory posits that spiral arms are not material entities, but shifting areas of congestion. They’re like traffic jams, with stars moving in and out of rush-hour traffic as they orbit the galactic center. First proposed in the 1960s, the theory suggests the pitch angles of each spiral arm should shift with the wavelength of the galaxy’s image. Previous analysis failed to identify a correlation between pitch angle and wavelength, but in studying imagery from the NASA Spitzer Space Telescope at two infrared wavelengths, Pour-Imani and research partner Daniel Kennefick identified pitch angles in accordance with density wave theory.

After billions of years wandering in the proverbial wilderness, two galaxies, Pisces A and B, are returning to cosmic civilization.

Recently, the Hubble Space Telescope caught a glimpse of the two galaxies, having made their way into a much denser collection of galaxies — a hub of cosmic activity astronomers are calling the “big city.”

The wilderness the two galaxies wandered in from is called the Local Void. The lonely region spans some 150 million light-years.

Their return to civilization — which actually began about 100 million years ago — triggered a burst of creativity. A plethora of young blue stars suggests the two galaxies are birthing new stars at an accelerated clip. Researchers suspect the uptick in star formation was fueled by big city’s denser supply of intergalactic gas.

“These Hubble images may be snapshots of what present-day dwarf galaxies may have been like at earlier epochs,” lead researcher Erik Tollerud, an astronomer at the Space Telescope Science Institute in Baltimore, said in a news release. “Studying these and other similar galaxies can provide further clues to dwarf galaxy formation and evolution.”

Though it is difficult to predict, researchers say the two galaxies’ star formation could soon slow. If the dwarf galaxies get pulled into orbit around a larger galaxy, their gas supply is likely to be cut off.

“The galaxies could even probably stop forming stars altogether, because they will stop getting new gas to make stars,” Tollerud said. “So they will use up their existing gas. But it’s hard to tell right now exactly when that would happen, so it’s a reasonable guess that the star formation will ramp up at least for a while.”

The story of the two loner galaxies was detailed in a new paper published in the Astrophysical Journal.