Lyman α absorption beyond the disc of simulated spiral galaxies

We present an analysis of the origin and properties of the circumgalactic medium (CGM) in a suite of 11 cosmological zoom simulations resembling present-day spiral galaxies. On average the galaxies retain about 50 per cent of the cosmic fraction in baryons, almost equally divided into disc (interstellar medium) gas, cool CGM gas and warm-hot CGM gas. At radii smaller than 50 kpc the CGM is dominated by recycled warm-hot gas injected from the central galaxy, while at larger radii it is dominated by cool gas accreted on to the halo. The recycled gas typically accounts for one-third of the CGM mass. We introduce the novel publicly available analysis tool pygad to compute ion abundances and mock absorption spectra. For Lyman α absorption, we find good agreement of the simulated equivalent width (EW) distribution and observations out to large radii. Disc galaxies with quiescent assembly histories show significantly more absorption along the disc major axis. By comparing the EW and H i column densities, we find that CGM Lyman α absorbers are best represented by an effective line width b ≈ 50–70 km s−1 that increases mildly with halo mass, larger than typically assumed