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Milky Way May Have Dark Matter Satellite Galaxies 174

Posted by CmdrTaco
from the whirling-and-twirling dept.
rubycodez writes "Berkeley astronomer Sukanya Chakrabarti has detected perturbations in the gases surrounding our Milky Way and concludes there is a satellite 'Galaxy X' 250,000 light years away that is mostly dark matter, but that may contain dwarf stars visible in infrared. She expects many more such dark matter satellites to the Milky Way to be discovered using her technique."
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Milky Way May Have Dark Matter Satellite Galaxies

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  • by roc97007 (608802) on Monday January 17, 2011 @01:27PM (#34906970) Journal

    "The creature from invisible Galaxy X"

    • by Black Parrot (19622) on Monday January 17, 2011 @01:59PM (#34907438)

      "The creature from invisible Galaxy X"

      There was an interesting musing by the author of a recent Scientific American about how dark matter may interact with its own kind by forces other than the ones that cause normal matter to interact with its own kind. According to the musing (which the author rejects), dark matter operating under such forces could form complex systems, maybe even an unseen parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on us.

      • by ron_ivi (607351)

        It'd be weird if one of those dark planets zoomed through our solar system closely enough to have it's gravity affect the earth.

        Imagine one zipping by and touching one of our oceans, and a bunch of the water'll & a few people, etc might be sucked up into that dark-gravity-source.

      • by mangu (126918)

        dark matter may interact with its own kind by forces other than the ones that cause normal matter to interact with its own kind. According to the musing (which the author rejects), dark matter operating under such forces could form complex systems, maybe even an unseen parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on us.

        A plot for an SF story: every time the universe branches due to wavefunction collapse [wikipedia.org] a copy of the universe is created which still interacts with the universe through gravitation but not through the other forces.

        Local effects of this are extremely difficult to measure, but they can be perceived as a fifth force [wikipedia.org] that appears, for instance in the Pioneer anomaly [wikipedia.org].

        I wish my writing skills were good enough to write this story...

      • by jbeaupre (752124) on Monday January 17, 2011 @07:12PM (#34910640)

        parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on us.

        Are you talking about women? I've actually seen them. No joke!

      • by lgw (121541)

        I'm pretty sure that's not possible, because dark matter around our galaxy forms a sphere, not a disk. You get a disk when there's friction between the particles -they start clumping, averaging out angular velocity until most of the mass lies in a disk perpindicular to the overall average axis of rotation. With no friction, and no clumping, everything stays distributed in a sphere.

        But that's at current energy levels. Who knows - in the early universe or far future (if there's another state change down to

      • by ShakaUVM (157947)

        There was an interesting musing by the author of a recent Scientific American about how dark matter may interact with its own kind by forces other than the ones that cause normal matter to interact with its own kind. According to the musing (which the author rejects), dark matter operating under such forces could form complex systems, maybe even an unseen parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on u

        • by khayman80 (824400)

          Last year Tanuki64 [slashdot.org] asked a similar question, and I referred him to an anecdote [dumbscientist.com] from my senior year of physics undergrad. In 2004, I presented a similar idea to an astrophysicist in my department:

          I wonder if "dark matter" is the result of gravitational interactions with galaxies in parallel universes. Suppose parallel universes exist in the same physical "space" we inhabit, and only interact with each other (and us) via gravity. The galaxies in different universes would then clump together, but their disks

          • by ShakaUVM (157947)

            Interesting, Khayman. Why do you say it's inconsistent with the Bullet Cluster data? It seems to me that if you had two clusters on top of each other, one in our 3-brane, and one in a neighboring one, and they collided with a cluster just in our 3-brane, that the result would be more or less consistent with the result of the purported dark matter separating from the normal matter.

