The large-scale distribution of cool gas around luminous red galaxies

We present a measurement of the correlation function between luminous red galaxies (LRGs) and cool gas traced by Mg ii lambda lambda 2796, 2803 absorption, on scales ranging from about 30 kpc to 20 Mpc. The measurement is based on cross-correlating the positions of about one million red galaxies at z similar to 0.5 and the flux decrements induced in the spectra of about 10(5) background quasars from the Sloan Digital Sky Survey. We find that: (i) this galaxy-gas correlation reveals a change of slope on scales of about 1 Mpc, consistent with the expected transition from a dark matter halo dominated environment to a regime 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 a gas-to-mass ratio, we find the standard halo model to provide an accurate description of the gas distribution over three orders of magnitude in scale. Within this framework, we estimate the average host halo mass of LRGs to be about 10(13.5) M-circle dot, in agreement with other methods. We also find the Mg ii gas-to-mass ratio around LRGs to be consistent with the cosmic mean value estimated on Mpc scales. Combining our galaxy-gas correlation and the galaxy-mass correlation function from galaxy-galaxy lensing analyses, we can directly measure the Mg ii gas-to-mass ratio as a function of scale and reach the same conclusion. (ii) From linewidth estimates, we show that the velocity dispersion of the gas clouds also shows the expected one- and two-halo behaviours. On large scales the gas distribution follows the Hubble flow, whereas on small scales we observe the velocity dispersion of the Mg ii gas clouds to be lower than that of collisionless dark matter particles within their host halo.