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ABSTRACT JWST has revealed a large population of UV-bright galaxies at $$z\gtrsim 10$$ and possibly overly massive galaxies at $$z\gtrsim 7$$, challenging standard galaxy formation models in the ΛCDM cosmology. We use an empirical galaxy formation model to explore the potential of alleviating these tensions through an Early Dark Energy (EDE) model, originally proposed to solve the Hubble tension. Our benchmark model demonstrates excellent agreement with the UV luminosity functions (UVLFs) at $$4\lesssim z \lesssim 10$$ in both ΛCDM and EDE cosmologies. In the EDE cosmology, the UVLF measurements at $$z\simeq 12$$ based on spectroscopically confirmed galaxies (eight galaxies at $$z\simeq 11\!-\!13.5$$) exhibit no tension with the benchmark model. Photometric constraints at $$12 \lesssim z\lesssim 16$$ can be fully explained within EDE via either moderately increased star-formation efficiencies ($$\epsilon _{\ast}\sim 3\!-\!10\ \hbox{per cent}$$ at $$M_{\rm halo}\sim 10^{10.5}{\, \rm M_\odot }$$) or enhanced UV variabilities ($$\sigma _{\rm UV}\sim 0.8\!-\!1.3$$ mag at $$M_{\rm halo}\sim 10^{10.5}{\, \rm M_\odot }$$) that are within the scatter of hydrodynamical simulation predictions. A similar agreement is difficult to achieve in $$\Lambda$$CDM, especially at $$z\gtrsim 14$$, where the required $$\sigma _{\rm UV}$$ exceeds the maximum value seen in simulations. Furthermore, the implausibly large cosmic stellar mass densities inferred from some JWST observations are no longer in tension with cosmology when the EDE is considered. Our findings highlight EDE as an intriguing unified solution to a fundamental problem in cosmology and the recent tensions raised by JWST observations. Data at the highest redshifts reached by JWST will be crucial for differentiating modified galaxy formation physics from new cosmological physics.
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The impact of UV variability on the abundance of bright galaxies at z ≥ 9
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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- PAR ID:
- 10452340
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 525
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 3254-3261
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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