  • Cue the chocolate milk and dark chocolate jokes. I'm too busy, otherwise I'd think of some. C'mon /., don't let us down!
    • Re: (Score:3, Funny)

      by rubycodez (864176)
      I prefer my women like dark matter: attractive, hard to catch, dark, and mysterious.
  • by Fractal Dice (696349) on Monday January 17, 2011 @01:36PM (#34907092) Journal
    How do you tell the difference between a blob of dark matter and a black hole? With all the small galaxies the Milky Way has swallowed over its lifetime, would it not be reasonable to find some relic black holes that have swung back out after being stripped of most of their surrounding gas/stars? Or, when "dark matter" is being talked about in this situation, is a black hole simply one of the possible candidates to supply the mystery mass?
    • Blackholes would prob ably get quite bright from time to time and (if nothing else) leave a halo of glowing matter behind. Also, dark matter can be quite spread out and turns into really huge blobs, rather than point masses. I imagine her technique looked for things like that. I don't think the article says anything really useful about the technique. Hopefully they cover it on Naked Astronomy.
    • by Colonel Korn (1258968) on Monday January 17, 2011 @01:46PM (#34907262)

      How do you tell the difference between a blob of dark matter and a black hole? With all the small galaxies the Milky Way has swallowed over its lifetime, would it not be reasonable to find some relic black holes that have swung back out after being stripped of most of their surrounding gas/stars? Or, when "dark matter" is being talked about in this situation, is a black hole simply one of the possible candidates to supply the mystery mass?

      I think we'd expect to see the kind of supermassive black hole that could be mistaken for a dwarf galaxy. The processes that form black holes of that size mean that there would probably still be a lot of material in the vicinity, if not actively accreting then still getting pulled around, compressed, and prompting star formation. Also, I think nearby galactic-sized black holes would probably make for some pretty wicked and obvious gravitational lensing.

      Alternatively, the detected mass might be a large number of small black holes. I doubt it, but I'm not an astronomer. Luckily, further observation will give us answers.

      • I think we'd expect to see the kind of supermassive black hole that could be mistaken for a dwarf galaxy. The processes that form black holes of that size mean that there would probably still be a lot of material in the vicinity, if not actively accreting then still getting pulled around, compressed, and prompting star formation

        Going out on a limb... I would expect dark matter to form "dark black holes" if you brought enough mass of it together in a small enough space.

        For that matter, I would expect dark matter to get caught up into "normal" black holes, and normal matter into "dark" black holes, if you brought one kind of matter near the other kind of black hole.

        I suspect a physicist would say it's impossible to distinguish a "normal" black hole from a "dark" black hole.

        • Well, black holes are always dark. No one shines.

          Now, about distinguishing a galaxy from a black hole (the original question). On this situation it is possible. Since our galaxy (with gas everywhere) is overlaping the dark galaxy, if it was a black hole some gas would fall near it and make the usual X-ray emmisions.

        • by TexVex (669445)

          Going out on a limb... I would expect dark matter to form "dark black holes" if you brought enough mass of it together in a small enough space.

          I'm no expert, but here's what I think I know:

          Dark matter only interacts gravitationally. Dark matter does not collide with matter, nor does it collide with other dark matter. This means it can't form clumps. No dark matter clumps means no dark matter gravity wells, meaning no dark matter black holes.

          A galaxy's dark matter, then, is a diffuse cloud of invis

          • by EdZ (755139)

            Dark matter only interacts gravitationally.

            OK.

            Dark matter does not collide with matter

            Not so sure on this. Does not interact maybe, but gravitational attraction should bring particles close to each other.

            nor does it collide with other dark matter.

            Why not?

            Now, obviously dark matter particles would be unable to escape a black hole just like anything else. However, the vast majority of dark matter will never interact with a black hole -- it will just orbit forever.

            Now this just smacks of nonsense. Why would dark matter magically start orbiting a black hole whereas normal matter, with the additional benefit or photon and particle wind to push it away, would not?

            • Collisions are interactions. If dark matter doesn't interact with anything, then it doesn't collide with anything.

              Also, dark matter wouldn't magically start orbiting black holes, it would just start orbiting it like normal matter. However, when a lot of normal matter clusters around a black hole it starts interacting thus giving up energy and falling deeper into the gravitational well.

