Why aren't brown dwarfs the dark matter?

  • There seems to be not enough matter to account for the fact that the speed of some stars located far from the Galaxy center is almost the same speed as those stars found nearer to the center of the Galaxy, therefore it seems there should be even more matter: so this missing matter is termed dark matter. There is a kind of stellar object that isn't massive enough to produce its own light but is many times more massive than Jupiter, a.k.a brown dwarf. Since it isn't bright enough to be detected by our deep space telescope nor the earth bound counterparts, they should be numerous in galaxies as I believe these objects sits in the middle range between planets and stars. However it seems the scientific community had ruled out this potential dark matter candidate without scanning the unobservable Milky Way galaxy as well as billions other with different light spectrums. Why aren't brown dwarfs considered candidates for the mysterious dark matter?

  • ProfRob

    ProfRob Correct answer

    7 years ago

    Two reasons.

    1. We know from looking at galaxy rotation curves and the motion of galaxies in clusters and from gravitational lensing, that the amount of "dark matter" is some 30% of the density of the universe. But on the other hand, estimates of the abundances of deuterium, helium, tritium and lithium produced in the big bang indicate that only 5% of the density of the universe can be in the form of normal matter (e.g atoms, protons, neutrons etc). As brown dwarfs are made of these things, they can only contribute to this small percentage and cannot be responsible for most of the dark matter (the same is true for anything made of normal, "baryonic matter").

    2. Astronomers have counted how many brown dwarfs there are, both in the local neighbourhood and in star clusters (see for example Andersen et al. 2008; Kirkpatrick et al. 2011; Burningham et al. 2013; . In other words we can see brown dwarfs - they emit most of their light in the infrared part of the spectrum, and surveys like SDSS, 2MASS and WISE have uncovered them in their thousands. It turns out that there is about 1 brown dwarf for every 4 more massive stars. So, although numerous, they contribute a very tiny fraction of the normal matter in our Galaxy. In addition, microlensing experiments towards the Galactic bulge and Magellanic clouds suggest that although there are lots of brown dwarfs elsewhere in our Galaxy (e.g. Alcock et al. 2000; Novati et al. 2008), there are not enough to make much of a contribution to dark matter, and so the results from our solar neighbourhood appear representative of the Galaxy as a whole.

    I had thought he only reason we know about exo-gas giants was the wobble they induce in the stars they orbit. But how about brown dwarfs not orbiting a star? If we can't see brown dwarfs, how were they counted? Some cites would be helpful.

    @HopDavid We can see brown dwarfs. I'll put some references in.

    IIRC, you can also estimate brown dwarf density by checking to see how often the objects eclipse more distant stars.

    @WayfaringStranger That's news to me - are you sure you don't mean microlensing, which is what I am talking about. Transits occur for brown dwarfs in *orbit* around stars, but as their radii are indistinguishable from giant planets, then you would not be able to tell a brown dwarf from a planetary mass object. Could you point out a project that finds free-floating brown dwarfs via the eclipse of background stars?

    @RobJeffries Can't point one out. Read this some years ago when people were claiming dark matter *might* amount to nothing more than rogue planets and small stars; normal matter in condensed form, that was simply too dark to see. By knowing the needed density to match galactic rotation curves, you could predict how often stars should be eclipsed by such large, dark objects. Photometry didn't find as many single-time events as needed. This was before WMAP data, so the world has moved on. Microlensing *may* have been involved, but as I said, I'm not finding any current info.

    @WayfaringStranger Hmm. I think you don't RC. I think the effect of such a transit is in fact a gravitational lensing magnification (by a large factor if the alignment is perfect), not an eclipse. This *is* what the microlensing surveys such as MACHO and MOA detect. Here is a paper about transits http://adsabs.harvard.edu/abs/2004ApJ...604..379D in the MACHO survey: I can't immediately see any mention of anything but binaries and planets orbiting the star they transit.

    @RobJeffries Could be, it was a while ago. Might've been bad journalism too. That's pretty common.

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