All Galaxies Rotate Once Every Billion Years

By Jake Parks | March 14, 2018 11:35 am
Grand spiral galaxy (NGC 1232). (Credit: FORS/8.2-meter VLT Antu/ESO)

Grand spiral galaxy (NGC 1232). (Credit: FORS/8.2-meter VLT Antu/ESO)

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.”

Turn, Turn, Turn

To carry out the study, the researchers measured the radial velocities of neutral hydrogen in the outer discs 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 disc galaxies take roughly a billion years to complete one rotation.

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.

“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,” said Meurer. “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.”

Considering new generations of radio telescopes — like the long-anticipated Square Kilometer Array (SKA) — will produce massive amounts of data, the fact that researchers now have a good idea where a galaxy’s edge lies should also help them significantly reduce the power required to sort through such data.

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


[This post originally appeared on]

MORE ABOUT: cosmology
  • John Thompson

    Are they sure?
    Seems very unlikely that the smallest galaxies would have about the same period of rotation as the largest ones.
    No mention of why.
    If it’s true then my guess is it has to do with some limitation or property of gravity (if for instance all larger galaxies have more massive cores/central regions then they could hold more distant stars and the speed of rotation would be faster due to increased gravity – the opposite for the less dense core galaxies – closer stars and slower speed of rotation).
    I guess we just have to wait for confirmation and the theories on why….

  • -7/4

    Too much for a coincidence. If these measurements are correct then something very fundamental is acting here and as usual we have no hint of what it might be. Should make us humble.

  • Uncle Al

    Does dark matter or Milgrom acceleration (MA) satisfy the Tully-Fisher relation and disk galaxies’ identical angular rotation? MA is invariant 1.2×10^(-10) m/s², everywhere and everywhen (red shift).

    Big Bang 6.1×10^(-10) excess matter over antimatter violates exact Noetherian baryon number conservation. Sakharov conditions, trace vacuum chiral anisotropy toward hadrons, creates Noetherian leakage. Noetherian angular momentum conservation demands exact vacuum achiral isotropy. Sakharov creates a net matter, MA-coupled universe.

    Observe ppb Sakharov in commercial apparatus, brightspec(.)com, on a bench top, in one hour. Given left foot,vacuum, extreme mirror image molecule shoes embed with different energies. Rotational spectra will not be exactly identical.

    … one analytical hour rewrites 50 theory years. Look.

  • OWilson

    The current theory that all parts of a galaxy, inner and outer, rotate at exactly the same speed, flies in the face of everything we have learned about gravity and motion over the last 200 years.

    It also undermines our understanding of vortex motion, which causes the whirlpool, and cyclone effect on Earth, and the nature of those “lagging” spiral arms.

    It can all easily be explained, though. Just suspend belief!

    Invoke some magic stuff that holds it all in place that we can never see, feel or measure, then our egos and hubris are not offended.

    The big mistake they made though was to call it Dark (Darth?) Matter and Dark Energy.

    Had they called it “interacting baryonic spontaneus virtual quanta duality”, we might not have seen that the magician apparently sawing the woman in half, had twins scrunched up in that opaque box! :)

    • John Thompson

      I too find it very hard to believe.
      This isn’t my field, but it seems to me that in order for this to be true, the dense central region (generally containing super-massive black holes) would have to be much more massive to “hold” a larger galaxy together at the far higher speeds that a larger galaxy would have to rotate to have the same 1 billion year period as a much smaller galaxy.
      Of course I’m not even getting into dark matter – and I’m mostly thinking about spiral or elliptical galaxies.
      I certainly do not think it is likely that irregular galaxies – and all the galaxies colliding with other galaxies – would have the 1 billion year rotation.
      I also wonder about dwarf galaxies where other studies show they have similar central mass, and they vary in size greatly – so how would they have the same rotational time period of 1 billion years?

      • OWilson

        A billion is a nice round figure!

        Like the Big Bang that suddenly occured to give us everything we need.

        No loose ends! :)

        But your questions are serious and logical!

    • Sic Semper Tyrannis

      “The current theory that all parts of a galaxy, inner and outer, rotate at exactly the same speed”

      That’s not a theory. It’s an observation. Theories explain observations in a predictable way. There isn’t a theory to explain the motion of galaxies, so scientists use dark matter as a place holder until they can understand what exactly is going on.

      I’m not a scientist, but my idiot’s inclination is that we have gravity backwards. I think it’s actually the expansion of space that is impeded by the presence of mass and that is what we perceive as gravity. Gravity is mass resisting the accelerating pressure of space’s expansion and that is why it is indecipherable from acceleration. This would also impose limits to gravity which is why the rate of stars orbiting in a galaxy is uniform even across separate galaxies.

      I’m an idiot, though. So take that untested hypothesis with a grain of salt.

  • John Thompson

    OK, so the largest galaxies – giant ellipticals – they are generally the result of galaxy collisions.
    But because they are combinations of galaxies, the stars in them do not all rotate the same way – obits are described as chaotic and random.
    So how do you claim a “rotation” period of 1 billion years for those when there is no discernible rotational pattern???

    • Isaac42

      The article specifically says disk galaxies. It does not address ellipticals.

  • Kurt Stocklmeir

    God made the universe – when God made the universe God wanted galaxies to spin at a certain speed – all forces gets more strong as they travel – gravity gets more strong as it travels – there is not any dark matter – increase of energy associated with gravitons is associated with spin of galaxies – magnetic fields get more strong as they travel – for years I have talked about this – magnetic fields of galaxies are a lot like gravity of galaxies – with lot of galaxies there are small magnetic fields around center of galaxy and big magnetic fields outside galaxy – with a lot of galaxies magnetic field decreases a lot less than 1/rrr through the galaxy – magnetic fields between galaxies decrease a lot less fast than 1/rrr – magnetic fields of galaxies are strong a big distance away from clusters of galaxies Kurt Stocklmeir


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