ΛCDM not dead yet: massive high-z Balmer break galaxies are less common than previously reported

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 Λ cold dark matter (ΛCDM) 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 Near Infrared Imager and Slitless Spectrograph (NIRISS) Unbiased Cluster Survey (CANUCS), and found a sample of 19 sources over five independent CANUCS fields that cover a total effective area of ~60 arcmin² at z ~ 8. However, (1) our spectral energy distribution 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-equivalent width emission lines in star-forming galaxies rather than Balmer breaks in massive, evolved systems. Additionally, (3) field-to-field variance leads to differences of ~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 ΛCDM paradigm.