Measuring the galaxy–mass and galaxy–dust correlations through magnification and reddening

We present a simultaneous detection of gravitational magnification and dust reddening effects due to galactic haloes and large-scale structure. The measurement is based on correlating the brightness of ~85 000 quasars at z > 1 with the position of 24 million galaxies at z ~ 0.3 derived from the Sloan Digital Sky Survey and is used to constrain the galaxy-mass and galaxy-dust correlation functions up to cosmological scales. The presence of dust is detected from 20 kpc to several Mpc, and we find its projected density to follow: Σ dust ~ r -0.8 p , a distribution similar to mass. On large scales, its wavelength dependence is described by R v ≃ 4.9 ± 3.2, consistent with interstellar dust. This, in turn, implies a cosmic dust density of Ω dust ≃ 5 x 10 -6 , roughly half of which comes from dust in haloes of ~L* galaxies. We estimate the resulting opacity of the Universe for various evolutionary models and find 〈A v 〉 ~ 0.03 mag up to z = 0.5. We present magnification measurements, corrected for dust extinction, from which the galaxy-mass correlation function is inferred to give the mean surface mass density profile around galaxies Σ ~ 30 (θ/1 arcmin) -0.8 h M ⊙ pc -2 up to a radius of 10 Mpc, in agreement with gravitational shear estimates.