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

We measure the dark matter halo masses of <z> ≈ 2.36 UV colour-selected star-forming galaxies by matching the observed median H I Lyα 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 log₁₀M_min/M_⊙ = 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. ≈30 × 30 arcsec. In addition, we find that the observed anisotropy in the 2D H I Lyα 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.