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Dark Matter’s ‘Building Blocks’

Although we often talk about the Magellanic Clouds as satellites of the Milky Way, recent research seems to point to a different conclusion. The dwarf galaxies may be moving too fast to be bound to our own, cities of stars simply flowing past us in the night. Be that as it may, the Milky Way still has over twenty other dwarf galaxies in orbit around it, eighteen of which have been the subject of recent work aimed at calculating their masses. The odd results have striking implications for dark matter.

For the dwarf galaxies around us vary greatly in brightness, from a thousand times the luminosity of the Sun to a billion times that amount. You would assume that the brightest dwarf galaxy would have the greatest mass, while the faintest would show the least. The surprise is that all the dwarf galaxies have roughly the same mass, some ten million times the mass of the Sun within their central 300 parsecs. Here’s Manoj Kaplinghat (University of California at Irvine) with a helpful comparison:

“Suppose you are an alien flying over Earth and identifying urban areas from the concentration of lights in the night. From the brightness of the lights, you may surmise, for example, that more humans live in Los Angeles than in Mumbai, but this is not the case. What we have discovered is more extreme and akin to saying that all metro areas, even those that are barely visible at night to the aliens, have a population of about 10 million.”

What’s going on? The dwarf galaxies, about half of them found within the past few years by the Sloan Digital Sky Survey, are primarily made up of dark matter, the ratio of dark to normal matter becoming as high as ten thousand to one (the latter are the most dark-matter dominated objects known). Even the faintest dwarf galaxies, all but invisible to us, contain huge amounts of dark matter. The researchers believe that clumps of dark matter can exist that contain no stars at all, and that a minimum dark matter mass exists — about ten million times the mass of the Sun — that allows stars within the dark matter to form into galaxies.

Image: Satellite galaxies studied by UCI researchers that are within 500,000 light-years of the Milky Way. Credit: J. Bullock/M. Geha/UCI.

Small, gravitationally bound clumps of dark matter are known as haloes, and there’s no reason why haloes can’t be smaller than the minimum size needed for galaxy formation. Indeed, the Milky Way may be teeming with them. Consider this, from the team’s letter to Nature:

The mass of the smallest dark matter halo is determined by the particle properties of dark matter. Dark matter candidates characterized as cold dark matter can form haloes that are many orders of magnitude smaller than the least luminous haloes that we infer from observations. Cosmological simulations of cold dark matter predict that galaxies like the Milky Way should be teeming with thousands of dark matter haloes with masses ∼ 106 M , with a steadily increasing number as we go to the smallest masses. A large class of dark matter candidates characterized as “warm” would predict fewer of these small haloes.

Are we getting any closer to an understanding of the elusive particle that makes up dark matter? A key to understanding the stuff at the microscopic level may be operations at the Large Hadron Collider, scheduled to become operational this year. But even as we ponder the possibilities of actually creating dark matter in the lab, the astronomical outlook is all about building a larger dataset of low-luminosity galaxies. From the paper:

Future imaging surveys of stars in the Milky Way will provide a more complete census of low-luminosity Milky Way satellites, with the prospects of determining whether astrophysics or fundamental dark matter physics is responsible for setting the common mass scale. In particular, the masses for the faintest dwarf galaxies will become more strongly constrained with more line-of-sight velocity data. This will sharpen the observational picture of galaxy formation on these small-scales and provide data around which theories of galaxy formation may be built.

The paper is Strigari et al., “A common mass scale for satellite galaxies of the Milky Way,” Nature 454 (28 August 2008), pp. 1096-1097 (abstract).

Comments on this entry are closed.

  • James M. Essig September 2, 2008, 14:09

    Hi Folks;

    This is a very interesting article.

    I wonder if there might exist super-massive black holes that were created by dark matter accumulation. We know of the black hole current record holder which I believe I have read has a mass of about 3.4 billion solar masses. Perhaps dark matter has produced larger monsters yet, perhaps in the 10s of billions of solar masses plus.

