M31’s Odd Rotation Curve

Early on in astronomical history, galactic rotation curves were expected to be simple; they should operate much like the solar system in which inner objects orbit faster and outer objects slower. To the surprise of many astronomers, when rotation curves were eventually worked out, they appeared mostly flat. The conclusion was that the mass we see was only a small fraction of the total mass and that a mysterious Dark Matter must be holding the galaxies together, forcing them to rotate more like a solid body.

Recent observations of the Andromeda Galaxy’s (M31) rotation curve has shown that there may yet be more to learn. In the outermost edges of the galaxy, the rotation rate has been shown to increase. And M31 isn’t alone. According to Noordermeer et al. (2007) “in some cases, such as UGC 2953, UGC 3993 or UGC 11670 there are indications that the rotation curves start to rise again at the outer edges of the HI discs.” A new paper by a team of Spanish astronomers attempts to explain this oddity.

Although many spiral galaxies have been discovered with the odd rising rotational velocities near their outer edges, Andromeda is both one of the most prominent and the closest. Detailed studies from Corbelli et al. (2010) and Chemin et al. (2009), mapped out the rise in HI gas, showing that the velocity increases some 50 km/s in the outer 7 kiloparsecs mapped. This makes up a significant fraction of the total radius given the studies extended to only ~38 kiloparsecs. While conventional models with Dark Matter are able to reproduce the rotational velocities of the inner portions of the galaxy, they have not explained this outer feature and instead predict that it should slowly fall off.

The new study, led by B. Ruiz-Granados and J.A. Rubino-Martin from the Instituto de Astrofisica de Canarias, attempts to explain this oddity using a force with which astronomers are very familiar: Magnetic fields. This force has been shown to decrease less rapidly than others over galactic distances and in particular, studies of M31’s magnetic field shows that it slowly changes angle with distance from the center of the galaxy. This slowly changing angle works in such a manner as to decrease the angle between the field and the direction of motion of particles within it. As a result, “the field becomes more tightly wound with increasing galactocentric distance” making the decrease in strength even slower.

Although galactic magnetic fields are weak by most standards, the sheer amount of matter they can affect and the charged nature of many gas clouds means that even weak fields may play an important role. M31’s magnetic field has been estimated to be ~4.6 microGauss. When a magnetic field with this value is added into the modeling equations, the team found that it greatly improved the fit of models to the observed rotation curve, matching the increase in rotational velocity.

The team notes that this finding is still speculative as the understanding of the magnetic fields at such distances is based solely on modeling. Although the magnetic field has been explored for the inner portions of the galaxy (roughly the inner 15 kiloparsecs), no direct measurement has yet been made in the regions in question. However, this model makes strict observational predictions which could be confirmed by future missions LOFAR and SKA.

14 Replies to “M31’s Odd Rotation Curve”

  1. If I get this right, it’s about the motion of galactic gas, not stars. Is it reasonable to identify the gas-only motion with galactic rotation? What would the gas/stars ratio in these regions? Are there any stars at all?

  2. Manu: You’re correct. Both the Corbelli and Chemin studies were tracking hydrogen gas. The gas should be coupled to stars in that region since the stars form from the gas. Thus, the stars would have the same velocity as the gas from which it formed. I’m not certain if there are many stars in the region though. 38 kpc is the distance out the studies looked at, and the diameter is estimated to be 141 kpc (so radius of ~70 kpc) which would put this ~2/3 the way out from the center at which point there would likely still be a good number of stars.

  3. Thanks!
    So these stars would uncouple from the gas motion as soon as born, being – hopefully 😉 insensitive to the magnetic field. They would have highly eccentric orbits; what would their cumulated movement at any one radius appear relatively to the gas?

  4. I would expect them to decouple, but given that they were already imparted with the extra velocity, I wouldn’t expect them to have significantly different orbits than the gas. Even if they did, the cumulative average should end up the same, but perhaps with a larger standard deviation from the scatter.

  5. Just a question how does they discover the speeding up for something that takes millions of years to rotate?

  6. Olaf, I am by no means an expert but the article refers to the relative speed of one portion of the galactic disk as compared to an inner portion .. i.e. Outer is rotating faster than inner.

    They are not referring to the speed of the outer disk increasing over time. (i.e. millions of years)

  7. You can determine the rotation of a galaxy by the differential Doppler shift of material and luminous matter. This would largely be stars of course, but free gas in the galaxy will have its unique spectrum one can pick out.


  8. If we see a galaxy, we see the light of its stars, so we usually think that a galaxy is made up of stars. We know that besides the stars, there is some thin hydrogen gas between them. This gas usually is interpreted as some leftover from the galaxy´s star production, or as blown out into the interstellar space by stellar winds, novae and supernovae. As we have some clear understanding of these dynamic processes, we can estimate the amount of this gas fairly well, in some galaxies it it makes up to 30 % of the total masses of the galaxy´s stars.
    But a galaxy is more than the stars and the gas between them. There is a lot of gas in its outskirts. The VLA´s THINGS (The H1 Nearby Galaxy Survey) project revealed that the hydrogen gas clouds of many nearby spiral galaxies reach out up to three times further out than the radius of the farthest rim visible in deepest high-resolution exposures. Further, THINGS revealed that the galaxies´ spiral structure is continued by that gas, which means that the spiral arms are much longer than we can see them to be.
    I checked the THINGS picture of e. g. M74, overlayed it with the deepest high-resolution picture of that galaxy I could find in the net, and brought both to scale. I saw that what we usully see to be the whole galaxy is nothing but its brighter inner part where its stars shine. This volume evidently is about one eighths of the volume of the galaxy as revealed by THINGS. This galaxy is much larger as we can see it.
    I repeated the procedure with THINGS pictures of NGC 2903. With this galaxy, the diameter of its outer hydrogen volume is five to six times the diameter of its disk visible on the best high-reolution deep-sky picture of that galaxy .

    These clouds may not be dense enough to develop stars, so they are not visible. But they have a mass. As the outer hydrogen volume is 8 times to 25 times greater than the visible volume, the mass of the outer hydrogen clouds may by far exceed the mass of (core plus disk).

    To some respect, the stars of a galaxy may indicate as much about the true nature of the galaxy as does the spray on the crest of a wave in the ocean seen at dark night indicate about the tue nature of the wave…

  9. Could all of this additional gas that was previously unseen help explain some of the missing mass, to which dark matter is attributed?

    Is it possible that there is simply a lot more matter than we thought?

  10. Olaf, It is written a bit oddly, for it rotates faster at increased radius, and is written in a way which makes it sound like an acceleration.

    GBendt, The above study carries out 7 and 38 kilo parsecs, which does not seem to cover the much large distance or volume of the THINGS study.


  11. We could have a look at it by going at near light speed, but when we come back there would no one be here to report back because billions of earth years would have passed.

  12. Dark Gnat: Although the gas doesn’t emit much in visible light, it is visible in other portions of the spectrum. If it’s warm, it gives off IR and some visible. Cold gives off 21cm radio emissions. Thus, we do know how much there is and this is taken into consideration when taking the mass of a galaxy. Even then, it’s still not NEAR enough to explain dark matter.

  13. I concur with that. All this might do is to divide up the non-dark energy content of the universe, about 27%, from 4.5% luminous 22.5% dark matter to 6% luminous matter and 21% dark matter. It is a worthy correction, but not enough to overturn the Lambda dark matter theory.


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