Lensing by galaxies in CNOC2 fields

Abstract

We have observed two blank fields of approximately 30 × 23 arcmin2 using the William Herschel Telescope. The fields have been studied as part of the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2), and spectroscopic redshifts are available for 1125 galaxies in the two fields. We measured the lensing signal caused by large-scale structure, and found that the result is consistent with current, more accurate measurements. We study the galaxy—galaxy lensing signal of three overlapping samples of lenses (one with and two without redshift information), and detect a significant signal in all cases. The estimates for the velocity dispersion of an galaxy agree well for the various samples. The best-fitting singular isothermal sphere model to the ensemble-averaged tangential distortion around the galaxies with redshifts yields a velocity dispersion of σ*= 130+15−17 km s−1, or a circular velocity of V*c= 184+22−25 km s−1 for an L*B galaxy, in good agreement with other studies. We use a maximum-likelihood analysis, where a parametrized mass model is compared with the data, to study the extent of galaxy dark matter haloes. Making use of all available data, we find σ*= 111 ± 12 km s−1 (68.3 per cent confidence, marginalized over the truncation parameter s) for a truncated isothermal sphere model in which all galaxies have the same mass-to-light ratio. The value of the truncation parameter s is not constrained that well, and we find s*= 260+124−73h−1 kpc (68.3 per cent confidence, marginalized over σ*), with a 99.7 per cent confidence lower limit of 80 h−1 kpc. Interestingly, our results provide a 95 per cent confidence upper limit of 556 h−1 kpc. The galaxy—galaxy lensing analysis allows us to estimate the average mass-to-light ratio of the field, which can be used to estimate Ωm. The current result, however, depends strongly on the assumed scaling relation for s.