Why Dark Matter Rules Mini-Galaxies!

“A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.” -Max Planck

(For Alan L., from the comments on this post.)

When you look out at the night sky, with the deepest, sharpest eyes possible, what is it that you see?

Image credit: NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, M. Rutkowski, R. O'Connell, P. McCarthy, N. Hathi, R. Ryan, H. Yan, and A. Koekemoer.

Galaxies! Lit by hundreds of billions of suns each (and that’s just on average), they not only illuminate the Universe, but they also trace out the great cosmic web that composes the large-scale structure of the Universe.

Image credit: 2dF Galaxy Redshift Survey.

How does it get to be that way? Well, we know the laws of physics, so we can put our ingredients for a Universe into a simulation and see what we get out.

Image credit: National Center for Supercomputer Applications, Andrey Kravtsov and Anatoly Klypin.

And the simulations that match up with our observations the best are ones where we have about five times as much dark matter as normal (protons, neutrons, and electrons) matter.

So we can look at individual galaxy clusters, and measure their mass using a variety of methods (like gravitational lensing, below), as well as their total normal matter content.

Image credit: NASA, Andrew Fruchter and the ERO Team, STScI.

And for every large galaxy cluster, we get (more or less) that same ratio: about 15-17% of the mass is “normal” matter, with about 83-85% dark matter.

In theory, gravity treats everything with mass equally, and once the Universe cools to the point where radiation (photons, neutrinos, etc.) is unimportant, we should form all the structure in the Universe — from the largest scales down to the smallest scales — with the same ratios of dark matter to normal (baryonic) matter.

And while large galaxies — like Messier 104 — definitely exhibit the expected ratio of dark matter to normal matter, things get a little sketchy when we start going down to smaller galaxies. Their rotation curves start to do something that, well, make us a little uncomfortable.

Image credit: The University of Sheffield.

It looks like there’s too much dark matter! More precisely, it looks like there isn’t quite enough normal matter, as Messier 33 — the third largest galaxy (behind Andromeda and the Milky Way) in our local group — shows.

And if we start to look at even smaller galaxies, things get even more extreme.

Image credit: ESO/Digitized Sky Survey 2, as is the image below.

Because dwarf galaxies, like the Fornax dwarf galaxy above (or the Sculptor dwarf galaxy below), have less than 1% normal matter, and more than 99% dark matter!

Well, just earlier this week I told you about the smallest mini-galaxy ever discovered. Let’s take a look.

Image credit: Marla Geha and Keck Observatories.

With just over 1,000 stars and yet a mass of something like 600,000 solar masses, Segue 1 is the most dark-matter-dominated object found to date!

Some people contend that this discrepancy is cause for alarm, and some even reject the idea of dark matter because of it! And while it surprised me when I first learned about it just a few days ago, it shouldn’t have.

Image credit: Goddard Space Flight Center / NASA.

Because the dark matter/normal ratio starts out the same on all scales, but then you form stars! And if you run your simulations of the Universe and include star formation, you find something remarkable.

Video credit: Marcelo Alvarez, Ralf Kaehler, and Tom Abel, KIPAC, Stanford University.

Forming stars for the first time not only released a tremendous amount of visible light, illuminating the Universe for the first time since the early stages of the Big Bang, it also did two other remarkable things:

  1. The ultraviolet radiation released re-ionizes the Universe, knocking the electrons off of protons in the intergalactic medium, as the video above shows. And…
  2. The radiation causes a small but non-negligible amount of pressure to permeate the Universe.

“Big deal,” you say. “Why should I care about a small amount of radiation pressure?”

Image credit: Tony Virgo.

Well, radiation pressure is the very thing responsible for bending this comet’s tail. In other words, it can cause atoms to accelerate, and exerts a force on them. That’s the normal matter. But photons do not interact with dark matter!

So what happens when you put a source of radiation pressure in or near something with normal matter and dark matter? Well, the dark matter stays put no matter what. But the normal matter?

Image credit: Graham Laurenson.