    Perhaps some of the large scale mass currents observed within the observable universe such as that associated with the so-called “great attractor” (I am not sure what they currently call this beast) can be associated or are associated with stupendously large black holes. I know a popular book written by Kip Thorne about a decade or so ago gave example of what could happen to material objects falling into a 100 trillion solar mass or even a quadrillion solar mass black hole. I believe, if I am not mistaken, that Thorne did not actually say that such huge black holes existed, but with some of the random highly directional mass flow currents that have been observed within the observable universe, I wonder if such gargantuan black holes may be at least in part the cause of such.

    It is interesting that cold dark matter may overwhelmingly dominate the mass energy of so-called dwarf galaxies. For one, the presence of luminous dwarf galaxies located ubiquitously in otherwise intergalactic space will provide more interesting “geography” for manned missions to explore. Secondly, we might find that there are whole planets, stars, and even bodily ETI composed of Cold Dark Matter. Thirdly, we might find that there are several different forms of CDM as well as Warm Dark Matter.

    It will be interesting to see what that “Bad Boy” LHC produces when it is fired up. As I mentioned on at a previous thread, I saw an excellent program on the Science Channel within the U.S. on the LHC this past weekend. We can all have high hopes that great discoveries will be made this fall at CERN.



  • Freederick September 3, 2008, 6:38

    Rather than positing undetectable massive particles (“dark matter”) to account for the mass discrepancy observed in galaxies, would it not be possible to posit additional dimensions? A galaxy extending into a fourth spatial dimension would be largely undetectable to instruments operating within a 3-D (+time) space–we could only see, or otherwise interact, with a slice of the higher-dimensional object. Yet the detectable mass–certainly the inertial mass–would include the whole object.

    A more graceful explanation of the concept may be found here: http://www.gwywyr.com/texts/harem.html

    This effect could also account for the distribution of “dark mass” varying from the observable distribution of mass, as a function of the 4-D shape of the object. For instance, a slice through a 4-toroidal object near its rim would manifest its observable mass at the center (where the slicing space intersects the 4-toroid) but the “dark mass” would be concentrated to two opposite sides of the observable mass, hinting at the continuation of the 4-ring.


  • Benjamin September 3, 2008, 7:42

    One of the most exciting discoveries – perhaps the most exciting possible discovery – would be to learn that dark matter has complex interactions like baryonic mater. This would be fantastically exciting. The problem is, these interactions would surely be producing some detectable particle flux, photons, light leptons, something; therefore, while I very much hope for an exciting Copernicus-crushing discovery that the universe is teeming with dark matter worlds, the idea of planets or ETI composed of dark matter looks shaky. If there were radiant bodies to support this, let alone the complex interactions to constitute it, surely we would have observed them.

  • george scaglione September 3, 2008, 17:22

    jim,freed,ben, good ideas one and all there is soooo much we have yet to learn but i cannot help but think that a site like this is just about the perfect place for people of interest to start looking around,talking together sharing ideas! in the old days we would have had to communicate by snail mail and lol that would have been impossible because we would not know of each others existance! but again,all good ideas that just remind me how much yet remains to be covered ! maybe the LHC will be of some help – if i am right it should go on line full tilt boogey in about a week! thanks one and all your friend george

  • ljk September 18, 2008, 12:46

    Astronomers Discover Most Dark Matter-Dominated Galaxy in Universe

    Published: September 18, 2008

    Segue 1 is 50 times dimmer than the star cluster pictured above but is 1000 times more massive, meaning most of its mass must be made up of dark matter. (Credit: Sloan Digital Sky Survey)

    New Haven, Conn. — A team led by a Yale University astronomer has discovered the least luminous, most dark matter-filled galaxy known to exist.

    The galaxy, called Segue 1, is one of about two dozen small satellite galaxies orbiting our own Milky Way galaxy. The ultra-faint galaxy is a billion times less bright than the Milky Way, according to the team’s results, to be published in an upcoming issue of The Astrophysical Journal (ApJ). But despite its small number of visible stars, Segue 1 is nearly a thousand times more massive than it appears, meaning most of its mass must come from dark matter.

    “I’m excited about this object,” said Marla Geha, an assistant professor of astronomy at Yale and the paper’s lead author. “Segue 1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass.”

    Full article here: