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ABSTRACT JWST observations have revealed a population of galaxies bright enough that potentially challenge standard galaxy formation models in the Λ cold dark matter (ΛCDM) cosmology. Using a minimal empirical framework, we investigate the influence of variability on the rest-frame ultra-violet (UV) luminosity function of galaxies at z ≥ 9. Our study differentiates between the median UV radiation yield and the variability of UV luminosities of galaxies at a fixed dark matter halo mass. We primarily focus on the latter effect, which depends on halo assembly and galaxy formation processes and can significantly increase the abundance of UV-bright galaxies due to the upscatter of galaxies in lower-mass haloes. We find that a relatively low level of variability, σUV ≈ 0.75 mag, matches the observational constraints at z ≈ 9. However, increasingly larger σUV is necessary when moving to higher redshifts, reaching $$\sigma _{\rm UV} \approx 2.0\, (2.5)\, {\rm mag}$$ at z ≈ 12 (16). This implied variability is consistent with expectations of physical processes in high-redshift galaxies such as bursty star formation and dust clearance during strong feedback cycles. Photometric constraints from JWST at z ≳ 9 therefore can be reconciled with a standard ΛCDM-based galaxy formation model calibrated at lower redshifts without the need for adjustments to the median UV radiation yield.
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Balancing the efficiency and stochasticity of star formation with dust extinction in z ≳ 10 galaxies observed by JWST
ABSTRACT Early observations with JWST indicate an overabundance of bright galaxies at redshifts z ≳ 10 relative to Hubble-calibrated model predictions. More puzzling still is the apparent lack of evolution in the abundance of such objects between z ∼ 9 and the highest redshifts yet probed, z ∼ 13–17. In this study, we first show that, despite a poor match with JWST luminosity functions (LFs), semi-empirical models calibrated to rest-ultraviolet LFs and colours at 4 ≲ z ≲ 8 are largely consistent with constraints on the properties of individual JWST galaxies, including their stellar masses, ages, and spectral slopes. We then show that order-of-magnitude scatter in the star formation rate of galaxies (at fixed halo mass) can indeed boost the abundance of bright galaxies, provided that star formation is more efficient than expected in low-mass haloes. However, this solution to the abundance problem introduces tension elsewhere: because it relies on the upscattering of low-mass haloes into bright magnitude bins, one expects typical ages, masses, and spectral slopes to be much lower than constraints from galaxies observed thus far. This tension can be alleviated by non-negligible reddening, suggesting that – if the first batch of photometrically selected candidates are confirmed – star formation and dust production could be more efficient than expected in galaxies at z ≳ 10.
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- Award ID(s):
- 1812458
- PAR ID:
- 10486647
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 519
- Issue:
- 1
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 843 to 853
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mnras/stac3578
