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Session 86 - Measurement of Cosmological Parameters.
Oral session, Wednesday, January 17
1st Floor, La Villita Assembly Building

[86.03] Where is the Dark Matter?

N. A. Bahcall (Princeton U.)

How much dark matter is there in the universe? Where is it located? These are two of the most fundamental questions in cosmology. We use optical and X-ray mass determinations of galaxies, groups, and clusters of galaxies to suggest that most of the dark matter may reside in very large halos around galaxies, typically extending to 200kpc for bright galaxies. We show that the mass-to-light ratio of galaxy systems does not increase significantly with linear scale beyond 200kpc. Rather, the total mass of large scale systems such as groups and rich clusters of galaxies can on average be accounted for by the total mass of their individual galaxies (including their large halos, which may be stripped-off but still remain in the clusters), plus the mass of the hot intracluster gas. This also suggests, if confirmed by future observations, that the total mass-density of the universe is low : omega \ 0.2 - 0.3.

Program listing for Wednesday