An SDSS-V 3D Tomographic Na I Map of the Interstellar Medium: An Initial Study Towards the Smith Cloud

Abstract High velocity clouds supply the Milky Way with gas that sustains star formation over cosmic timescales. Precise distance measurements are therefore essential to quantify their mass inflow rates and gauge their exact contribution to the Galaxy’s gas supply. We use a sample of 594 SDSS-V BOSS stellar spectra within 10° of the high-velocity Smith Cloud (SC) to trace Na I absorption and dust extinction as functions of distance. By fitting interstellar-medium-corrected MaStar templates to each spectrum, we isolate residual equivalent widths and extinction and then compare trends in the SC region to a same-latitude control field. Stars beyond 1 kpc toward the SC exhibit a significant Na I equivalent width excess (>0.2 Å, >3 σ ) relative to the control. Two-component linear fits of Na I equivalent width and A V against both low and high-velocity H I column densities show that the low-velocity component is strongly correlated with both quantities, while the high-velocity term is marginally significant in extinction and Na I , consistent with a patchy, low dust-to-gas ratio. Given that the excess Na I begins at distances <2 kpc uniquely in the direction of the SC, and previous estimates of the SC place it at 12.4 ± 1.3 kpc, further investigation of its distance is warranted.