Properties of Galactic Outflows Driven by Starburst at Cosmic Noon: Insights from Hydrodynamical Simulations

We investigate starburst-driven galactic outflows in low-mass galaxies ($9.0 < \log(M_/M_\odot) < 10.0$) at cosmic noon using high-resolution 3D hydrodynamical simulations based on a framework that can reproduce the multiphase outflows in M82. The simulations produce starbursts lasting 20-30 Myr, with peak star formation rates of 2-68 M$\odot ,\rm{yr}^{-1}$. Outflow properties vary strongly with time, radial distance to galaxy center, stellar mass, and gas fraction, exhibiting velocities of 50-1000 $,\rm{km,s}^{-1}$, mass outflow rates of 0.3-20 M$\odot ,\rm{yr}^{-1}$, and mass loading factors, $η_\mathrm{M}$, of 0.24-6.26. The cool phase ($8000 < T \le 2 \times 10^4$ K) dominates the outflow, and properties of the cool and warm phases are broadly consistent with observations. At $M_= 10^{9.5},M_\odot$, average $η_\mathrm{M}$ for the total, cool, and warm phases are $\sim$1.2, 0.75, and 0.25, respectively. The mass loading factor decreases with increasing galaxy stellar mass, but increases with star formation rate. Given strong temporal and spatial evolution, scaling slopes from limited samples should be treated with caution. Our total $η_\mathrm{M}$ values are higher than FIRE-2 by 0.06 dex but lower than EAGLE and TNG50 by 0.50 and 0.84 dex. Accounting for methodological differences in outflow measurement reduces these gaps to 0.2-0.4 dex, suggesting that part of the discrepancy between observations and simulations reported in the literature may arise from inconsistent definitions and measurement methods, though differences in individual phases persist. Larger observational and simulation samples, together with consistent methods for measuring outflow properties, are required to draw robust conclusions about the scaling relations of galactic outflows.

💡 Research Summary

This paper presents a systematic investigation of starburst‑driven galactic outflows in low‑mass galaxies (stellar masses 10⁹–10¹⁰ M☉) at the epoch of peak cosmic star formation (z ≈ 1–2). Building on a simulation framework that successfully reproduces the multiphase wind of the nearby starburst galaxy M82, the authors run a suite of 14 high‑resolution, three‑dimensional hydrodynamical simulations of isolated disk galaxies with gas fractions ranging from 30 % to 85 %. The simulations are performed with the Athena++ code, include static dark‑matter, stellar disk and bulge potentials, self‑gravity of the gas, metal‑dependent radiative cooling/heating (via the Grackle library), and a sink‑particle model for star formation and stellar feedback (supernovae, radiation pressure, etc.). Metal enrichment is tracked with passive scalars, and the initial metallicity is set to 0.02 Z☉.

Starbursts in the models last 20–30 Myr and reach peak star‑formation rates (SFRs) of 2–68 M☉ yr⁻¹. The resulting winds are highly time‑variable and show strong radial dependence. The authors separate the outflow into three temperature phases: cool (8 × 10³–2 × 10⁴ K), warm (2 × 10⁴–5 × 10⁵ K), and hot (>5 × 10⁵ K). The cool phase dominates the mass budget, carrying 60–80 % of the total outflow mass, while the hot phase carries most of the kinetic and thermal energy.

Key quantitative results include:

• Outflow velocities ranging from 50 km s⁻¹ near the galaxy centre to >1 000 km s⁻¹ at several kiloparsecs.
• Mass‑outflow rates of 0.3–20 M☉ yr⁻¹, peaking during the starburst.
• Mass‑loading factors (η_M ≡ Ṁ_out/SFR) spanning 0.24–6.26, with an average η_M ≈ 1.2 for a galaxy of M_* = 10⁹·⁵ M☉. Phase‑specific loading factors are ≈ 0.75 (cool) and ≈ 0.25 (warm); the hot phase contributes negligibly to mass loading.
• A clear trend of decreasing η_M with increasing stellar mass (η_M ∝ M_*^{-0.3 – ‑0.5}) and increasing η_M with higher SFR (η_M ∝ SFR^{0.4 – 0.6}). However, the authors emphasize that these scaling relations are highly sensitive to the specific time snapshot and radial shell considered, cautioning against over‑interpretation of limited observational samples.

When compared with large‑scale cosmological simulations, the authors find that their total η_M values are 0.06 dex higher than those reported by FIRE‑2, but 0.5 dex (EAGLE) and 0.84 dex (TNG50) lower. By re‑measuring outflows in the comparison simulations using the same velocity cuts, binding‑energy criteria, and geometric definitions employed here, the discrepancies shrink to 0.2–0.4 dex. This exercise demonstrates that a substantial part of the long‑standing tension between observations and simulations may arise from inconsistent outflow definitions rather than purely physical model differences, although phase‑by‑phase discrepancies remain.

The paper concludes that high‑resolution, idealized simulations are essential for dissecting the physics of multiphase winds: the clustering of supernovae, the inhomogeneous ISM, and the spatial distribution of star formation all critically shape wind properties. The authors advocate for (1) larger, more diverse simulation suites that include cosmological inflows and mergers, (2) coordinated observational campaigns that adopt uniform outflow measurement protocols, and (3) systematic cross‑comparison of simulation outputs using standardized definitions. Such efforts will be crucial for refining feedback prescriptions in galaxy formation models and for achieving a coherent picture of how starburst‑driven winds regulate galaxy evolution during the cosmic noon epoch.