Key takeaways

  • Researchers found that all disk galaxies, no matter their size, complete one rotation every billion years.
  • This regularity in galaxy rotation helps scientists understand the mechanics driving galaxies.
  • By measuring the radial velocities of neutral hydrogen in various galaxies, researchers determined the consistent rotational period.
  • Surprisingly, the outer edges of galaxies contain both old and young stars along with interstellar gas.
  • Knowing the rotational period and edges of galaxies will aid astronomers in efficiently processing data from upcoming radio telescopes like the Square Kilometer Array.

In a research published in The Monthly Notices of the Royal Astronomical Society, astronomers discovered that all disk galaxies, regardless of size or mass, rotate once every billion years.

“It’s not Swiss watch precision,” remarked Gerhardt Meurer, an astronomer at the International Centre for Radio Astronomy Research (ICRAR), in a news statement. “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 allows us to better understand the mechanics that drive them,” he added. “You won’t find a dense galaxy rotating quickly, while another with the same size but lower density is rotating more slowly.”

To conduct the study, the researchers measured the radial velocities of neutral hydrogen in the outer disks of a wide range of galaxies, from minuscule dwarf irregulars to enormous spirals. These galaxies vary in size and rotational velocity by up to a factor of thirty. Using these velocity data, the researchers were able to determine the rotational period of their sample galaxies, leading them to the conclusion that the outer rims of all disk galaxies take around a billion years to complete one rotation. However, the researchers emphasize that further study is needed to show that the clock-like spin rate is a ubiquitous feature of disk galaxies rather than a product of selection bias.

According to theoretical models, the researchers anticipated to detect sparse concentrations of newborn stars and interstellar gas on the edges of these galaxies. Instead, scientists detected a large number of considerably older stars merging with the young stars and gas.

“This is an important result because knowing where a galaxy ends means we astronomers can limit our observations and not waste time, effort, and computer processing power on studying data from beyond that point,” Meurer stated. “So 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.”

Given that new generations of radio telescopes, such as the long-awaited Square Kilometer Array (SKA), will generate massive amounts of data, the fact that researchers now have a good idea of where a galaxy’s edge is should help them significantly reduce the power required to process such data.

“When the SKA comes online in the next decade,” Meurer added, “we’ll need as much assistance as we can get to characterize the billions of galaxies these telescopes will soon make available to us.”

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