The simulated H I sky at low redshift

Context. Observations of intergalactic neutral hydrogen can provide a wealth of information about structure and galaxy formation, potentially tracing accretion and feedback processes on Mpc scales. Below a column density of N-HI similar to 10(19) cm(-2), the "edge" or typical observational limit for H I emission from galaxies, simulations predict a cosmic web of extended emission and filamentary structures. Current observations of this regime are limited by telescope sensitivity, which will soon advance substantially.Aims. We study the distribution of neutral hydrogen and its 21-cm emission properties in a cosmological hydrodynamic simulation, to gain more insight into the distribution of H I below N-HI similar to 10(19) cm(-2). Such Lyman limit systems are expected to trace out the cosmic web and are relatively unexplored.Methods. Beginning with a 32 h(-1) Mpc simulation, we extract the neutral hydrogen component by determining the neutral fraction, including a post-processed correction for self-shielding based on the thermal pressure. We take molecular hydrogen into account, assuming an average density ratio Omega(H2)/Omega(HI) = 0.3 at z = 0. The statistical properties of the H I emission are compared with observations, to assess the reliability of the simulation. We then make predictions for upcoming surveys.Results. The simulated H I distribution robustly describes the full column density range between N-HI similar to 10(14) and N-HI similar to 10(21) cm(-2) and agrees very well with available measurements from observations. Furthermore there is good correspondence in the statistics when looking at the two-point correlation function and the H I mass function. The reconstructed maps are used to simulate observations of existing and future telescopes by adding noise and accounting for the sensitivity of the telescopes.Conclusions. The general agreement in statistical properties of H I suggests that neutral hydrogen, as modelled in this hydrodynamic simulation, is a fair representation of neutral hydrogen in the Universe. Our method can be applied to other simulations, to compare different models of accretion and feedback. Future H I observations will be able to probe the regions where galaxies connect to the cosmic web.