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Space Science

All Disk Galaxies Rotate Once Every Billion Years (astronomy.com) 89

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.
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All Disk Galaxies Rotate Once Every Billion Years

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  • by Anonymous Coward

    That's interesting.

  • “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.”

    OK, but why? It seems counter-intuitive that dense galaxies and sparse galaxies, big galaxies and small galaxies, would all rotate at roughly the same speed. The astronomy.com article is light on details and the Royal Astronomical

    • Re:Why? (Score:5, Insightful)

      by iggymanz ( 596061 ) on Thursday March 15, 2018 @12:14PM (#56264709)

      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

    • “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.”

      OK, but why? It seems counter-intuitive that dense galaxies and sparse galaxies, big galaxies and small galaxies, would all rotate at roughly the same speed. The astronomy.com article is light on details and the Royal Astronomical Society's abstract is somewhat incomprehensible to a layman like myself.

      Can someone explain?

      Obviously, greater minds than mine were the ones to calculate this, but I can't help but wonder... could they have made a mistake? It's like when they found a particle that went faster than light- and it turned out they measured it wrong. When scientists announce something truly strange (no matter how smart they are), the knee jerk reaction is "are they right"?

      If they are right, it's unlikely that this is all a coincidence.

    • Most likely gravity behaves differently at very (very) long distances than the usual inverse square. Kinda like how things behave differently at very very high speeds (i.e. special relativity)

      I haven't figured out exactly how this works or even any way to prove it happens, but when I do work it out I'll let you guys know.

    • It's what you would expect if there was a jet which ran through all of them, which was driving the galaxies.
    • “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.”

      OK, but why? It seems counter-intuitive that dense galaxies and sparse galaxies, big galaxies and small galaxies, would all rotate at roughly the same speed. The astronomy.com article is light on details and the Royal Astronomical Society's abstract is somewhat incomprehensible to a layman like myself.

      Can someone explain?

      More than likely, it just means that these are large complicated systems and the results that come from observation don't match up with the theoretical model somebody created. It's just a sign to go back to the drawing board and see what they missed and come up with a new model. That the angular velocity of galaxies are somewhat close to each other is probably just the result of certain terms canceling out. The real world works that way a great deal and can appear "elegant" without too much deeper meaning.

      H

  • Oversimplified (Score:5, Interesting)

    by lgw ( 121541 ) on Thursday March 15, 2018 @12:15PM (#56264713) Journal

    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.

    • by lgw ( 121541 )

      Also, I apparently can't type "ratio".

    • 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.

      That was considered decades ago. The problem with thinking of galaxies in the classic 1/r^2 gravitational sense is that it implies that the spiral structure we observe in so many galaxies should have destroyed itself within a few billion years (a few rotations). There's something else going on with how they rotate that we don't understand. This apparent cons

      • Something to consider [sciencedaily.com]:

        Strong variations in density indicate that the electrically charged part of Saturn's atmosphere (the so-called ionosphere) has a strong coupling to the visible rings that consist primarily of ice particles. The ice particles are also electrically charged.

        "It is as though the small ice particles in the D-ring suck up electrons from the ionosphere," says Jan-Erik Wahlund. "As a result of the coupling, electrical flows of gas to and from the rings along the magnetic field of Saturn cause

      • Also one needs to note that they are talking about the the visible edge of the disk which is defined by where the disk star population ends. This is not the edge of galactic system as their is a massive halo of dark matter extending far out from the visible disk.

        In the conclusion section of the paper they make the following key observation Continuous cosmic accretion provides a natural explanation for the RV relation and is their preferred explanation, but the paper is not seeking to establish that.

      • by mcarp ( 409487 )
        from the wikipedia article you cited:

        "The leading theory regarding the spokes' composition is that they consist of microscopic dust particles suspended away from the main ring by electrostatic repulsion, as they rotate almost synchronously with the magnetosphere of Saturn."

        This is NOT like galactic rotation or any kind of like a solid disk. These formations are influenced by the rotation of Saturn's magnetosphere which you should expect to rotate as a coherent unit.
  • Okay, so the larger the galaxy, the faster it must spin to complete a rotation in the same time as a smaller galaxy. The more mass a galaxy has means more for dark matter to gravitationally interact with it.. could it be repelling it somehow in order to accelerate it? Or attracting it?

  • i mean really,
    #define kGALAXY_ROT_SPD 1000000000
    ?

  • Well, I am sure you can find a spot or a metric where the numbers fit, like the researchers here did, but galaxies are not rigid and thus does not have a constant rotations per billion year for every part of it.

  • In addition to selection bias, as the summary noted, there is also the matter of sample size, compared to the entire universe. The margin of error would have to be very large.

    Also, can one really estimate motion in terms of billion-year cycles from studies conducted over, at most, a few years?

  • Obviously the Intergalactic Police are making sure they do not travel faster than the posted galactic speed limit.

  • It all has to do with Gravity, once we figure that out then the rest will fall into place.

    Does the universe rotate around the point where the Big Bang happened?

  • That existence is pretty much what I am expecting some scientist to figure out a long time from now, get supremely depressed, and end it all taking his secrets with him.

    So we tell time?
    Well no, the Universe tells time, you do practically nothing.
    What does he use it for? Wouldn't he always know what time it is?
    Mostly just as an accessory, he likes how it looks. I mean you have an iPhone don't you?
    Oh my God.
    Exactly.

  • "But instead, they discovered a significant population of much older stars mingling with the young stars and gas."

    Sounds like an Oscars after-party.

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