[論文レビュー] The THESAN-ZOOM project: Star-formation efficiencies in high-redshift galaxies

Xuejian Shen, Rahul Kannan+11 more|ArXiv.org|Mar 3, 2025

Astronomy and Astrophysical Research被引用数 11

ひとこと要約

THESAN-ZOOMシミュレーションは高赤方偏移銀河におけるハロー規模および銀河規模の星形成効率を解析し、質量による二重べき関数のハローSFE、普遍的な Kennicutt–Schmidt関係、モデルバリエーションに対する頑健性を明らかにする。

ABSTRACT

Recent JWST observations hint at unexpectedly intense cosmic star-formation in the early Universe, often attributed to enhanced star-formation efficiencies (SFEs). Here, we analyze the SFE in THESAN-ZOOM, a novel zoom-in radiation-hydrodynamic simulation campaign of high-redshift ($z \gtrsim 3$) galaxies employing a state-of-the-art galaxy formation model resolving the multiphase interstellar medium (ISM). The halo-scale SFE ($ε^{\ast}_{ m halo}$) - the fraction of baryons accreted by a halo that are converted to stars - follows a double power-law dependence on halo mass, with a mild redshift evolution above $M_{ m halo} \gtrsim 10^{9.5}\,{ m M}_{\odot}$. The power-law slope is roughly $1/3$ at large halo masses, consistent with expectations when gas outflows are momentum-driven. At lower masses, the slope is roughly $2/3$ and is more aligned with the energy-driven outflow scenario. $ε^{\ast}_{ m halo}$ is a factor of $2-3$ larger than commonly assumed in empirical galaxy-formation models at $M_{ m halo} \lesssim 10^{11}\,{ m M}_{\odot}$. On galactic (kpc) scales, the Kennicutt-Schmidt (KS) relation of neutral gas is universal in THESAN-ZOOM, following $Σ_{ m SFR} \propto Σ_{ m gas}^2$, indicative of a turbulent energy balance in the ISM maintained by stellar feedback. The rise of $ε^{\ast}_{ m halo}$ with halo mass can be traced primarily to increasing gas surface densities in massive galaxies, while the underlying KS relation and neutral, star-forming gas fraction remain unchanged. Although the increase in $ε^{\ast}_{ m halo}$ with redshift is relatively modest, it is sufficient to explain the large observed number density of UV-bright galaxies at $z \gtrsim 12$. However, reproducing the brightest sources at $M_{ m UV} \lesssim -21$ may require extrapolating the SFE beyond the halo mass range directly covered by THESAN-ZOOM.

研究の動機と目的

• Quantify the halo-scale star-formation efficiency (SFE) as a function of halo mass and redshift in high-z galaxies.
• Characterize the Kennicutt–Schmidt relation for neutral gas on kpc scales within thesan-zoom galaxies.
• Assess the redshift evolution of halo-scale SFE and its impact on the observed abundance of UV-bright galaxies.
• Evaluate how different physical/numerical model variants affect galaxy formation outcomes in the simulations.

提案手法

• Use thesan-zoom zoom-in radiation-hydrodynamic simulations with AREPO and a multi-band radiation transfer (RT) scheme.
• Model star formation in dense, self-gravitating gas with a cell-level SFE per free-fall time (epsilon_ff) and stochastic star particle creation.
• Include explicit stellar feedback channels (SNe, winds, ESF) and radiative feedback via a non-equilibrium thermochemical network.
• Implement seven radiation bins and self-consistent, spatially inhomogeneous external radiation fields during reionization.
• Measure halo-scale SFE as epsilon_halo* = SFR / (f_b * M_dot_halo) over dynamical times to map to the stellar-to-halo-mass relation, and analyze KS relation Sigma_SFR ∝ Sigma_gas^2.

実験結果

リサーチクエスチョン

• RQ1 What is the functional form of the halo-scale SFE as a function of halo mass and redshift in high-z galaxies?
• RQ2 Is the Kennicutt–Schmidt relation universal for neutral gas in high-redshift, star-forming regions within thesan-zoom galaxies?
• RQ3 How do different physical implementations and numerical variants affect the stellar-to-halo-mass relation and SFE in the simulations?
• RQ4 Can the observed abundance of UV-bright galaxies at very high redshift be explained by the inferred halo-scale SFE evolution?

主な発見

• Halo-scale SFE follows a double power-law in halo mass with mild redshift evolution above M_halo ~ 10^9.5 M_sun.
• The high-mass slope is roughly 1/3, consistent with momentum-driven outflows; the low-mass slope is roughly 2/3, aligned with energy-driven outflows.
• epsilon_halo* is 2–3 times larger than commonly assumed in empirical models at M_halo < 10^11 M_sun.
• On kpc scales, the KS relation of neutral gas is universal in thesan-zoom with Sigma_SFR ∝ Sigma_gas^2, suggesting turbulent energy balance from stellar feedback.
• The rise of epsilon_halo* with halo mass is driven primarily by increasing gas surface densities in massive galaxies, while the KS relation and neutral gas fraction remain unchanged; results are robust to resolution and feedback recipe variations.