Description
The NaI D λλ5891, 5897 doublet is a key tracer of the motions and gas content of the interstellar medium (ISM). It is one of the only strong transitions sensitive to cool (< 1000 K), neutral gas that is accessible at rest-frame optical wavelengths. Here we explore the spatially-resolved empirical relationship between NaI D absorption from the ISM, total dust extinction, and SFR in a large nearby galaxy sample for the first time. Our sample consists of 1,796 nearby spiral galaxies selected from the SDSS-IV/MaNGA integral field spectroscopic survey. For this analysis, we perform spectral fitting with three independent spectral libraries to estimate the contribution of stellar atmospheres to the total observed NaI D absorption strength. We demonstrate that spectral fits that rely on empirical stellar spectra (rather than theoretical spectra) systematically overestimate the NaI D absorption due to stars, and posit that this is likely due to contamination of these libraries by NaI-absorbing gas in the Milky Way. We investigate trends between the NaI D ISM equivalent width (EW), the dust attenuation measured toward HII regions (AV,gas) and from the stellar continua (AV,stars), and the local SFR density as a function of radius. Our exploration has revealed that (1) the global NaI D ISM EW is most tightly correlated with a galaxy’s median AV,gas, and exhibits weaker correlations with stellar mass and total SFR; (2) global NaI D ISM EW exhibits no significant correlation with total neutral hydrogen gas mass; and (3) local NaI D ISM EW exhibits a strong relationship with local AV,gas, and weaker correlations with local SFR surface density. Moreover, the slope and intercept of the AV,gas − NaI D ISM EW relationship remain consistent across all radii and stellar masses. Together, these results suggest that NaI D EW is a poor tracer of total neutral hydrogen content, and instead demonstrate that it arises primarily in cold, dust-enshrouded regions that are host to ongoing star formation.
