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 dynamical mass and gas-flow properties of a sample of 16 sub-L^⋆ star-forming galaxies at 4 ≤ z ≤ 7.6, using high-resolution James Webb Space Telescope Near InfraRed Spectrograph (NIRSpec) observations. From the velocity dispersion of the (resolved) emission lines (σ_gas (km s^-1) ≃ 38 - 96) and the galaxy size (r_e = 400 - 960 pc), we estimate dynamical masses of log M_dyn / M_⊙ = 9.25 - 10.25. Stellar-to-dynamical mass ratios are low (log M_⋆ / M_dyn ∈ [-0.5, -2]) and decrease with increasing star formation rate surface density (Σ_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 SFR_10 / SFR_100 ≥ 1, with elevated Σ_SFR and low M_⋆ / M_dyn. This links high gas surface densities to increased outflow incidence and lower M_⋆ / M_dyn. With moderate outflow velocities (v_flow (km s^-1) = 150 - 250) and mass outflow rates (Ṁ_flow / M_⊙ yr^-1 = 0.2 - 5), these high-redshift galaxies appear more efficient at removing baryons than low-redshift galaxies with similar M_⋆, showing mass-loading factors of Ṁ_flow / SFR = 0.04 - 0.4. For their dynamical mass, outflow velocities exceed the escape velocities, meaning they may eventually enrich the circumgalactic medium.