Feedback and dynamical masses in high-$z$ galaxies: the advent of high-resolution NIRSpec spectroscopy

Abstract

Stellar feedback is an essential step in the baryon cycle of galaxies, but it remains unconstrained beyond Cosmic Noon. We study the ionized gas kinematics, dynamical mass and gas-flow properties of a sample of 16 sub-$L^{\star }$ star-forming galaxies at $4\le z\le 7.6$, using high-resolution JWST/NIRSpec observations. The emission lines are resolved, with velocity dispersions (${\sigma }_{\mathrm{gas}}(\mathrm{km}{\mathrm{s}}^{-1})\simeq 38-96$) comparable to more massive galaxies at Cosmic Noon. From ${\sigma }_{\mathrm{gas}}$ and the galaxy size ($r_e=400-960$pc), we estimate the dynamical mass to be $\log M_{\mathrm{dyn}}/M_{\odot }=9.25-10.25$. Stellar-to-dynamical mass ratios are low ($\log M_{\star }/M_{\mathrm{dyn}}\in [-0.5,-2]$) and decrease with increasing SFR surface density (${\Sigma }_{\mathrm{SFR}}$). We estimate the gas surface densities assuming a star-formation law, but the gas masses do not balance the baryon-to-dynamical mass ratios, which would require a decrease in the star-formation efficiency. We find evidence of ionized outflows in five out of the sixteen galaxies, based on the need of broad components to reproduce the emission-line wings. We only observe outflows from galaxies undergoing recent bursts of star formation $\mathrm{SF}{\mathrm{R}}_{10}/SF{\mathrm{R}}_{100}\ge 1$, with elevated ${\Sigma }_{\mathrm{SFR}}$ and low $M_{\star }/M_{\mathrm{dyn}}$. This links high gas surface densities to increased outflow incidence and lower $M_{\star }/M_{\mathrm{dyn}}$. With moderate outflow velocities ($v_{\mathrm{flow}}(\mathrm{km}{\mathrm{s}}^{-1})=150-250$) and mass outflow rates (${\dot{M}}_{\mathrm{flow}}/{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}=0.2-5$), these high-redshift galaxies appear more efficient at removing baryons than low-redshift galaxies with similar $M_{\star }$, showing mass loading-factors of ${\dot{M}}_{\mathrm{flow}}/\mathrm{SFR}=0.04-0.4$. For their given dynamical mass, the outflow velocities exceed the escape velocities, meaning that they may eventually enrich the Circumgalactic Medium.

15 pages, 10 figures, 3 tables; accepted for publication in MNRAS on September 26, 2025

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