[LINK] O/t: All disk galaxies rotate once every billion years

[Stephen Loosley]

All disk galaxies rotate once every billion years

By Jake Parks  |  Published: Tuesday, March 13, 2018
http://www.astronomy.com/news/2018/03/all-galaxies-rotate-once-every-billion-years


In a study published March 9 in The Monthly Notices of the Royal Astronomical Society, astronomers announced the discovery that all disk galaxies rotate about once every billion years, no matter their size or mass.

“It’s not Swiss watch precision,” said Gerhardt Meurer, an astronomer from the International Centre for Radio Astronomy Research (ICRAR), in a press release. “But regardless of whether a galaxy is very big or very small, if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round.”

“Discovering such regularity in galaxies really helps us to better understand the mechanics that make them tick,” he said. “You won’t find a dense galaxy rotating quickly, while another with the same size but lower density is rotating more slowly.”

Based on theoretical models, the researchers also expected to find only sparse populations of young stars and interstellar gas on the outskirts of these galaxies. But instead, they discovered a significant population of much older stars mingling with the young stars and gas.

Meurer said, “Because of this work, we now know that galaxies rotate once every billion years, with a sharp edge that’s populated with a mixture of interstellar gas [and] both old and young stars.”

To carry out the study, the researchers measured the radial velocities of neutral hydrogen in the outer disks of a plethora of galaxies — ranging from small dwarf irregulars to massive spirals. These galaxies differed in both size and rotational velocity by up to a factor of 30. With these radial velocity measurements, the researchers were able to calculate the rotational period of their sample galaxies, which led them to conclude that the outer rims of all disk galaxies take approximately a billion years to complete one rotation. The researchers also note however that further research is required to confirm the clock-like spin rate is a universal trait of disk galaxies and not just a result of a selection bias.

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Cheers,
Stephen