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 log10Mmin/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.