Tuesday, February 07, 2006

Cool Dark Matter

Paul Cook has more on the Mystery of Not-So-Cold Dark Matter over on Tangent Space

I have to confess that I was expecting this, and am looking forward to the paper.
Gerry is a former collaborator and I've talked to him about the mystery of the dwarf spheroidals.

We also have a proposed SIM Key Project to nail this down, although if SIM is descoped he project might not be doable, or more likely will be bumped by the local astrometry and planet hunts which will take more time on a smaller telescope.

Short version: dwarf spheroidals show evidence of a disproportionately high fraction of dark matter; this is expected since their gravitational potential is shallow, so baryons(normal matter) are easily expelled, by say type II supernovae, and thus star formation is interrupted or halted.

But, if you look at them in detail, two things jump out - there are too few dwarf galaxies, by ~ 2 orders of magnitude, and their properties cut off abruptly.
Simplest explanation is the Cold Dark Matter is not cold. Rather it has a residual irreducable velocity dispersion of ~ 10 km/sec, corresponding to O(104) K, and a scale of ~ 300 pc and mass of ~> 3d7 solar masses.
Or equivalently a characteristic density cut off.

Which is peculiar.

Might be we're missing something about baryon cooling and there are a lot of lower mass baryon free dwarfs out there ("lumpy halos"), or we're getting data on the physics of Cold Dark Matter.

Some ideas about that, but not in a blog.


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