It depends on how deep your gravitational potential well is! If you’ve got a lot of mass in a large galaxy (or very large cluster of galaxies), most of the normal matter stays put, as this extra pressure is insufficient to kick the normal matter out. But the smaller and lower-mass your galaxy (or dwarf galaxy, or mini-galaxy) is, the more normal matter gets expelled!

And that’s why dark matter rules mini-galaxies, and the tinier you are, the more dark matter rules you!


  1. #1 CR
    August 5, 2011

    Would the effect of this radiation pressure have different effects on different normal matter particles/molecules?

    I realize most everything is hydrogen at this point, but I’m wondering if it would affect electrons more than protons or neutrons, for example, or something else that would show up as tending to form different kinds of stars/nebulae/etc. in small vs. large galaxies, or maybe forming them earlier or later.

  2. #2 João Carlos
    August 5, 2011

    I think I’m missing something… When you say: “dark matter/normal ratio starts out the same on all scales, but then you form stars!“, are you implying dark matter is also fused in the core of the early stars?

  3. #3 D. C. Sessions
    August 5, 2011

    And here I figured that the honking ginormous black holes in large galaxies would eat more dark matter than the cores of smaller galaxies.

  4. #4 Ethan Siegel
    August 5, 2011

    CR @1, it will probably be slightly more difficult to expel helium (about 8% of the atoms, by number) than hydrogen, and ions will experience a different effect from neutral atoms. But by far the dominant effect is the radiation pressure I described in the post.

    João @2, no, there is no implication of that. Only normal matter can form objects with high densities like stars; dark matter has no way to lose that angular momentum. I am implying that the formation of stars causes sufficient radiation pressure to sometimes kick normal matter out, but that it can never kick dark matter out.

    D.C. Sessions @3. black holes, like everything else, do a lousy job at eating dark matter. Dark matter haloes are much… umm… fluffier than you’re imagining.

  5. #5 Alan L.
    August 5, 2011

    Thanks, Ethan, for your expanded explanation about the reason for the variable Dark to Baryonic Matter ratios that occur in galaxies of different sizes and masses. That’s certainly cleared up a few things in my mind since I first encountered Stacy McGaugh’s comments last year.

    Perhaps I have a character defect of some kind but now I’ve developed an irresistible urge to refer to mini galaxies as a Quasi Great Voids.

  6. #6 CR
    August 6, 2011

    CR @1, it will probably be slightly more difficult to expel helium (about 8% of the atoms, by number) than hydrogen, and ions will experience a different effect from neutral atoms. But by far the dominant effect is the radiation pressure I described in the post.

    Thanks for the response. I figured it wouldn’t be much of an effect, but I was curious. In any case this is interesting. Not what I would’ve expected, but then again, I didn’t expect there to be such a difference in normal matter/dark matter ratios anyway.

    Thanks again for having such an awesome blog!

  7. #7 SCHWAR_A
    August 6, 2011

    But photons do not interact with dark matter!

    How can that be? We actually can see in gravitational lense constellations that photons coming from galaxies behind an object do bend corresponding to the sum of baryonic AND dark matter of the lensing object.

    And: this effect is also used to generate dark-matter-maps from the universe…

  8. #8 feedayeen
    August 6, 2011

    “But photons do not interact with dark matter!How can that be? We actually can see in gravitational lense constellations that photons coming from galaxies behind an object do bend corresponding to the sum of baryonic AND dark matter of the lensing object.

    And: this effect is also used to generate dark-matter-maps from the universe…”

    Dark matter doesn’t directly interact with photons, this means that it doesn’t absorb or radiate them. Because of this, energy can’t be transfered either to or away from the matter via this route. Photons however are perfectly happy to interact with gravity and dark matter still has mass so by extension it interacts with photons by creating gravity.

  9. #9 Torbjörn Larsson, OM
    August 6, 2011

    Ah, thanks! I didn’t know about the size vs ratio trend; this makes so much more sense than the haphazard mechanism of “collisions will strip the stars” analogy from cluster collisions!

  10. #10 Martin
    August 6, 2011

    OK…I’m pretty much on board with dark matter and not MOND. It definitely works at galactic and sub-galactic scales.


    Why is it not needed when doing orbital calculations of solar system objects?

