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 } \sim 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 21cm emission properties in a cosmological hydrodynamic simulation , to gain more insights into the distribution of H i below N _ { HI } \sim 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 into account molecular hydrogen , assuming an average density ratio \Omega _ { H _ { 2 } } / \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 } \sim 10 ^ { 14 } and N _ { HI } \sim 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 taking account of the sensitivity of the telescopes .

Conclusions : The general agreement in statistical properties of H i suggests that neutral hydrogen as modeled in this hydrodynamic simulation is a fair representation of that 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 .