CosmoCoffee

Another interesting paper this week! It seems that our local halo doesn't seem to conform to expectations from cosmology: it's too oblate and has far to few satellites anyway. But I though we do understand "small scale" structure of cosmological kind, i.e. various models to populate DM halos are in agreement with 2dF and SDSS (for example various cell counting exercises, bias from lensing, etc.). So what's wrong with Milky Way?
(Authors do write down a plausible theory in the last section, but nevertheless, our halo is an atypical one!)

Yes, this looks interesting -- authors note that the positions of many of the MW's satellites appear to be almost co-planar, which would seem to suggest a common origin for at least some of the satellites. I don't think this is true for M31's satellites, though.

Should we lose any sleep over this? If several of these satellites were in a loosely bound group when accreted by the MW, wouldn't you expect them to share an orbital plane? It's amusing that other work (i.e. astro-ph/0304267 by Andy Gould) has argued that there's too many cold streams in the MW stellar halo to be compatible with CDM, while this work says there's too few.

I'm not sure if the Milky Way is such an oddball from the CDM viewpoint. For years, there was much hand-wringing over the Sgr stream indicating a perfectly spherical halo, but this result seems to have evaporated (astro-ph/0407565). Similarly, there was consternation that the radial distribution of MW satellites was too concentrated relative to CDM expectations, but this problem also seems to have gone away (astro-ph/0401088).

No need to loose any sleep about this. All of this is in agreement with CDM with an "if" (see below).

Papers that compare MW satellite anisotropy to the CDM predictions are:
Kang et al. 2005, AA 437, 383
Zentner et al. 2005, ApJ 629, 1
Liebeskind et al. 2005, MNRAS in press

The main result is that everything is consistent, IF (and it's a big if) MW disk is almost perpendicular to the major axis of its outer DM halo. This is not something one expects
naively, and is not something that comes out naturally in galaxy formation simulations
(Bailin et al. 2005).

For M31, the situation is more uncertain, but see Zentner et al. 2005 and Koch & Grebel 2005 (astro-ph/0509258)