All disk galaxies rotate once every billion years

In a study published March 14 in The Monthly Notices of the Royal Astronomical Society, astronomers announced the discovery that all 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.”

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 roughly a billion years to complete one rotation. However, the researchers note 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 selection bias.   

Source: www.astronomy.com/news/2018/03/all-galaxies-rotate-once-every-billion-years