    • by Monkeedude1212 (1560403) on Monday January 17, 2011 @01:54PM (#34907378) Journal

      How do you tell the difference between a blob of dark matter and a black hole?

      Gravitational pull is probably the biggest factor. A black hole simply gets so massive that at one point the gravitational pull is so strong that not even light can escape. It will have objects orbitting around it like planets orbit stars except at distances far greater than a star would normally hold.

      Dark Matter, on the other hand, simply seems to have the gravitational pull of a regular star, but doesn't emit any light.

      One thing to note is that when we observe things out there, it's not just a 2D plane we're observing but a great deal of depth is involved. When observing a black hole, the light behind the black hole will get sucked into the black hole if it happens to cross the event horizon. This will create a nice black circle in the sky. However dark matter, on the other hand, would not stop the light behind it from reaching our eyes, it might bend it a little but nothing too extreme.

      • by Fuseboy (414663)

        That doesn't sound quite right - because of the inverse-squared falloff of gravity, once you're a certain distance away, black holes and stars aren't much different in how material orbits around them.

        One difference is that that black holes often spew high-energy x-rays as infalling matter is crushed, whereas dark matter is - well - dark.

      • by Tim C (15259) on Monday January 17, 2011 @02:34PM (#34907978)

        A black hole simply gets so massive that at one point the gravitational pull is so strong that not even light can escape.

        Actually that's a good working definition of a black hole - if its gravity weren't that strong, it wouldn't be one.

        It will have objects orbitting around it like planets orbit stars

        Yes...

        except at distances far greater than a star would normally hold.
         
        ...and no, not necessarily. That depends entirely upon the mass of the hole. The gravitational field of a black hole at a given distance is no different than that generated by a star of the same mass at the same distance; the difference is that the hole is so much smaller that you can get much closer to its centre. That vastly reduces the r in GM/(r^2), thus increasing the maximum gravity that can be experienced.

        When observing a black hole, the light behind the black hole will get sucked into the black hole if it happens to cross the event horizon. This will create a nice black circle in the sky.

        The situation is a little more complicated than that thanks to gravitational lensing, but essentially you're correct - a black hole will block light, while dark matter does not.

        • Mmmmm, you are saying that a photon hitting a particle of dark matter simply passes through it with no chance of being absorbed?
          • This is exactly what is meant by the word "dark" in "dark matter". It precisely means that its coupling to photons is zero (except, of course, for higher order loop effects in the Feynman diagrams, but those would be incredibly, perhaps immeasurably, small).
            • by c++0xFF (1758032)

              To be fair, there's two definitions of dark matter. One is as you said -- matter that has little or no interaction with photons.

              The other is baryonic dark matter [wikipedia.org] which is simply normal matter that isn't emitting many photons, so we don't see it. And if we can't see it, we can't include it in our tally of mass.

              But, it seems that baryonic dark matter can only account for a small percentage of the total dark matter in the universe, so it's usually ignored.

              • Umm he didn't say "little or no interaction" with photons - he said "It precisely means that its coupling to photons is zero", no interaction with photons. Big difference.
                • by c++0xFF (1758032)

                  Yeah, I debated on whether to include the "little or no" bit. We should probably ignore interactions due to "higher order loop effects" ... but he threw it in there, so I did too. Probably a mistake.

                  But my point holds: there are two kinds of dark matter: ordinary matter that we normally deal with (but not emitting much light) or something a bit more "exotic" than that (which doesn't even interact with light, much less emit any). It's basically the difference between MACHOs and WIMPs.

                  • Well my point was (completely internalized and so probably not obvious) really that we don't know what dark matter is, or how many kinds of dark matter there may be, and we really don't know whether there is no interaction (outside of gravity) with particles such as photons. Head on collision between photon and dark matter particle - is there an actual collision or do they not exist as far as the other is concerned or something else? It seems that, at least in some cases, there is an actual collision which
            • Ummm well I hate to rely upon Wikipedia as the cite for an opposing opinion but....

              http://en.wikipedia.org/wiki/Dark_matter [wikipedia.org]

              Don't stop at the first paragraph or two...

