Bridging Theory and Observations: Insights into Star Formation Efficiency and Dust Attenuation in $z > 5$ Galaxies

Daisuke Toyouchi, Hidenobu Yajima, Andrea Ferrara, Kentaro Nagamine

First listed 2025-02-18 | Last updated 2025-02-18

Abstract

We investigate early galaxy evolution by modeling self-consistently their radially-resolved evolution of gas, stars, heavy elements, and dust content. Our model successfully reproduces various observed properties of JWST-identified galaxies at $z > 5$, including sizes, stellar masses, star formation rates (SFR), metallicities, and dust-to-stellar mass ratios. We show that the star formation efficiency (SFE), $f_\ast \equiv {\rm SFR}/(f_{\rm b} \dot{M}_{\rm h})$, is regulated by the global equilibrium between cosmological gas inflows, star formation, and gas outflows. Our model predicts $f_\ast \lesssim 20~\%$ for galaxies with halo masses of $M_{\rm h} \sim 10^{11-12}\, M_\odot$ down to $z = 5$, allowing them to reach intrinsic UV magnitudes of $M_{\rm UV} \lesssim -22~{\rm mag}$; when dust attenuation is ignored, the predicted UV luminosity function (LF) at $z \sim 12$ agrees well with observations. However, our model also suggests that these galaxies would be heavily obscured by dust, with high optical depths at 1500~Å~of $τ_{1500} \gtrsim 10$, causing the dust-attenuated UV LF to fall significantly below the observed one. This discrepancy highlights the need for mechanisms that mitigate strong dust attenuation, such as dust evacuation from star-forming regions and/or preferential production of large dust grains. Further exploration of these processes is essential for understanding the early stages of galaxy evolution.

Short digest

Radially resolved disk-evolution modeling links gas inflow, star formation, outflows, enrichment, and dust to reproduce z>5 galaxy sizes, stellar masses, SFRs, metallicities, and dust-to-stellar mass ratios. The model finds SFE f_* governed by inflow–SF–outflow equilibrium with f_* ≲ 20% for M_h ~10^11–10^12 Msun down to z=5, enabling intrinsic M_UV ≲ −22 and a dust-free z≈12 UV LF consistent with observations. Yet the same galaxies are predicted to be extremely opaque (τ_1500 ≳ 10), which would depress the UV LF below the observed counts. The tension motivates mechanisms that lower effective attenuation, such as dust evacuation from star-forming regions or preferentially large grains.

Key figures to inspect

• SFE versus halo mass and redshift: inspect how f_* peaks and saturates below ~20% for M_h ~10^11–10^12 Msun, illustrating the inflow–SF–outflow equilibrium that sets efficiencies.
• UV luminosity functions at z≈12: compare intrinsic and dust-attenuated LFs against JWST measurements to quantify by how much τ_1500-driven extinction underpredicts the bright end.
• Dust optical depth distributions: τ_1500 versus M_UV (or metallicity) showing the predicted heavy columns and where attenuation-free scenarios must intervene.
• Size–mass (and size–redshift) relations: check the modeled compact disks against JWST size measurements and note cases requiring reduced inflow angular-momentum extent.
• Dust-to-stellar mass ratio versus metallicity or M_*: verify consistency with observed dust budgets and assess how grain-size assumptions shift inferred opacities.