How do massive galaxies in the early Universe stop forming stars?

This project uses JWST/NIRSpec spectroscopy and NIRCam photometry to search for neutral gas outflows in galaxies at z = 2.0-5.0. By tracing Na I D absorption in stacked spectra, I investigate whether cool neutral gas flows are linked to the rapid quenching of massive galaxies during the epoch when they are actively transitioning from star-forming to quiescent systems.

• Constructed a galaxy sample at 2 < z < 5 from JWST/NIRSpec spectroscopy and NIRCam photometry, separating quiescent, intermediate, and star-forming populations using sSFR.
• Built matched spectroscopic-photometric datasets and performed SED fitting with Prospector to derive physical parameters and best-fit stellar continuum models.
• Subtracted stellar continuum models from observed spectra to isolate residual interstellar absorption features.
• Stacked residual spectra by galaxy type and redshift bin to enhance weak Na I D absorption signals that are difficult to detect in individual high-redshift spectra.
• Measured physical properties of neutral gas outflow from stacked absorption profiles to characterize neutral gas kinematics across different evolutionary states.

• Neutral-gas outflows are ubiquitous in quiescent galaxies at z = 2–5.
• Quiescent galaxies show stronger outflows than non-quiescent systems, reaching 310–570 km s⁻¹ and 9–30 M_☉ yr⁻¹.
• Their extreme mass-loading factors cannot be explained by residual star formation alone.
• AGN activity or fast shocks are plausible drivers, although the dominant mechanism remains uncertain.
• These galaxies are not passive, gas-exhausted systems; continued gas removal may help maintain their quiescence.

Massive galaxies in the early Universe appear to have assembled their stellar mass and stopped forming stars on surprisingly short timescales. JWST has revealed an unexpectedly abundant population of massive quiescent galaxies at z > 3, making it urgent to understand what physical mechanisms can suppress star formation so rapidly.

A key uncertainty is whether quenching is primarily driven by stellar feedback, AGN activity, heating of infalling gas, environmental effects, or some combination of these processes. Neutral gas outflows provide a direct observational window into this problem because they can remove or disturb the cold gas reservoir that fuels star formation.

The Na I D absorption doublet is especially useful for tracing cool neutral gas along the line of sight to the stellar continuum. However, at high redshift, the feature is often too weak to detect in individual galaxy spectra. This motivates a statistical stacking approach: by combining residual spectra across galaxies with similar evolutionary states, weak absorption signatures can be studied at the population level.

This project connects directly to my broader research goal of understanding massive-galaxy quenching as part of the baryon cycle. By comparing star-forming, intermediate, and quiescent systems, I aim to test how gas removal or redistribution may accompany the transition from active star formation to quiescence in early massive galaxies.