groups, environments

I spoke with Blanton about his argument that almost all relationships between galaxies and their environments can be explained as coming from the masses of the gravitationally bound objects ("groups" in my vocabulary, "haloes" in the new economy) in which they reside, and their locations within. Really his results are about multiplicity, not mass, but the punchline is mass.

The one exception is very small-scale clustering or close pairs. It appears that there is color-related substructure within groups. Not surprising, but very interesting. The challenge is to demonstrate it in a clear way.

Berlind and I reviewed the figures for Berlind's groups paper, which is beautiful. He shows that you cannot find groups with high completeness in a survey (eg, SDSS) and also simultaneously meet the following three criteria:

• get the multiplicity distribution right
• get the velocity dispersions right
• get the angular sizes right

This is particularly interesting because many investigators at the present day tune up to get the last two things right and then write papers about the multiplicity and mass functions. So, as long as he can get the word out, he has a pretty important result. And its not a negative result, because he does have a catalog that gets the multiplicity function right.