Virial masses and the baryon fraction in galaxies

We have measured the weak-lensing signal as a function of rest-frame B-, V-, and R-band luminosity for a sample of "isolated'' galaxies. These results are based on four-band photometry from the Red-Sequence Cluster Survey, enabling us to determine photometric redshifts for a large number of galaxies. We select a secure sample of lenses with photometric redshifts 0: 2 < z < 0: 4 and study the relation between the virial mass and baryonic contents. In addition, we discuss the implications of the derived photometric redshift distribution for published cosmic shear studies. The virial masses are derived from a fit to the observed lensing signal. For a galaxy with a fiducial luminosity of 10(10) h(-2) L(B,)circle dot, we obtain a mass M-vir = 9.9(-1.3)(+1.5) x 10(11) M circle dot The virial mass as a function of luminosity is consistent with a power law proportional to L-1.5, with similar slopes for the three filters considered here. These findings are in excellent agreement with results from the Sloan Digital Sky Survey and semianalytic models of galaxy formation. We measure the fraction of mass in stars and the baryon fraction in galaxies by comparing the virial mass-to-light ratio to predicted stellar mass-to-light ratios. We find that star formation is inefficient in converting baryons into stars, with late-type galaxies converting similar to 33% and early-type galaxies converting only similar to 14% of baryons into stars. Our results imply that the progenitors of early-type galaxies must have low stellar mass fractions, suggestive of a high formation redshift.