JWST Spectra Conclusively Show an Excess of Neutral Gas Outflows in Quiescent Galaxies at z=2-5

Abstract: Galaxies exhibit a broad range of star formation activity, from actively star-forming to quiescent systems. Yet, the mechanisms responsible for the rapid shutdown and continued suppression of star formation remain poorly understood, particularly for the quiescent galaxies recently uncovered by JWST at $z&gt;3$. Neutral gas outflows provide a direct tracer of gas removal and regulation in quiescent galaxies. We therefore present a NaID $λ\lambda5891,5897$ absorption--line stacking analysis of 274 galaxies at $z=2$--5 drawn from the DAWN JWST Archive (DJA) and the EMBER JWST program, using JWST/NIRSpec medium-resolution grating spectroscopy and NIRCam photometry. NaID absorption is detected in all quiescent galaxy stacks spanning $z=2$--5, providing the first statistical evidence that neutral gas outflows are a ubiquitous feature of quiescent galaxies in the early Universe. Compared to the non-quiescent galaxy stacks, the quiescent stacks show mass loading factors higher by 2--4 dex, with mass outflow rates $\dot{M}<em>{\rm out} \sim 9$--$30\,M</em>\odot\,{\rm yr}<sup>{-1}$ elevated by $\sim0.5$--$1$ dex and outflow velocities $v_{\rm out} \sim 310$--$570\,{\rm km\,s}<sup>{-1}$ higher by a factor of $\sim2$. The extreme mass loading factor values and declining star formation histories strongly indicate that these outflows cannot be driven by current star formation alone. In the statistically reliable quiescent redshift bins at $2 \leq z &lt; 3$ and $3 \leq z &lt; 4$, the elevated $[\mathrm{N\,II}]/\mathrm{H}α$ and $[\mathrm{O\,III}]/\mathrm{H}β$ ratios are consistent with a possible AGN contribution, suggesting that non-stellar feedback may play a role in sustaining quiescence in galaxies at these epochs.