$\Lambda$CDM not dead yet: massive high-z Balmer break galaxies are less common than previously reported

TL;DR

This study re-evaluates high-redshift galaxy candidates from early JWST data, finding they are less massive and less common than previously thought, supporting the continued validity of the standard $\ ext{Lambda}$CDM cosmological model.

Contribution

The paper provides a systematic analysis of double-break galaxy candidates, demonstrating that previous claims of massive, evolved high-redshift galaxies are likely due to star-forming galaxies with emission lines, reaffirming the standard cosmological paradigm.

Findings

High-redshift candidates are not exceptionally massive or rare.

Spectroscopy shows high-redshift sources are star-forming galaxies with emission lines.

Field-to-field variance affects stellar mass estimates, indicating sample bias.

Abstract

Early JWST observations that targeted so-called double-break sources (attributed to Lyman and Balmer breaks at $z>7$), reported a previously unknown population of very massive, evolved high-redshift galaxies. This surprising discovery led to a flurry of attempts to explain these objects' unexpected existence including invoking alternatives to the standard $\Lambda$CDM cosmological paradigm. To test these early results, we adopted the same double-break candidate galaxy selection criteria to search for such objects in the JWST images of the CAnadian NIRISS Unbiased Cluster Survey (CANUCS), and found a sample of 19 sources over five independent CANUCS fields that cover a total effective area of $\sim60\,$arcmin$^2$ at $z\sim8$. However, (1) our SED fits do not yield exceptionally high stellar masses for our candidates, while (2) spectroscopy of five of the candidates shows that while all five are at high redshifts, their red colours are due to high-EW emission lines in star-forming galaxies rather than Balmer breaks in massive, evolved systems. Additionally, (3) field-to-field variance leads to differences of $\sim 1.5$ dex in the maximum stellar masses measured in the different fields, suggesting that the early single-field JWST observations may have suffered from cosmic variance and/or sample bias. Finally, (4) we show that the presence of even a single massive outlier can dominate conclusions from small samples such as those in early JWST observations. In conclusion, we find that the double-break sources in CANUCS are not sufficiently massive or numerous to warrant questioning the standard $\Lambda$CDM paradigm.