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We present the first measurement of the large-scale cross-correlation ofLyman alpha forest absorption and Damped Lyman alpha systems (DLA), using the9th Data Release of the Baryon Oscillation Spectroscopic Survey (BOSS). Thecross-correlation is clearly detected on scales up to 40 Mpc/h and is wellfitted by the linear theory prediction of the standard Cold Dark Matter modelof structure formation with the expected redshift distortions, confirming itsorigin in the gravitational evolution of structure. The amplitude of theDLA-Lyman alpha cross-correlation depends on only one free parameter, the biasfactor of the DLA systems, once the Lyman alpha forest bias factors are knownfrom independent Lyman alpha forest correlation measurements. We measure theDLA bias factor to be b_D = (2.17 +/- 0.20) beta_F^{0.22}, where the Lymanalpha forest redshift distortion parameter beta_F is expected to be aboveunity. This bias factor implies a typical host halo mass for DLAs that is muchlarger than expected in present DLA models, and is reproduced if the DLA crosssection scales with halo mass as M_h^alpha, with alpha= 1.1 +/- 0.1 forbeta_F=1. Matching the observed DLA bias factor and rate of incidence requiresthat atomic gas remains extended in massive halos over larger areas thanpredicted in present simulations of galaxy formation, with typical DLA propersizes larger than 20 kpc in host halos of masses ~ 10^12 solar masses. We inferthat typical galaxies at z ~ 2 to 3 are surrounded by systems of atomic cloudsthat are much more extended than the luminous parts of galaxies and contain ~10% of the baryons in the host halo.

The large-scale cross-correlation of Damped Lyman Alpha Systems with the Lyman Alpha Forest: First Measurements from BOSS