The Scatter of the Many Outweighs the Scatter of the Few: Systematic Error Asymmetry in Steeply-Falling Mass Functions for High-Redshift JWST Galaxies

TL;DR

The paper addresses whether JWST-detected massive high-redshift galaxies challenge ΛCDM by linking the linear power spectrum to inferred star-formation efficiencies via abundance matching. It extends the framework to include random sample variance, asymmetric scatter from the steep high-mass halo tail, and systematic uncertainties in SED-based stellar masses, showing that systematics can move inferred efficiencies toward ΛCDM expectations. The analysis finds a central quadrature-averaged efficiency of $\epsilon_{\mathrm{avg}} = 0.018 \pm 0.004$, with most objects remaining compatible with $\epsilon \lesssim 0.4$, suggesting no need for new physics given present uncertainties. The work provides an openly available JWSTEG framework to test future detections and emphasizes that spectroscopic confirmations can tighten systematic errors, potentially further validating ΛCDM at the earliest epochs.

Abstract

The discovery of massive, high redshift galaxies with JWST has been argued to challenge $Λ$CDM: such systems would require extremely rare halos and baryon-to-stellar-mass conversion efficiencies unphysically approaching--or exceeding--100%. If confirmed at galaxy formation forbidden efficiencies, these galaxies could signal new physics beyond standard cosmological structure formation. We develop a galaxy model framework that ties the linear power spectrum to the inferred efficiencies of galaxy growth in order to test the structure formation models. In addition, we incorporate multiple sources of error, including (i) observational sample variance, (ii) asymmetric scatter induced by the steepness of the high-mass halo tail, and (iii) systematic uncertainties in stellar mass estimates. We find that the inferred efficiency of star formation is dominated by systematic uncertainties on the spectral energy distribution inferred stellar mass of the JWST detected galaxies. The systematic uncertainty augments the asymmetry in scatter that largely brings the inferred efficiencies to be in line with that expected from early galaxy formation models. Our framework can be used to test $Λ$CDM as errors are reduced and further detections are made.

Figures (5)

• Figure 1: Comparison of halo mass functions at redshifts 10 (solid lines) and 25 (dashed lines) Sheth:1999mnWarren:2005eyTinker:2008ffReed:2006rwYung:2025ttv. Among these, the Reed and Yung HMFs are calibrated for use at these $z$. There are clear significant differences of the HMFs at smaller masses for $z=10$. However, Reed et al. & Yung et al. are not very discrepant at the high-$M$ tail, especially when converting to cumulative comoving stellar mass density (see Fig. \ref{['fig:MBK Reed vs Yung and sample var']}).
• Figure 2: A massive Labbé et al. galaxy at $z = 10.4$ shown with cumulative comoving stellar-mass density curves computed using the Reed et al. and Yung et al. HMFs. Solid curves correspond to a baryon-to-stellar-mass conversion efficiency of $\epsilon = 1$, while dashed curves show $\epsilon = 0.5$. Random errors for the galaxy are shown in gray, and random plus sample-variance errors in black. At the high-mass, high-$z$ regime probed by JWST, these HMFs yield only minor differences, and we adopt Yung et al. for subsequent calculations.
• Figure 3: Systematic uncertainties shown as two sets of curves. The purple stellar-mass density curves are computed using Eq. (\ref{['eq:rhostarLH']}) and shift upward relative to Fig. \ref{['fig:MBK Reed vs Yung and sample var']} after we account for asymmetry in the random scatter. Error bars on the galaxy point represent random errors plus sample variance. The orange curves indicate the upper (dark orange) and lower (light orange) systematic bounds when inferring the efficiency associated with the galaxy. See text for further details. Comparing the galaxy’s central value with these curves shows that the total uncertainty is dominated by systematics.
• Figure 4: Efficiency versus galaxy mass for our sample of 31 galaxies from Table \ref{['tab:gal data']}. The solid horizontal line marks a $100\%$ conversion efficiency, and the dashed line indicates $50\%$. Random errors are shown in gray, and random plus sample variance errors are shown in black. For nearly all of the galaxies, the contribution from sample variance is minimal.
• Figure 5: Efficiency versus galaxy mass (left) and efficiency versus $z$ (right) for the 31 galaxies from Table \ref{['tab:gal data']}. Purple error bars represent random errors plus sample variance, while orange error bars indicate systematic uncertainties. Dashed orange bars correspond to galaxies for which the systematic uncertainty was estimated. As described in the text, we calculate the inferred average $\epsilon_\mathrm{avg} = 0.018 \pm 0.004$, shown as a dotted line.