JADES: The incidence rate and properties of galactic outflows in low-mass galaxies across 3 < z < 9

TL;DR

This study leverages JWST/NIRSpec R2700 spectroscopy to detect and characterize ionised galactic outflows in 52 low-mass galaxies ($M_\ o10^{6.7-9.2}\,M_\odot$) at $3<z<9$, marking the first census of such winds in the early universe. Using a rigorous two-Gaussian line-fitting approach on rest-frame optical lines, the authors derive outflow incidences, velocities, and mass-loading factors, and compare them with local dwarfs and cosmological simulations. They find a significant, but geometry- and S/N–dependent, incidence ($\sim25\%$) with median outflow velocity $\sim350\,\mathrm{km\,s^{-1}}$ and median mass-loading factor $\eta\approx2$, with outflows often exceeding the escape velocity and potentially enriching the CGM/IGM. The results imply strong, galaxy-scale feedback in the early low-mass population, contributing to regulation of star formation and metal-enrichment during the first 2 Gyr of cosmic time, while also highlighting uncertainties from multiphase gas and outflow geometry. JWST/NIRSpec opens a path to expanding this demographic across broader mass and redshift ranges and with spatially resolved spectroscopy.

Abstract

We investigate the incidence and properties of ionized gas outflows in a sample of 52 galaxies with stellar mass between $10^7$ M$_{\odot}$ and $10^9$ M$_{\odot}$ observed with ultra-deep JWST/NIRSpec MSA spectroscopy as part of the JWST Advanced Deep Extragalactic Survey (JADES). The high-spectral resolution (R2700) NIRSpec observations allowed us to identify for the first time the signature of outflows in the rest-frame optical nebular lines in low-mass galaxies at $z>3$. The incidence fraction of ionized outflows, traced by broad components, is about 25-40$\%$ depending on the intensity of the emission lines. The low incidence fraction might be due to both the sensitivity limit and the fact that outflows are not isotropic but have a limited opening angle which results in a detection only when this is directed toward our line of sight. Evidence for outflows increases slightly with stellar mass and star-formation rate. The median velocity and mass loading factor (i.e., the ratio between mass outflow rate and star formation rate) of the outflowing ionized gas are 350 km s$^{-1}$ and $η=2.0^{+1.6}_{-1.5}$, respectively. These are 1.5 and 100 times higher, respectively than the typical values observed in local dwarf galaxies. These outflows are able to escape the gravitational potential of the galaxy and enrich the circum-galactic medium and, potentially, the inter-galactic medium. Our results indicate that outflows can significantly impact the star formation activity in low-mass galaxies within the first 2 Gyr of the Universe.

Figures (14)

• Figure 1: Star formation rate vs. stellar mass ($M_\star$) for the 52 JADES galaxies at $3.5<z<8.5$. The selected galaxies are detected in [O$\rm~III$], H$\alpha$, or in the NIRSpec G395H and F290LP bands with a S/N $>$ 5. Targets detected in [O$\rm~III$] and H$\alpha$ are reported in blue and orange, respectively. Galaxies detected in both [O$\rm~III$] and H$\alpha$ are illustrated with circles. Half of the marks are shown in blue and the other half in orange. The right and upper panels show the number of targets in bins of SFR and $M_\star$, respectively.
• Figure 2: S/N of the overall nebular line profiles as a function of redshift. All the detected lines in our sample are reported as grey circles, while the emission lines with evidence of the broad component are indicated with blue ([O$\rm~III$]) and orange (H$\alpha$) squares. The dashed grey lines indicate [O$\rm~III$] and H$\alpha$ lines observed in the same galaxy. The top panels show the distribution of galaxies and the incidence of outflow in regular redshift bins. The right panels report the distribution of emission lines with S/N $>5$ and the incidence of broad components in irregular bins of S/N, respectively. Each irregular bin of S/N contains a similar number of lines, except for the last bin at the highest S/N values. The dashed red line marks an incidence of 25$\%$.
• Figure 3: Outflow detection rate vs. S/N of the overall line profile. The cumulative number of targets and detected outflows is reported in the top panel, and the incidence rate is illustrated in the bottom panel.
• Figure 4: Incidence of outflows as a function of host galaxy properties. The bottom panels from left to right report the detection rate as a function of stellar mass, star formation rate estimated over the last 10 Myr, and specific star formation rate. The binomial uncertainties are reported as error bars for each bin. The uncertainties on the galaxy properties are smaller than the bin sizes. The top panels illustrate the number of targets in each bin. Incidence values are not reported for bins with fewer than five targets.
• Figure 5: Cosmic evolution of ionised outflow velocities from rest-frame optical emission lines in low-mass ($M_\star<10^9~{\rm M_\odot}$ ) galaxies. The blue and orange squares indicate the outflows from JADES. The red star represents the median outflow velocity of our sample. The grey circles are the outflow velocities from a sample of local low-mass galaxies Marasco:2023 and re-estimated consistently with our definition (see details in the text). The larger empty circle is their median. The diamond shows a tentative outflow detection by stacking spectra of low-mass galaxies at $z=1.2-2.6$Concas:2022. The squares report the outflow velocities detected in four low-mass galaxies at $z\sim3$Llerena:2023. The dotted, dashed, and solid black lines are the predictions from the TNG50 simulation for three different stellar masses: $M_{\rm \star}=10^{8.5},10^{8}, and10^{7.5}~{\rm M_{\odot}}$Nelson:2019.