Tracing Interstellar Gas in Quiescent Galaxies Using Na I D and Ca II H&K Absorption

For over two decades, Na I D (λ lambda5891.58, 5897.56 Å) absorption has been the primary optical tracer of cool, neutral gas in galaxies. In contrast, Ca II H&K (λλ 3934.78, 3969.59 Å), which traces both neutral and ionized gas, has been comparatively unexplored. Here, we present a comparative study of the Na I D and Ca II H&K interstellar absorption lines using spatially resolved MaNGA IFU spectroscopy of 140 red geyser galaxies (local quiescent systems hosting low-luminosity AGN with bisymmetric ionized outflows) and a control sample of 140 quiescent galaxies. Using Gaussian fits to the Na I D and Ca II K lines, we measure gas velocity and dispersion. We find that the Ca II reservoirs are up to 6 times larger in area, twice as radially extended, and have 1.7 - 1.8x broader line widths than Na I D. Furthermore, Na I D shows a direct dependence on dust reservoir size, but the correlation vanishes for Ca II. In only 20% of galaxies do the Na I D detections extend beyond dust regions, but this fraction jumps to 70% for Ca II. Using these findings, we argue that dust plays a significant role in shielding Na I D from ionizing radiation, whereas Ca II, with its higher ionization potential, is not as dependent on shielding and can more easily survive beyond regions of dust. Na I D and Ca II trace distinct gas components, and studying both together provides a more complete picture of gas flows in galaxies.