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We present a measurement of the correlation function between luminous redgalaxies and cool gas traced by Mg II \lambda \lambda 2796, 2803 absorption, onscales ranging from about 30 kpc to 20 Mpc. The measurement is based oncross-correlating the positions of about one million red galaxies at z~0.5 andthe flux decrements induced in the spectra of about 10^5 background quasarsfrom the Sloan Digital Sky Survey. We find that: (i) This galaxy-gascorrelation reveals a change of slope on scales of about 1 Mpc, consistent withthe expected transition from a dark matter halo dominated environment to aregime where clustering is dominated by halo-halo correlations. Assuming that,on average, the distribution of Mg II gas follows that of dark matter up to agas-to-mass ratio, we find the standard halo model to provide an accuratedescription of the gas distribution over three orders of magnitude in scale.Within this framework we estimate the average host halo mass of luminous redgalaxies to be about 10^{13.5} M_solar, in agreement with other methods. Wealso find the Mg II gas-to-mass ratio around LRGs to be consistent with thecosmic value estimated on Mpc scales. Combining our galaxy-gas correlation andthe galaxy-mass correlation function from galaxy-galaxy lensing analyses we candirectly measure the Mg II gas-to-mass ratio as a function of scale and reachthe same conclusion. (ii) From line-width estimates, we show that the velocitydispersion of the gas clouds also shows the expected 1- and 2-halo behaviors.On large scales the gas distribution follows the Hubble flow, whereas on smallscales we observe the velocity dispersion of the Mg II gas clouds to be lowerthan that of collisionless dark matter particles within their host halo. Thisis in line with the fact that cool clouds are subject to the pressure of thevirialized hot gas.

The Large-scale Distribution of Cool Gas around Luminous Red Galaxies