We investigate galaxy sizes at redshift $z\gtrsim 6$ with the cosmological radiation-magnetohydrodynamic simulation suite thesan(-hr). These simulations simultaneously capture reionization of the large-scale intergalactic medium and resolved galaxy properties. The intrinsic sizes ($r^{\ast }_{1/2}$) of simulated galaxies increase moderately with stellar mass at $M_{\ast } \lesssim 10^{8}{\, \rm M_\odot}$ and decrease fast at larger masses, resulting in a hump feature at $M_{\ast }\sim 10^{8}{\, \rm M_\odot}$ that is insensitive to redshift. Low-mass galaxies are in the initial phase of size growth and are better described by a spherical shell model with feedback-driven outflows competing with the cold inflowing gas streams. In contrast, massive galaxies fit better with the disc formation model. They generally experience a phase of rapid compaction and gas depletion, likely driven by internal disc instability rather than external processes. We identify four compact quenched galaxies in the $(95.5\, {\rm cMpc})^{3}$ volume of thesan-1 at $z\simeq 6$ and their quenching follows reaching a characteristic stellar surface density akin to the massive compact galaxies at cosmic noon. Compared to observations, we find that the median ultraviolet effective radius ($R^{\rm UV}_{\rm eff}$) of simulated galaxies is at least three times larger than the observed ones at $M_{\ast }\lesssim 10^{9}{\, \rm M_\odot}$ or $M_{\rm UV}\gtrsim -20$ at $6 \lesssim z \lesssim 10$. The population of compact galaxies ($R^{\rm UV}_{\rm eff}\lesssim 300\, {\rm pc}$) galaxies at $M_{\ast }\sim 10^{8}{\, \rm M_\odot}$ is missing in our simulations. This inconsistency persists across many other cosmological simulations with different galaxy formation models and demonstrates the potential of using galaxy morphology to constrain physics of galaxy formation at high redshifts.
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.
more » « less- PAR ID:
- 10541753
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 533
- Issue:
- 4
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 3923-3936
- Size(s):
- p. 3923-3936
- Sponsoring Org:
- National Science Foundation
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