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

TL;DR

Using JWST/NIRSpec high-resolution spectroscopy, this study characterizes ionized-gas kinematics, dynamical masses, and gas flows in 16 sub-L* galaxies at z≈4–7.6 from the GO1871 program. Intrinsic gas velocity dispersions are $σ_{\rm gas} ≈ 38$–$96$ km s$^{-1}$, and dynamical masses span $\log M_{\rm dyn}/M_⊙ ≈ 9.25$–$10.25$, yielding low $M_\star/M_{\rm dyn}$ ratios ($\log M_\star/M_{\rm dyn} ≈ -0.5$ to $-2$) that anticorrelate with $Σ_{\rm SFR}$. Five galaxies show ionized outflows with $v_{\rm flow} ≈ 150$–$250$ km s$^{-1}$, $\dot{M}_{\rm flow} ≈ 0.2$–$5$ $M_⊙$ yr$^{-1}$, and $η ≈ 0.04$–$0.4$, with some winds exceeding the escape velocity for their dynamical masses, implying CGM enrichment; tentative inflow signatures are present in three cases. Overall, the results demonstrate efficient baryon cycling and feedback in early galaxies, enabled by JWST’s resolving power, with implications for galaxy assembly and CGM metal enrichment during the Epoch of Reionization.

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 $4\leq z\leq7.6$, using high-resolution JWST/NIRSpec observations. The emission lines are resolved, with velocity dispersions ($σ_{\rm gas}{\rm~(km~s^{-1})}\simeq38-96$) comparable to more massive galaxies at Cosmic Noon. From $σ_{\rm gas}$ and the galaxy size ($r_e=400-960~$pc), we estimate the dynamical mass to be $\log M_{\rm dyn}/M_{\odot}=9.25-10.25$. Stellar-to-dynamical mass ratios are low ($\log M_{\star}/M_{\rm dyn}\in[-0.5,-2]$) and decrease with increasing SFR surface density ($Σ_{\rm SFR}$). 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 ${\rm SFR_{10}/SFR_{100}\geq1}$, with elevated $Σ_{\rm SFR}$ 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 ($\dot{M}_{\rm flow}/{\rm M_{\odot} yr^{-1}}=0.2-5$), 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 given dynamical mass, the outflow velocities exceed the escape velocities, meaning that they may eventually enrich the Circumgalactic Medium.

Figures (10)

• Figure 1: Examples of NIRCam data and morphological modeling for GO1871 gas-flow candidates. For each galaxy the ID is given, and the left panels show $3\times 3$ arcsec$^{2}$ cutouts in the corresponding NIRCam band, with the white bar spanning 1 arcsec length. Sérsic or point-like best-fit models are shown in the right panels, using the pysersic code (see Sect. \ref{['sub:nircam']}). Yellow circles indicate the best-fit effective radius ($r_e$).
• Figure 2: Examples of NIRSpec spectra for GO1871 gas-flow candidates. Each panel contains the high-resolution G395H / F290LP 2D (top) and extracted 1D spectra (bottom), showing the[O iii]$\lambda\lambda$4960,5008 doublet (indicated by white and black vertical lines at the top and bottom of each panel, respectively). The red lines show the best simultaneous fits to both emission lines plus the continuum (see Sect. \ref{['sub:nirspec']}). The narrow, yellow Gaussians reproduce the static component of the ionized gas, and purple Gaussians model the gas-flows over the broad wings of the [O iii] lines. Best-fit values for the velocity dispersion of each component ($\sigma_{\rm narrow}, \sigma_{\rm broad}$), as well as the broad-to-narrow flux ratios ($F_{\rm B}/F_{\rm N}$) are labeled in the insets. Galaxy $1871-12$ has an associated fainter companion at 0.36 pMpc Gazagnes2025: two pairs of [O iii] doublets are clearly resolved in the NIRSpec spectra.
• Figure 3: Overview of physical properties for GO1871 galaxies. Hatched symbols indicate gas-flow candidates, and the IMBH candidate from Chisholm2024 is displayed with a spiral. Top-left: UV magnitude versus redshift. The dashed line shows the characteristic $M{\rm UV}$M_ UV$^*$ for $z=7$ galaxies Bouwens2021. Top-right: SFR (from H$\beta$) as a function of stellar mass, together with the star-forming main sequence at $z=7$ from Popesso2023. Bottom-left: rest-optical size-mass relation, as traced by the effective radius ($r_e$) in comparison with the Allen2025 best-fit relation at $5 \leq z \leq 6$ (dashed-dotted). Bottom-right: SFR surface-density (_ SFR $\Sigma_{\rm SFR}$) versus burstiness parameter (SFR$_{10}$/SFR$_{100}$), defined as the ratio between SFRs averaged over 10 and 100 Myr. The properties of GO1871 galaxies are representative of typical high-$z$ galaxies in the EoR.
• Figure 4: $O_{32}$ (a proxy of the ionization state) versus $R_{23}$. The blue diamond and square show the average for $z = 7-8$ galaxies from the CEERS Sanders2023 and JADES surveys Cameron2023. The grey 2D histogram in the background shows the density of SDSS galaxies at $z \simeq 0$Brinchmann2008. The space of parameters occupied by the compilation of extremely metal-poor galaxies (XMPs) from Izotov2024, is illustrated with a grey shaded band. With high ionization parameters and low metallicities, high-$z$ star-forming galaxies share the locus in this diagram with local analogs such as Green Pea, Bluberries and EELGs.
• Figure 5: Evolution of the ionized gas velocity dispersion ($\sigma_{\rm gas}$) with redshift. Open circles show the measurements from FS2018 at $z \simeq 2$, and the solid (dashed) line indicates the linear fit (extrapolation) by Ubler2019. Other NIRSpec studies (using [O iii] and H$\alpha$) at similar redshifts to this work are shown with hexagons deGraaff2024. Ionized gas in high-$z$ SFGs have comparable (or lower) velocity dispersions than Cosmic Noon galaxies.