Weak-lensing study of low-mass galaxy groups: Implications for Omega(m)

We report on the first measurement of the average mass and mass-to-light ra tio of galaxy groups by analyzing the weak-lensing signal induced by these systems. The groups, which have velocity dispersions of 50-400 km s(-1), ha ve been selected from the Canadian Network for Observational Cosmology Fiel d Galaxy Redshift Survey (CNOC2). This survey allows the identification of a large number of groups with redshifts ranging from z = 0.12 to 0.55, idea l for a weak-lensing analysis of their mass distribution. For our analysis we use a sample of 50 groups that are selected on the basis of a careful dy namical analysis of group candidates. We detect a signal at the 99% confide nce limit. The best-fit singular isothermal sphere model yields an Einstein radius. r(E) = 0".72 +/- 0".29. This corresponds to a velocity dispersion of [sigma (2)](1/2) = 274(-59)(+48) km s(-1) (using photometric redshift di stributions for the source galaxies), which is in good agreement with the d ynamical estimate. Under the assumption that the light traces the mass, we find an average mass-to-light ratio of 191 +/- 83 h (M)./L-B. in the rest-f rame B band. Unlike dynamical estimates, this result is insensitive to prob lems associated with determining group membership. After correction of the observed mass-to-light ratio for luminosity evolution to z = 0, we find 254 +/- 110 h M./L-B., lower than what is found for rich clusters. We use the observed mass-to-light ratio to estimate the matter density of the universe , for which we find Omega (m) = 0.19 +/- 0.10 (Omega (Lambda) = 0), in good agreement with other recent estimates. For a closed universe (Omega (m) Omega (Lambda) = 1), we obtain Omega (m) = 0.13 +/- 0.07.