  11. #11 Peter Fred
    August 6, 2011

    @SCHWAR_A Thanks for pointing out that (some kinds) of light- bending theoretically implies that photons do interact with the putative dark matter. Being a 30-year dark matter doubter, I never thought of this contradiction. Since Ethan has brought up the twin topics of radiation pressure and dark matter, I feel once again to bring up my slant on the Kuhnian Crisis that the flat rotations curves and cosmic acceleration has placed us in for the last 40 years. With a Kuhnian Crisis a serious anomaly has occurred such as the discovery of the photoelectric effect or the ultraviolet catastrophe. A paradigm shift then eventually emerges that reorientates the thinking that scientists have had. History tells us that this new idea is unexpected and unlike anything in the text book, which our revered physicists have memorized to a fault. Comets tails from Kepler on down have instilled in physicists the idea that radiation can only be repulsive and never attractive. Newton’s knowledge of the behavior of comet tails was probably the reason why Newton never consider that radiation, which varies as the square of he distance from a body was the actual source of the attractive gravitational force. Instead he proposed that some yet-to-be-specified, innate property of the mass of a body was the source of the attractive gravitation force. This notion which has been with us for 300 years is about as unphysical and counter-intuitive as the notion of geocentric orbital which was entertained for 1500 years. Of course I have experiments showing that radiation is attractive which are quickly dismissed by our highly trained and inculcated scientists. The radiation I use in my experiments is infrared radiation–the kind of radiation that the gas giants are most likely to be bathed in. The shorter-wavelength-light that we on Earth, Mercury and Venus experience have a much greater capacity to burn off the atmospheric gas. Mercury and our slow-revolving Moon hardly have an atmosphere.
    We have have had a long history from Newton to Einstein from switching back and forth as to whether light is particle or a wave. Now we have assumed that is both. Maybe its time we consider the idea that radiation can be both repulsive and attractive. For anyone interested in replicating my simple inexpensive experiments that represent a serious anomaly go to

  12. #12 Lassi Hippeläinen
    August 6, 2011

    And for every large galaxy cluster, we get (more or less) that same ratio: about 15-17% of the mass is “normal” matter, with about 83-85% dark matter.
    With just over 1,000 stars and yet a mass of something like 600,000 solar masses, Segue 1 is the most dark-matter-dominated object found to date!

    So it should have about 100’000 solar masses of baryonic matter, but only 1000 stars are seen. The rest should be star remnants that have run out of fuel, or black holes. Would it be possible to see this unseen matter indirectly, e.g. via occultations?

  13. #13 Ema Nymton
    August 6, 2011


    More than 12 hours until our first kook sighting.

    You may be losing your touch, Ethan.

  14. #14 SCHWAR_A
    August 8, 2011


    Please tell me if I’m wrong, but I would say that any object always will experience an acceleration, following a way through its environment, which leads to the mean temperature of the accelerated object itself, to get a balanced state within its environment. It even can torque the object like a light-mill.

    Could you produce an horizontal experiment with a sphere arranged as a swing, which will be pulled towards the heat element located at one side? A simple measurement could be done with camera and background-lattice, more sophisticated with mirror and LASER.

    Interesting would be after a certain time of the experiment, whether you can then find a swing-mountpoint somewhere between the heat-source and the ice, which will not show any swing-deviation. This would then be the temperature-balanced location.

    Sure you also could try to vary the length of the wooden dowel to find such a location vertically, but because there is no vacuum I would like to avoid disturbances due to environmental air convections.

    Nevertheless this behaviour could be interesting related to gas clouds: they do not only collaps by gravity but also by re-distributing hotter molecules towards the center and thus they collaps and heat up faster than due to gravity only.

  15. #15 ObsessiveMathsFreak
    August 8, 2011

    This appears absurd to me. If Dark matter is increasingly dominant down to even clusters of 1000 stars, why is it not appreciable at the solar system level, particularly in regard to comets? Would any beings in solar systems within this cluster come to different conclusions about Gravity than our own for example?

  16. #16 SCHWAR_A
    August 8, 2011


    That’s why we say that Dark Matter is a halo around a system. A testmass completely ‘within’ the halo cannot ‘feel’ its gravity, like You would be weightless in the earth’s center. The more you put a testmass to the outer zones, the more it has some part of the dark matter halo mass in its inner sphere and thus ‘feels’ its effect: additional force towards the center of the system.

