Galactic outflows and star-formation quenching: a JWST perspective

Outflows driven by active galactic nuclei (AGN) are widely considered a key mechanism for regulating and quenching star formation in massive galaxies. However, direct observational evidence linking outflows to quenching has remained elusive for decades. This article reviews recent findings for massive galaxies at cosmic noon ( $z\sim 2$ ), based on sensitive James Webb Space Telescope (JWST) observations tracing both star formation histories (through Balmer absorption lines) and neutral outflows (through blueshifted Na i d absorption). Neutral outflows are found in $\sim 25\text{ - }50\%$ of massive ( $log(M_{\ast }/M_{\odot })>10$ ) galaxies, independent of star formation rate. Outflows from star-forming galaxies are consistent with being powered by star-formation and/or AGN activity, whereas outflows from quiescent systems are generally attributed to current or previous AGN activity. There is growing evidence that recent rapid quenching of star-formation is associated with powerful AGN-driven outflows with mass loading factors $\eta \gtrsim 1\text{ - }10$ . The neutral gas dominates the outflow mass budget but in most cases its velocity is below the halo escape velocity, indicating that this gas may form part of a galactic fountain. Detections of neutral gas inflows and residual cold gas reservoirs in quiescent galaxies suggest that other feedback mechanisms such as AGN heating are required to keep galaxies quenched over long timescales. We highlight the potential of multiphase outflow measurements to constrain feedback models, describe the main systematic uncertainties in mass outflow rate measurements, and discuss how the $z\sim 2$ results inform our understanding of the formation of the earliest massive quiescent galaxies.