A Census of Na D–Traced Neutral Interstellar Medium and Outflows at 0.6 < z < 4

Abstract We present a statistical census of the Na D–traced neutral interstellar medium (ISM) and outflows in 309 galaxies at 0.6 < z < 4 using JWST/NIRSpec medium-resolution spectroscopy from the SMILES, JADES, Blue Jay, and Aurora surveys. After subtracting the stellar continuum, we model Na D λλ 5890, 5896 and identify 73 Na D ISM absorptions, including 63 robust (4 in broad-line active galactic nuclei, AGN) and 10 tentative detections. Of the robust detections, 88% are found in massive galaxies ( log ( M * / M ⊙ ) > 10 ), and 12% are in lower-mass systems. At high mass, ISM absorption is seen in both star-forming and quiescent galaxies, whereas in lower-mass systems, it is observed only in star-forming galaxies. In massive quiescent galaxies, Na D detectability appears linked to star formation history: it is preferentially detected in older systems with larger 4000 Å breaks and younger, rapidly quenching galaxies with strong Balmer absorption H δ A . We identify Na D outflows in 25 galaxies, revealing a possible dichotomy in driving mechanisms between star-forming and quiescent galaxies. In star-forming galaxies, outflow properties correlate with star formation properties, consistent with a star-formation-driven origin. In quiescent galaxies, however, outflows are not associated with residual star formation and often require more energy than such star formation can provide. Together with the high AGN fraction among outflow-detected quiescent galaxies, this suggests that AGN dominate Na D–traced neutral outflows in cosmic noon quiescent systems. We further identify four quiescent galaxies with possible AGN fossil outflows, suggesting that AGN-driven outflows can persist beyond the active accretion phase and may help maintain quiescence.