There and back again: Neutral outflows in z ∼ 3.5 quiescent galaxies

Neutral gas outflows play a crucial role in the baryon cycle of galaxies, regulating their evolution by removing gas and redistributing energy and momentum into the surrounding medium. Their properties provide key insights into the transition from star formation to quiescence. In this work, we investigate the neutral gas outflow properties of 23 massive ( M ★ = 10 10.1 − 11.6 M ⊙ ) quiescent galaxies (QGs) at z = 2.82–4.61, selected from the JWST NIRSpec (R ∼ 1000) spectroscopic and NIRCam imaging program DeepDive . We traced neutral gas outflows using the Na I doublet absorption lines and detected excess Na I D absorption in 13/23 (57%) targets. Of these, 7/23 (30%) displayed blueshifted excess absorption with velocity offsets |Δ v |≳150 km s −1 . The z ∼ 3.5 targets exhibited velocity offsets similar to those of their local massive quiescent counterparts and they are also in agreement with comparisons in the SFR–Δ v space. We derived mass outflow rates and identified, in particular, the most extreme neutral gas outflow rate log( Ṁ out /M ⊙ yr −1 ) = 2.68 ± 0.27 ever reported beyond the Local Universe, coincident with an X-ray active galactic nucleus (AGN). For all Na I D-detected systems, the inferred mass outflow rate could, in principle, suppress ongoing star formation (i.e., SFR ≤ Ṁ out ). However, the outflows are still unlikely to escape their hosts; in fact, they are suggestive of fountain-like recycling on relatively short timescales (∼3–180 Myr), depending on the assumed potential and launching radius. All Na I D-detected targets occupy the LI(N)ER region of the BPT diagram and/or are X-ray-detected. Still, we found no strong correlation between ongoing AGN activity and the neutral outflow: 2/4 broad-line/X-ray AGNs were not Na I D-detected. However, the outflows could be powered by fossil or episodic AGNs and one broad-line target shows a possible P-Cygni profile that would indicate strong outflows. Since, on average, neutral outflows alone are not able to permanently quench star formation by removing gas in our sample at z ∼ 3.5, the presence of gas cycling in and out of massive passive systems could be interpreted as a signature of feedback-regulated quenching-maintenance processes.