  17. #17 ObsessiveMathsFreak
    August 8, 2011

    But the entire system must be feeling the overall pull of the dark matter potentially locally. In these small clusters, would this be great enough to, say, affect the long term stability of planetary orbits or binary star systems? Would aliens living in this cluster ever conclude Kepler’s laws?

  18. #18 SCHWAR_A
    August 8, 2011


    Obviously, when observing systems, which are large enough, the Kepler’s laws are only valid when regarding dark matter: you have to add the more dark matter mass, the farther away the testmass is from the system’s centre. Only then the related rotation curve matches the observed.

    Thus a usual binary star system with narrow stars does not show a measurable effect.

    And, sure, the entire system ‘feels’ the pull, but due to the equal distribution of the dark matter in the halo there is no ‘effective’ momentum, unless you ‘dive into’ the halo and thus create an unbalanced momentum, which is increasing when you go farther away from the centre.

  19. #19 Mr. Gravity
    August 8, 2011

    A recent study linked a change (reduction)in radiation properties of radio-active materials here on earth
    occurring a couple of days after a major solar flare. Do you suppose this could be a result of radiation pressure from whatever caused the solar flare?

  20. #20 SCHWAR_A
    August 9, 2011


    Please offer the link to that study.

    I can only speculate: Assuming that decay occurs due to a critical energy state in the atom, i.e. the atom needs more energy from the outside to longer stay stable, then I would expect that specific radiation input to the atom could delay the decay…

  21. #21 OKThen
    August 11, 2011


    OK, I think I understand this post.
    Nothing to argue about the observations. Here’s what we see in galaxies versus minigalaxies and comets and how the dark matter hypothesis fits nicely. And yes, dark matter does not interact electromagnetically with photons (only gravitationally). And the idea that dark matter is fluffy.

    OK, so what is my problem.

    Dark matter has been described as a halo around galaxies. There is more dark matter at the edges of a galaxy than at the middle.

    As NASA says, “The strongest evidence for the existence of dark matter comes from studying galactic dynamics… nonluminous mass (dark matter) is less concentrated in the center of the galaxy… dark matter may be clumped around galaxies, in much the same way that we found matter clumped around voids in “The Spongy Universe.””

    Now the term mini galaxy, e.g the one in your post The Smallest mini galaxy in the Universe, seems to be just a fancy name for small globular cluster (or maybe small dwarf galaxy.)

    And of course globular clusters surround galaxies, which is to say they are in the halo of a galaxy. Hence, they are located where the highest concentrations of dark matter are calculated. (i.e. where observation forces the dark matter hypothesis upon us).

    So far I think I am in agreement with the observational facts. Educate me if I am not.

    Now I understand that there is no standard accepted model of how galaxies are formed and the relation between globular clusters and their black holes and a primary galaxy and its supermassive black hole.

    Do we know if a galaxy grows from a globular cluster; or if globular clusters accumulate to build galaxies; or if galaxies spin off globular clusters as a galaxy ages and grows. wiki says, “it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear.”

    So we don’t seem to know a causal sequence of events, the dynamics of evolution between globular clusters and galaxies.

    All we know is that globular clusters and minigalaxies are in the halo where the most dark matter of a major galaxy.

    So my problem in saying “dark matter rules mini-galaxies” is that such a statement is identical to saying “a halo of dark matter surrounds galaxies.” These two statements are tautological.

    OK, that’s my problem. But let me add one other little tinsey wintsey little problem. Radiation pressure.

    A galaxy has all of the stars and the luminescent mass in the center and all of the dark matter is in the halo around the dark matter. Oops, why has the dark matter moved outside the gravity well and the baryonic matter stayed in the gravity well?

    So my second problem, saying “dark matter rules mini-galaxies” may be true but I don’t think it can be explained by radiation pressure. Rather, it would seem that in a galaxy, radiation pressure correlates with the movement of dark matter away from the center.

