N(HI) in the local interstellar medium

Medium- to high-resolution UV absorption spectroscopy toward individual nearby stars is the primary method used, and incomplete spatial sampling of this complex medium makes a global map of the material difficult. Using HI column density measurements derived from HI Ly{alpha} spectroscopy toward 164 stars inside 100pc, we have generated 2D spatially interpolated N(HI) maps for different distance shells.

Based on the area-weighted sky averages, we find that sight lines inside 10pc typically have log_10_(N(HI)/cm^-2^)~17.9. For greater distance shells, log_10_(N(HI)/cm^-2^) increases to 18.3 (10-20pc), then to 18.4 (20-70pc), and finally to 18.6 (70-100pc). This last increase is likely associated with the detection of the Local Bubble boundary, thus making the plateau of column density from 20 to 70pc notable and suggestive of the rarity of warm LISM material beyond ~10-20pc.