A measurement of galaxy halo mass from the surrounding H i Lyα absorption

We measure the dark matter halo masses of << z >> approximate to 2.36 UV colour-selected star-forming galaxies by matching the observed median H i Ly alpha absorption around them, as observed in the spectra of background QSOs, to the absorption around haloes above a given mass in cosmological simulations. Focusing on transverse separations 0-2 proper Mpc (pMpc) and line-of-sight separations 154-616 km s(-1), we find a minimum halo mass of log(10)M(min)/M-circle dot = 11.6 +/- 0.2, which is in good agreement with published halo mass estimates from clustering analyses. We verified that the measured halo mass is insensitive to a change in the cosmological parameters (Wilkinson Microwave Anisotropy Probe 1 versus Wilkinson Microwave Anisotropy Probe 3) and to the inclusion of strong AGN feedback. One unique strength of this method is that it can be used in narrow field galaxy-QSO surveys, i.e. approximate to 30 x 30 arcsec. In addition, we find that the observed anisotropy in the 2D H i Ly alpha absorption distribution on scales of 1.5-2 pMpc is consistent with being a consequence of large-scale gas infall into the potential wells occupied by galaxies.