The JWST EXCELS survey: outflows in 1.5 < z < 5 quiescent and recently quenched galaxies are likely relics from episodic AGN activity

We investigate the presence and origin of neutral gas outflows and inflows in 13 post-starburst and quiescent galaxies at redshifts 1.8 ≤ z ≤ 4.6⁠, using James Webb Space Telescope NIRSpec spectroscopy from the EXCELS survey. Na d absorption profiles reveal that 3 out of 13 exhibit blueshifted absorption indicative of outflows, and a further 2 objects show signs of inflowing gas. Outflow velocities range from ≈300 to 1200 km s⁻¹⁠, and we find gas flows are detected exclusively in objects that quenched < 600 Myr ago. We derive mass outflow rates over two orders of magnitude higher than current levels of star formation in our sample, indicating that the winds are unlikely to be driven by supernovae. The majority of the outflow sample have anomalously high energy and momentum outflow rates compared to those predicted for current levels of star formation or active galactic nucleus (AGN) activity. We conclude that we are likely observing fossil outflows driven by previous, more luminous AGN activity which has since faded. We then compare with the eagle simulation to explore a potential ‘outflow cycle’, finding that our observations are consistent with a model in which z ~ 3 quiescent galaxies undergo short ≃ 5 Myr periods of AGN activity strong enough to drive outflows, which occur every ≃ 40 Myr on average. This AGN activity drives observable outflows that persist for up to ≃ 10 Myr after the AGN fades, followed by a ≃ 20 Myr lull, and a subsequent short inflow, which eventually re-ignites AGN activity, and the cycle repeats.