All Disk Galaxies Rotate Once Every Billion Years (astronomy.com)

According to a new study published in The Monthly Notices of the Royal Astronomical Society, astronomers discovered that all disk galaxies rotate about once every billion years, no matter their size or mass. Astronomy Magazine reports: 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. 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.

Comments

Re:Why?

[iggymanz]

No, we don't understand the mechanics of galactic rotation, it is not the rate expected from observed matter and gravity and so we postulate "dark matter".

Why indeed....big question

Oversimplified

[lgw]

TFS oversimplifies things a bit. The finding is that the outer edge of these galaxies rotates at about the same rate for all of them. That's not entirely surprising: the more massive the galaxy, the faster the rotation at any given distance, but also the more distant the outer rim. It also implies a similar ration of dark matter to familiar matter across these galaxies - which again isn't shocking, but is interesting if the ration has to be very similar. If it's confirmed they really do line up this closely that's probably big news for those modelling galaxy formation.