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

Abstract

We present the first measurement of the large-scale cross-correlation of Lyα forest absorption and Damped Lyman α systems (DLA),using the 9th Data Release of the Baryon Oscillation Spectroscopic Survey (BOSS). The cross-correlation is clearlydetected on scales up to 40h−1Mpc and is well fittedby the linear theory prediction of the standard Cold Dark Matter modelof structure formation with the expected redshift distortions,confirming its origin in the gravitational evolution of structure.The amplitude of the DLA-Lyα cross-correlation depends on only one freeparameter, the bias factor of the DLA systems, once the Lyα forest biasfactors are known from independent Lyα forest correlation measurements.We measure the DLA bias factor to be bD = (2.17±0.20)βF0.22, where the Lyα forest redshift distortion parameterβF is expected to be above unity. This bias factor implies atypical host halo mass for DLAs that is much larger than expected inpresent DLA models, and is reproduced if the DLA cross section scaleswith halo mass as Mhα, with α = 1.1±0.1 forβF = 1. Matching the observed DLA bias factor and rate of incidencerequires that atomic gas remains extended in massive halos over largerareas than predicted in present simulations of galaxy formation, withtypical DLA proper sizes larger than 20 kpc in host halos of masses ∼ 1012M☉. We infer that typical galaxies at z ≃ 2 to3 are surrounded by systems of atomic clouds that are much more extendedthan the luminous parts of galaxies and contain ∼ 10% of thebaryons in the host halo.