Baryon Fractions for Poor Clusters of Galaxies

We analyze x ray and optical data for six poor clusters of galaxies (MKW4, AWM4, AWM7, MKW4S, N79-299, and N67-336). For four of the systems, AWM4, AWM7, MKW4, and MKW4S, the x-ray emission is regular enough to measure the virial mass, gas mass, and temperature from the x-ray emission. We have obtained digitized plate scans for five of the poor clusters. We derive positions and photographic magnitudes for galaxies in a 30'x 30' region around each group center (1.5^deg^ x 1.5^deg^ for AWM7). From the x-ray surface brightness distribution, we construct models of the mass distribution. We use a modified V/V_max_ test [Avni & Bahcall, ApJ, 27,446(1980)] to test whether the galaxies trace the potential marked by the x-ray emitting gas. The galaxy distribution is consistent with the density distribution inferred from the x rays. The mass in galaxies is ~3h^-1^% of the total mass of the systems. The galaxies contribute significantly to the baryonic mass total. The ratio M_gas_/M_gal_ ~ 1.4 h^-1/2^, similar to the value for rich clusters. The baryon fraction in rich groups is ~0.08 (for H_0_ = 100), or about half that in rich clusters. This result has significant consequences for models of the formation of large-scale structure.