              The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly transparent.

              Which implies there is a part that does interact with electromagnetic radiation - photons.

              nonbaryonic dark matter includes neutrinos

              And of course neutrinos can interact with photons and bar

              • I am aware of and understand all this. I was trying to alleviate what I thought was a minor but fundamental and common misunderstanding regarding what "dark" in "dark matter" signifies. If I had known that you were just looking for an excuse to be pedantic about the existence of baryonic dark matter, I would not have posted anything. Yes, baryonic dark matter exists. Yes, baryonic dark matter couples to the photon. Happy?
                • I did not say, and neither did the Wikipedia article, that only baryonic dark matter interacts with photons or baryonic matter. So before you start trying to be condescending and tossing around labels like "pedantic" perhaps you ought to first try a little more reading and a little more thinking.
      • by c++0xFF (1758032) on Monday January 17, 2011 @03:29PM (#34908580)

        Say what?

        First off, the GPP has a decent question. The largest supermassive black holes are on the order of 10^9 solar masses, about the same mass as what was calculated for this satellite galaxy. So, I suppose it's at least plausible that it's a single black hole, if unlikely.

        But remember and repeat after me: a black hole has no more gravity than any other object of the same mass. As long as you stay away from the event horizon, that is. You need to rethink your first paragraph with that in mind.

        So, how would we tell the difference? Well, an X-ray source from the same location would be a good clue that it's a black hole, which says that it's feeding off of something. You should also be able to tell from the gravitational lensing -- dark matter is incredibly diffuse compared to a black hole. It would still bend light, but not quite in the same way, especially considering the distances involved.

        But what about a black circle in the sky? Well, the even horizon for such a black hole has the same diameter as the orbit of Pluto, if I remember right. Detectable, maybe, under the right conditions (but not by Hubble -- you'd need something with about 20x better resolution ... if I did the math right, which I probably didn't). But we have to capture it overlapping with some other body, such as a background galaxy. By then you'd be better off looking at the lensing effect, anyway. Here [wikimedia.org] is a classic simulation of what I'm talking about.

      • by bcrowell (177657)

        How do you tell the difference between a blob of dark matter and a black hole?

        Gravitational pull is probably the biggest factor. A black hole simply gets so massive that at one point the gravitational pull is so strong that not even light can escape. It will have objects orbitting around it like planets orbit stars except at distances far greater than a star would normally hold.

        Dark Matter, on the other hand, simply seems to have the gravitational pull of a regular star, but doesn't emit any light.

        No, this is completely wrong. A black hole doesn't have stronger gravity than the star or stars that it formed out of.

        One thing to note is that when we observe things out there, it's not just a 2D plane we're observing but a great deal of depth is involved. When observing a black hole, the light behind the black hole will get sucked into the black hole if it happens to cross the event horizon. This will create a nice black circle in the sky. However dark matter, on the other hand, would not stop the light behind it from reaching our eyes, it might bend it a little but nothing too extreme.

        This is also wrong. Gravitational lensing occurs both for black holes and for other objects that aren't black holes. The black hole in the sky that you're describing is not what is predicted for a black hole either.

        For anyone who wants to see the actual paper, here it is: http://xxx.lanl.gov/abs/1101.0815 [lanl.gov]

    • by vlm (69642)

      How do you tell the difference between a blob of dark matter and a black hole?

      In theory due to hawking radiation etc black holes temperature increases over time as it shinks (weird but true). Both probably live around the cosmic microwave blackbody limit.

      A big array of dark matter would be a hair above 2.7 kelvin and tending to thermalize down to 2.7, but a big array of black holes would tend to be a hair above 2.7 kelvin and tend to increase over time.

