An SDSS-V 3D Tomographic Na I Map of the ISM: 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 degrees of the high-velocity Smith Cloud (SC) to trace Na I absorption and dust extinction as functions of distance. By fitting ISM-corrected MaStar templates to each spectrum, we isolate residual equivalent widths and extinction 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 Angstroms, >3sigma) 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 Cloud, and previous estimates of the SC place it at 12.4 +/- 1.3 kpc, further investigation of its distance is warranted.

Figures (13)

• Figure 1: H I map of the main body of the Smith Cloud at $(l,b)=(43^\circ,-18^\circ)$, integrated over $v_{\rm LSR}=80$--150 km s$^{-1}$ from the Green Bank Telescope (private comm.). The sources coincident with strong 21-cm emission (594 BOSS sightlines), are marked and colored by distance.
• Figure 2: Example best-fit MaStar template to an observed MWM stellar spectrum. The large panel on the right shows the full-spectrum comparison, with the observed BOSS spectrum in blue and the best-fit, reddened, and scaled MaStar template in orange. The two subpanels on the left zoom in on key absorption features: the Na I D doublet (top) and the Ca II K region (bottom), again showing the BOSS spectrum in blue and the MaStar template in orange. The vertical black dashed lines in the Ca II K panel indicate the window used for equivalent-width estimation of the Ca II K line.
• Figure 3: Distribution of $\chi^2_\nu$ versus $\psi$, as defined in Eq \ref{['psi_eq']}, for the fitted stellar sample. The left panel shows results for stars fitted using Na I-corrected MaStar templates, while the right panel shows the same for Ca II-corrected templates. Both samples exhibit a concentration towards $\chi^2_\nu\sim1$ and a modest preference towards $\psi\sim 1$, the upper bound allowed by our fitting procedure. The vertical dashed line denotes the $\chi^{2}_{\nu}$ cutoff of 1.
• Figure 4: Dust extinction map of fitted SDSS-V BOSS stellar spectra (as of early July 2025), colormapped to the A$_{V}$ obtained from fitting stellar templates to the data using a Fitzpatrick+1999 extinction model. The black dashed line denotes a declination of 0$^{\circ}$, separating the APO (North) and LCO (South) observations. The plane of the Milky Way, as well as large dusty complexes at higher latitudes are clearly visible.
• Figure 5: All-sky map of residual Na I absorption from SDSS-V BOSS spectra after fitting stellar templates and removing contributions from stellar atmospheres. The structure closely follows the $A_V$ map, consistent with Na I tracing cold neutral gas associated with dust.