High-Redshift Galactic Outflows: Orientation Effects, Kinematics, and Metallicity in TNG50 and SERRA

TL;DR

We address the mismatch between observed high-redshift outflow incidence and simulation predictions by comparing two complementary simulations, TNG50 and SERRA, with JWST/JADES data. The study analyzes about 6e4 galaxies from TNG50 (z ~ 3–6) and 3e3 SERRA galaxies (z ~ 4–6) across M* from 10^7.5 to 10^11 Msun, identifying near-galaxy outflows via a Gaussian mixture model in a five-dimensional feature space. Outflow masses broadly agree with JWST within about 0.5 dex; outflow velocities are typically an order of magnitude lower; metallicity ratios Zout/Zgal are about 0.52 in TNG50 and 0.16 in SERRA. Orientation strongly affects detectability in TNG50, with face-on systems more detectable by roughly 15% (up to 40% for disc-like, massive galaxies), while SERRA shows a weaker orientation signal due to merger-driven dynamics. These findings help reconcile JWST results with simulations, highlight biases in traceable gas via optical lines, and guide improvements in feedback modeling for the early universe.

Abstract

Context: Recently, JWST/NIRSpec observations have provided the first detections of warm ionised outflows in low-mass galaxies at high redshifts (z>3), revealing an occurrence rate of 25-40% depending on the intensity of the emission lines. This fraction is lower than predicted by simulations, which suggest that fast outflowing gas should be a common feature of all star-forming galaxies in the early Universe. Aims: In order to better understand the discrepancies between simulations and observations, we identify and characterize outflows in high-redshift galaxies using the TNG50 cosmological and SERRA zoom-in simulations. Our study examines how outflow detectability depends on the line of sight, explores the properties of the fast gas, and investigates its relationship with key galactic properties. Methods: We analyse approximately 60000 galaxies from TNG50 and 3000 galaxies from SERRA over the redshift ranges z=3-5 and z=4-5, respectively, spanning stellar masses of Mstar=10^7.5-10^11Msun. Outflows in the immediate vicinity of each galaxy are identified using a Gaussian mixture model algorithm that uses the gas velocity, star-formation-rate, and location as input parameters. We subsequently compare the simulated outflows to those observed in the JWST/JADES NIRSpec survey. Results: Outflow masses in both TNG50 and SERRA broadly reproduce the JWST/JADES measurements within roughly 0.5dex, though simulations tend to predict slightly higher values, suggesting that optical emission lines capture only a fraction of the multiphase outflow. However, simulated outflow velocities are typically an order of magnitude lower than those inferred from observations. TNG50 indicates a clear orientation dependence as outflows in face-on galaxies are approximately 15% more likely to be detected than in edge-on systems, with this difference increasing to nearly 40% for more massive, disc-shaped galaxies.

Figures (12)

• Figure 1: Number of galaxies as a function of stellar mass analysed in this work. The blue and orange histograms report the distribution for TNG50 and SERRA, respectively. The shaded region indicated the mass range of galaxies observed in JADES.
• Figure 2: Two sample galaxies from the TNG50 (left) and SERRA (right) simulations respectively, shown in edge-on and face-on view together with their corresponding radial velocity maps. From top to bottom, each column shows the full projected gas of the galaxy, the fraction of gas identified as outflows, and the fraction of gas associated with the galaxy itself.
• Figure 3: Mass of outflowing gas as a function of galactic stellar mass, measured within a radius of $0.6"$. The green contours represent the 1, 2, and 3$\sigma$ distribution of SERRA galaxies, while the black contours represent the same for TNG50 galaxies. Black stars represent galaxy observations from JADES.
• Figure 4: Star formation rate as a function of stellar mass. The color bar indicates the average mass of outflowing gas in each bin for the total outflowing gas of TNG50 (left) galaxies, the outflowing gas within the aperture of TNG50 galaxies (centre), and the corresponding values for SERRA galaxies (right). Overplotted stars represent galaxies observed by JADES, with their colours representing the outflow mass following the same scale as the respective panels.
• Figure 5: Velocity of the 80th percentile of outflowing gas as a function of stellar mass. Velocities are measured within a radius corresponding to an observed radius of $0.6"$ for TNG50 galaxies (left) and SERRA galaxies (right). The colour bar indicates the average velocity value in each bin. Stars represent galaxies observed by JADES, with colours following the same scale as the histograms.