The circumgalactic medium in Lyman α: a new constraint on galactic outflow models

Galactic outflows are critical to our understanding of galaxy formation and evolution. However, the details of the underlying feedback process remain unclear. We compare Ly α observations of the circumgalactic medium (CGM) of Lyman Break Galaxies (LBGs) with mock observations of their simulated CGM. We use cosmological hydrodynamical ‘zoom-in’ simulations of an LBG which contains strong, momentum-driven galactic outflows. Simulation snapshots at z = 2.2 and z = 2.65 are used, corresponding to the available observational data. The simulation is post-processed with the radiative transfer code crash to account for the impact of ionizing photons on hydrogen gas surrounding the simulated LBG. We generate mock absorption line maps for comparison with data derived from observed close galaxy–galaxy pairs. We perform calculations of Ly α photons scattering through the CGM with our newly developed Monte Carlo code slaf, and compare to observations of diffuse Ly α haloes around LBGs. Our fiducial galactic outflow model comes closer to reproducing currently observed characteristics of the CGM in Ly α than a reference inefficient feedback model used for comparison. Nevertheless, our fiducial model still struggles to reproduce the observed data of the inner CGM (at impact parameter b < 30 kpc). Our results suggest that galactic outflows affect Ly α absorption and emission around galaxies mostly at impact parameters b < 50 kpc, while cold accretion flows dominate at larger distances. We discuss the implications of this result, and underline the potential constraining power of CGM observations – in emission and absorption – on galactic outflow models. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society