High-redshift galactic outflows: orientation effects, kinematics, and metallicity in TNG50 and SERRA

¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 50126, Pisa, Italy
² Dipartimento di Fisica “Enrico Fermi”, Universitá di Pisa, Largo Bruno Pontecorvo 3, Pisa, I-56127, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 December 2025
Accepted: 16 February 2026

Abstract

Context. Galactic outflows driven by central black holes and supernovae play a crucial role in the formation and evolution of galaxies. Recently, JWST/NIRSpec observations have provided the first detections of warm ionised outflows in low-mass (M_★ ∼ 10⁷ M_⊙) 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 theoretical models and 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 identified and characterised 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 analysed approximately 6 × 10⁴ galaxies from TNG50 and 3 × 10³ galaxies from SERRA over the redshift ranges z = 3 − 5 and z = 4 − 5, respectively, spanning stellar masses of M_★ = 10^7.5 − 10¹¹ M_⊙. Outflows in the immediate vicinity of each galaxy were identified using a Gaussian mixture model algorithm that uses the gas velocity, star formation rate, and location as input parameters. We subsequently compared 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 0.5 dex, 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 since outflows in face-on galaxies are approximately 15 percent more likely to be detected than in edge-on systems, with this difference increasing to nearly 40 percent for more massive, disc-shaped galaxies.