Determining field galaxy halo masses via the weak gravitational lensing effect

The dynamical properties of galaxies suggest they live in a cocoon or halo of dark matter, the nature of which remains unknown. To determine the mass and extent of these galactic halos one must observe some type of trace particle moving in the gravitational field of the galaxy, but such tracers are rare beyond about 30 h-1 kpc from the galactic center. We overcome this difficulty by measuring the effect of the galaxy's gravity on passing photons---an effect now called gravitational lensing.; We detect this weak gravitational lensing as the distortion of over 450,000 background galaxies (20 < R < 23) by 790 foreground galaxies (R < 18) selected from the Las Campanas Redshift Survey (LCRS). This is the first detection of weak lensing by field galaxies of known redshift, which imparts two great advantages: We reconstruct the shear profile of a typical galaxy halo in physical units (i.e. kpc rather than arcsec), and we investigate the dependence of halo mass on galaxy luminosity. This is also the first galaxy-galaxy lensing study for which the calibration errors due to uncertainty in the background galaxy redshift distribution and the seeing correction are negligible.; The shear profile we derive within a projected radius of 200 h-1 kpc of an L* galaxy is consistent with that of an isothermal sphere of circular velocity nu c = 164 +/- 20 km s-1 By combining this halo mass normalization with the higher S/N profile derived by Fischer et al. (2000) we place a lower limit of (2.7 +/- 0.6) x 1012 h-1 M⊙ on the mass of an L* galaxy halo, which agrees with the satellite galaxy findings of Zaritsky & White (1994). Assuming that M ∝ Lbeta we use the measured luminosity function of our LCRS foreground galaxies to determine the contribution of normal galaxies to the density of the Universe. We find that the mass within 260 h-1 kpc sets a lower limit of O ≥ 0.16 +/- 0.03 for the mass-follows-light value beta = 1 or O ≥ 0.24 +/- 0.06 for the Tully-Fisher and Faber-Jackson value beta = 0.5. Furthermore, our lensing data give a best fit value of beta = 1.4 +/- 0.4, and fitting the data results in a lower limit of O ≥ 0.12 +/- 0.05.