Detection of Cosmic Magnification with the Sloan Digital Sky Survey

We present an 8 sigma detection of cosmic magnification measured by thevariation of quasar density due to gravitational lensing by foreground largescale structure. To make this measurement we used 3800 square degrees ofphotometric observations from the Sloan Digital Sky Survey (SDSS) containing\~200,000 quasars and 13 million galaxies. Our measurement of the galaxy-quasarcross-correlation function exhibits the amplitude, angular dependence andchange in sign as a function of the slope of the observed quasar number countsthat is expected from magnification bias due to weak gravitational lensing. Weshow that observational uncertainties (stellar contamination, Galactic dustextinction, seeing variations and errors in the photometric redshifts) are wellcontrolled and do not significantly affect the lensing signal. By weighting thequasars with the number count slope, we combine the cross-correlation ofquasars for our full magnitude range and detect the lensing signal at >4 sigmain all five SDSS filters. Our measurements of cosmic magnification probe scalesranging from 60 kpc/h to 10 Mpc/h and are in good agreement with theoreticalpredictions based on the WMAP concordance cosmology. As with galaxy-galaxylensing, future measurements of cosmic magnification will provide usefulconstraints on the galaxy-mass power spectrum.Comment: 12 pages, 8 figures, 2 tables; accepted for publication in Ap