Clusters of Galaxies in the Local Universe

We use a matched filter algorithm to find and study clusters in both N-body simulations artificially populated with galaxies and the Two Micron All Sky Survey (2MASS). In addition to numerous checks of the matched filter algorithm, we present results on the halo multiplicity function and the cluster number function. For a subset of our identified clusters we have information on X-ray temperatures and luminosities that we cross-correlate with optical richness and galaxy velocity dispersions. With all quantities normalized by the spherical radius corresponding to a mass overdensity of Δ_M=200 or the equivalent galaxy number overdensity of Δ_N=200Ω^-1_M~=666, we find that the number of L>L_* galaxies in a cluster of mass M₂₀₀ is logN_*666=(1.44+/-0.17)+(1.10+/- 0.09)log(M₂₀₀h/10¹⁵M_solar), where the uncertainties are dominated by the scatter created by three choices for relating the observed quantities to the cluster mass. The region inside the virial radius has a K-band cluster mass-to-light ratio of (M/L)_K=(116+/-46)h, which is essentially independent of cluster mass. Integrating over all clusters more massive than M₂₀₀=10¹⁴ h^-1 M_solar, the virialized regions of clusters contain ~=7% of the local stellar luminosity, quite comparable to the mass fraction in such objects in currently popular ΛCDM models.