Article,
ΛCDM not dead yet: massive high-z Balmer break galaxies are less common than previously reported
Affiliations
- [1] Saint Mary's University [NORA names: Canada; America, North; OECD];
- [2] NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada [NORA names: Miscellaneous; Canada; America, North; OECD];
- [3] Kyoto University [NORA names: Japan; Asia, East; OECD];
- [4] Canada Research Chair. [NORA names: Miscellaneous];
- [5] York University [NORA names: Canada; America, North; OECD];
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Abstract
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.