    Such is my thinking. Please now tell me what I am missing or have screwed up in my dark matter thinking. I am not trying to formulate any alternative hypothesis; just trying to understand the current best thinking. Thanks.

  22. #22 Ethan Siegel
    August 11, 2011


    Dark matter has been described as a halo around galaxies. There is more dark matter at the edges of a galaxy than at the middle.

    Start here; your understanding of “what a halo is” is incorrect.

    Think of a large, diffuse cloud of mass that is densest at the center, and then the density decreases very slowly at first as you move away from the center, then turns over and decreases more rapidly, until eventually the background density of space is greater than the matter bound to your halo.

    Large objects like galaxies have large dark matter halos; “mini-galaxies” like this may very well form in isolation, but get gravitationally captured by larger galaxies over time.

    Wiki is wrong on at least one point: globular clusters are very unlikely to have older stars than our galaxy does; they merely don’t have populations of younger stars mucking up our observations.

  23. #23 OKThen
    August 11, 2011


    Your paragraph “Think of a large..” is a bit of a cipher. I might be understanding or not. I’ll think about it.

    The rest I understand. Thank you.

  24. #24 Mr, Gavity
    August 12, 2011

    @20 – Jenkins and Fishbind (sp) at Purdue did their studies back in 2006 when there was a large solar flare. Apparently this affect is also experienced when the sun is closer to the earth.

  25. #25 Mr Gravity
    August 12, 2011

    @20 – This link discusses the findings. Sounds like “radiation pressure” could provide them clues and possible answers.

  26. #26 forrest noble
    August 15, 2011

    In my opinion Dark matter is still no better than an ad hoc hypothesis, and much of what they are observing concerning the mechanics of dark matter is purely speculative.

    This something they are observing seemingly cannot just be remedied by a change in galaxy mechanics, such as an improvement of Newtonian mechanics or GR. This is because two galaxies which appear to be very similar concerning their visible size,form,density, and stellar distribution, may have very different stellar relationships and mechanics concerning their rotation curves.

    There is something unseen which causes unpredictable rotation curves of galaxies which also has other observable gravitational effects. I think this is all of the facts, the rest is just speculation.

    Even the idea that it is matter is just speculation. Vortex mechanics of a particulate aether of some kind would seemingly have the same effect. The size of such particulates could be plank size (very tiny) if it saturates the entire universe and moves in currents causing galaxy rotation and cluster rotation. Such currents could actually be the cause of gravity everywhere, not just some unknown form of matter.

  27. #27 SCHWAR_A
    August 15, 2011

    @OKThen (23)

    Imagine, each single mass in a system, each atom, each proton etc., has its own very, very weak dark matter halo. It starts at its centre and is spherically effective all around it, but with 1/r-distance-law, until its effect is less than that of the environmental space.

    The complete system’s halo thus is a superposition of all these single haloes.

    There is no possibility to detect these single weak effects in laboratories, but superposed we can detect it in the universe.

    @Mr.Gravity (25)

    Thanks! This could lead to build a new neutrino-detector:
    A sphere with radioactive material and related decay-rate-counters.
    Significant changes in the rates are then correlated with changing neutrino-streams in related directions.

    @forrest noble (26):

    Nobody states that “Dark Matter” actually is ‘matter’, just the opposite. But as it behaves gravitationally and this behaviour so far only belongs to mass (or more common ‘matter’), the working name is “Dark Matter”.

    There is something, it shows its effect, and we are very excited to see somewhen its actual mechanism…

    Go on with your ideas: Make calculations with them and present (post?) them to be compared to actual observations (= experiment). That’s science!

  28. #28 Nathan Myers
    August 16, 2011

    Is the preponderance of dark matter in them the reason the stars in these clusters don’t collapse into a black hole in short order? I imagine stars near the center almost unaffected by nearby stars, for the soup of dark matter they swim in.

  29. #29 SCHWAR_A
    August 18, 2011

    @Nathan Myers (28):
    There is a preponderance, but its effect is very, very weak and thus you can measure it only when the much bigger effect of usual mass gravity (decreasing with 1/r^2) is overruled.
    You only can measure higher velocity of stars far away from the galaxy’s centre, as if its mass would be higher for them…