      So basically something cold that tends to stay cold is probably dark matter and something that seems to be warming up more than reas

    • by Locke2005 (849178)
      A black hole would occlude light from behind it, making it detectable by a circular absense of light as it travels across the sky. Apparently a blob of "dark matter" either does not absorb light or has such a small diameter that it does not detectably occlude light. But if the dark matter was a point source of gravity, wouldn't we still see some gravitational lensing? It appears dark matter would have to be much less dense than a black hole. But then, I'm not astrophysicist.
    • How do you tell the difference between a blob of dark matter and a black hole? With all the small galaxies the Milky Way has swallowed over its lifetime, would it not be reasonable to find some relic black holes that have swung back out after being stripped of most of their surrounding gas/stars? Or, when "dark matter" is being talked about in this situation, is a black hole simply one of the possible candidates to supply the mystery mass?

      Good thinking - astronomers thought of it too.

      I don't know the details, but they've ruled out black holes, brown dwarfs, neutrinos (except the hypothetical sterile neutrino), and all the other "normal" matter anyone knows about.

    • Both interact with light solely through gravitation, but dark matter is constitutionally incapable of interacting with light. It's dark not because it holds onto light, but because light just passes through it the same way a piece of plastic ignores a magnetic field. (Actually, not quite the same, but it's close enough for the moment.)

      Black holes may or may not interact with light; what's inside a black hole is undefined. But when light falls on it, it passes the point of no return and never leaves.

      Light

    • by Chemisor (97276)

      > How do you tell the difference between a blob of dark matter and a black hole?

      Stick your hand in it. If you can pull it out again, it's dark matter.

  • by starglider29a (719559) on Monday January 17, 2011 @01:36PM (#34907094)
    What is the form of the dark matter? Does it coalesce into spherical bodies? Or does it homogenize into equidistant particles due to mutual repulsion? And if it is bound to the Milky Way by gravity, and itself bound to as a 'galaxy', does it exert cosmos expanding repulsion in an "inverse almost square" relationship? Is it 1/ (r- fudgeFactor)^2 or 1/ (r)^(2-fudgeFactor)?

    Seriously. I'm a rocket scientist, and I'm baffled by the mixed properties of 'dark matter'. Can we land a probe on it, or would baryonic space probes pass right through it?
    • by rainmouse (1784278) on Monday January 17, 2011 @01:45PM (#34907234)

      What is the form of the dark matter?

      Assuming it exists at all. There is much circumstantial evidence but some argue no direct proof yet (though NASA believe the have proof). Still this excerpt from NASA seems to imply that dark matter does not interact with matter except through gravity.

      "The hot gas in this collision was slowed by a drag force, similar to air resistance. In contrast, the dark matter was not slowed by the impact, because it does not interact directly with itself or the gas except through gravity. "

      Source: http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html [nasa.gov]

    • by gstoddart (321705)

      Seriously. I'm a rocket scientist, and I'm baffled by the mixed properties of 'dark matter'. Can we land a probe on it, or would baryonic space probes pass right through it?

      I think that's kind of the point, isn't it?

      We don't know WTF it is, or what it's made up of ... only that we can measure it's gravitational effects but can't directly figure out how to observer it.

      Beyond that, I've never heard anyone offer an good, testable explanation of what it is, merely what we think it isn't.

      • There are lots of testable explanations of what it might be. Many of them are being tested by experiments in progress today, or will be tested by experiments in the planning or building stages. For example, the lightest supersymmetric partner (LSP) is a good candidate for dark matter. Weakly interacting massive particles (yes, WIMPs) are another good candidate, and the LSP might in fact be a WIMP, depending on the values of the parameters describing supersymmetry (if it exists).

        Some proposed explanatio
        • by gstoddart (321705)

          Thanks, I think. I'll have to read a little bit before the whooshing sound stops though. :-P

          I take it "higher mass, lower momentum cold dark matter" wouldn't just be lots big rocks of really heavy elements, but essentially inert? Or would anything that big be generating something else that would rule it out?

          I believe people are sure of it needing to be accounted for ... but how we know to be looking for it in the first place is one of those things I just have to mostly take on faith.

          Something as remote as

    • Seriously. I'm a rocket scientist, and I'm baffled

      C'mon, man! It ain't brain surgery!

