The large-scale cross-correlation of Damped Lyman alpha systems with the Lyman alpha forest: first measurements from BOSS

We present the first measurement of the large-scale cross-correlation of Ly alpha forest absorption and Damped Lyman alpha systems (DLA), using the 9th Data Release of the Baryon Oscillation Spectroscopic Survey (BOSS). The cross-correlation is clearly detected on scales up to 40 h(-1)Mpc and is well fitted by the linear theory prediction of the standard Cold Dark Matter model of structure formation with the expected redshift distortions, confirming its origin in the gravitational evolution of structure. The amplitude of the DLA-Ly alpha cross-correlation depends on only one free parameter, the bias factor of the DLA systems, once the Ly alpha forest bias factors are known from independent Ly alpha forest correlation measurements. We measure the DLA bias factor to be b(D) = (2.17 +/- 0.20)beta(0.22)(F), where the Ly alpha forest redshift distortion parameter beta(F) is expected to be above unity. This bias factor implies a typical host halo mass for DLAs that is much larger than expected in present DLA models, and is reproduced if the DLA cross section scales with halo mass as M-h(alpha), with alpha = 1.1 +/- 0.1 for beta(F) = 1. Matching the observed DLA bias factor and rate of incidence requires that atomic gas remains extended in massive halos over larger areas than predicted in present simulations of galaxy formation, with typical DLA proper sizes larger than 20 kpc in host halos of masses similar to 10(12) M-circle dot. We infer that typical galaxies at z similar or equal to 2 to 3 are surrounded by systems of atomic clouds that are much more extended than the luminous parts of galaxies and contain similar to 10% of the baryons in the host halo.