Feedback and dynamical masses in high- <i>z</i> galaxies: the advent of high-resolution NIRSpec spectroscopy

Abstract

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.

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