    • by blair1q (305137)

      It doesn't emit or absorb electromagnetic radiation, or it wouldn't be "dark".

      I'm not sure how there would be "mutual repulsion" in a body of free particles unless they all had the same charge. But charge is mediated by photons, which are electromagnetic radiation (from radio on up to gamma rays). Without EM, it would take a new force.

      I'm kind of skeptical that such a dark-matter galaxy could exist. Galaxies are coalescence of gas by gravity. Why would there be a huge collection of particles affected by

      • by ghostdoc (1235612)

        That's a really good point actually.

        If Dark Matter attracts itself through gravity (which it must do to form Dark Galaxies), but is incapable of holding a charge (or, presumably, interacting with the Strong or Weak nuclear), then how does it repel itself enough to stay separate? Surely it should just collapse almost instantly into Dark Black Holes?

        Or are there Dark Black Holes out there?

        • by wanerious (712877)
          Because it's collision-less, there's no effective way for a cloud of mutually-attracting DM particles to lose energy other than gravitational radiation. Normal matter will collide and heat up (accretion disks), losing kinetic energy as well as potential, and coalesce into a smaller object. DM clouds, as I understand it, have a much larger timescale for collapse. They don't "clump" very fast.
    • Seriously. I'm a rocket scientist, and I'm baffled by the mixed properties of 'dark matter'. Can we land a probe on it, or would baryonic space probes pass right through it?

      Due to the presumed lack of electrostatic interaction, your probe could not "touch" it. You could orbit it or settle into a common center of gravity, but not land on it.

    • by arcsimm (1084173)
      I'm no scientist, but to my untrained nose dark matter has the same smell as "luminiferous aether" and "epicycles." Basically, it's a theoretical placeholder for something we don't fully grasp yet.
    • by blueg3 (192743)

      "Dark matter" isn't really a name for a particular kind of matter with known properties. Some observations about gravitational forces don't work out quite right, as if there was a bunch of matter that we can't see. We don't really know any properties of it other than that it has mass and is otherwise undetectable (so far). Hence, "dark matter".

    • There are a lot of different models for dark matter candidates. All of them have in common that the coupling of dark matter to photons is zero. Also the strong coupling constant (coupling to gluons) is zero. Some models have dark matter that interacts via the weak interaction (W and Z bosons), eg WIMPs. Other models have zero weak interaction. Some models propose brand new interactions among dark matter particles that normal baryonic matter does not couple to, but TTBOMK none of these have a coupling s

  • Wouldn't dark matter galaxies so close to ours result in the occlusion of galaxies behind them?

    Since a galaxy is mostly empty space -- wouldn't this result in a detectable degree of light variation?

    • Wouldn't dark matter galaxies so close to ours result in the occlusion of galaxies behind them?

      Since a galaxy is mostly empty space -- wouldn't this result in a detectable degree of light variation?

      No, for the dark matter hypothesis to work as an explanation of what it's supposed to explain, it doesn't have any kind of EML interaction with normal matter, so it can't obscure any more than it can be seen.

      It does bend space, and AIUI the best support for the DM hypothesis is the lensing effect it has at a distant cluster, but there may not be enough in the postulated satellite galaxies to cause any detectable lensing.

    • by Fnkmaster (89084)

      I believe the whole deal is that this dwarf dark matter galaxy is on the far side of the milky way and in the same plane (or nearly so) as the majority of the milky way, and thus is difficult to directly detect in terms of gravitational lensing effects. The article wasn't clear however on what the actual technique used was, but obviously it must be *some* form of gravitational lensing.

  • Ages ago I have seen 'The Elegant Universe'. There was a real nice explanation of several flavours of the string theory. One of the points was that gravity is so weak compared to the other forces because part of it wanders off into neighbouring universes. If this is true, why should only our gravity wander away from us? It would be expected, that at the same time gravity from other universes would come into ours. So dark matter is simply normal matter from a neighbouring universe from which we 'see' part of
  • will sue the astronomers who found it over copyright violations. They've had "Mikey Way Dark' out for years.

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