Feedback and dynamical masses in high- <i>z</i> 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 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 James Webb Space Telescope Near InfraRed Spectrograph (NIRSpec) observations. From the velocity dispersion of the (resolved) emission lines ($\sigma _{\rm gas}{\rm ~(km~s^{-1})}\simeq 38-96$) and the galaxy size ($r_e=400-960~$pc), we estimate dynamical masses of $\log M_{\rm dyn}/{\rm M}_{\odot }=9.25-10.25$. Stellar-to-dynamical mass ratios are low ($\log M_{\star }/M_{\rm dyn}\in [-0.5,-2]$) and decrease with increasing star formation rate surface density ($\Sigma _{\rm SFR}$). We estimate gas surface densities assuming a star formation law, but the gas masses do not balance the baryon-to-dynamical mass ratios, requiring a lower star formation efficiency. Evidence of ionized outflows is found in five galaxies, based on broad components reproducing the emission-line wings. We only observe outflows from galaxies undergoing recent bursts of star formation ${\rm SFR_{10}/SFR_{100}\ge 1}$, with elevated $\Sigma _{\rm SFR}$ and low $M_{\star }/M_{\rm dyn}$. This links high gas surface densities to increased outflow incidence and lower $M_{\star }/M_{\rm dyn}$. With moderate outflow velocities ($v_{\rm flow}{\rm ~(km~s^{-1})}=150-250$) and mass outflow rates ($\dot{M}_{\rm flow}/{\rm {\rm M}_{\odot } yr^{-1}}=0.2-5$), these high-redshift galaxies appear more efficient at removing baryons than low-redshift galaxies with similar $M_{\star }$, showing mass-loading factors of $\dot{M}_{\rm flow}/{\rm SFR}=0.04-0.4$. For their dynamical mass, outflow velocities exceed the escape velocities, meaning they may eventually enrich the circumgalactic medium.