Toward a 2D H <scp>I</scp> Map of the Local Interstellar Medium

Abstract Obtaining a complete census of gas in the local interstellar medium (LISM; &lt;100 pc) is challenging given the limited available tracers of the warm, partially ionized 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 H I column density measurements derived from H I Ly α spectroscopy toward 164 stars inside 100 pc, we have generated 2D spatially interpolated N (H I ) maps for different distance shells. Based on the area-weighted sky averages, we find that sight lines inside 10 pc typically have log 10 [ N (H I )/cm −2 ] ∼ 17.9. For greater distance shells, log 10 [ N (H I )/cm −2 ] increases to 18.3 (10–20 pc), then to 18.4 (20–70 pc), and finally to 18.6 (70–100 pc). This last increase is likely associated with the detection of the Local Bubble boundary, thus making the plateau of column density from 20 to 70 pc notable and suggestive of the rarity of warm LISM material beyond ∼10–20 pc. We estimate that the uncertainties associated with N (H I ) values inferred from the interpolated sky maps are approximately inversely correlated with the number of samples in each distance shell, and are in the range of 0.20–0.48 dex, compared to the 0.01–0.30 dex typically determined from direct Ly α observations. We discuss the impact of these uncertainties on interstellar medium corrections of extreme-UV and Ly α observations for nearby stars. Denser spatial sampling of the sky via UV absorption spectroscopy of nearby stars is required to improve the accuracy of these N (H I ) estimates.