Stress Testing $Λ$CDM with High-redshift Galaxy Candidates (2208.01611v2)

Abstract: Early data from JWST have revealed a bevy of high-redshift galaxy candidates with unexpectedly high stellar masses. An immediate concern is the consistency of these candidates with galaxy formation in the standard cosmological model. In the $\Lambda$CDM paradigm, the stellar mass ($M_\star$) of a galaxy is limited by the available baryonic reservoir of its host dark matter halo. The mass function of dark matter halos therefore imposes an absolute upper limit on the number density $n(>M_\star,z)$ and stellar mass density $\rho_{\star}(>M_\star,z)$ of galaxies more massive than $M_\star$ at any epoch $z$. Here I show that the most massive galaxy candidates in JWST observations at $z\sim 7-10$ lie at the very edge of these limits, indicating an important unresolved issue with the properties of galaxies derived from the observations, how galaxies form at early times in $\Lambda$CDM, or within this standard cosmology itself.

Examining the Constraints on Galaxies in a ΛCDM Universe with High-Redshift Candidates

The paper authored by Michael Boylan-Kolchin explores the tension between the observed properties of high-redshift galaxy candidates detected by the James Webb Space Telescope (JWST) and the predictions of galaxy formation models within the framework of Lambda Cold Dark Matter (ΛCDM) cosmology. The unexpected detection of high-redshift galaxies with extraordinary stellar masses challenges the assumptions about the baryonic and dark matter content in the ΛCDM paradigm.

Key Findings

The ΛCDM model presupposes that dark matter and baryons are thoroughly mixed at early times, with baryons collapsing into dark matter halos over time. The stellar mass of a galaxy within this framework is constrained by the available baryonic mass of its host dark matter halo, dictated by the cosmic baryon fraction. Consequently, the mass function of dark matter halos establishes limits on both the number density and stellar mass density of galaxies of a given mass at any cosmic epoch.

Boylan-Kolchin employs the Planck 2020 cosmological parameters and corresponding dark matter halo mass functions to examine the compatibility of the ΛCDM model with observations of galaxies at redshifts $z \sim 7-10$. He argues that if galaxies possess stellar contents beyond their theoretical baryonic allotment at these redshifts, it would imply either discrepancies in the JWST data analysis, assumptions in ΛCDM, or both.

Implications of Mass Limits and Stellar Efficiencies

The paper's analysis demonstrates that the high-redshift candidates identified by JWST lie at the extremity or even surpass the upper limits on stellar mass density derived from ΛCDM model predictions. For the most massive galaxies at redshifts $z \approx 7.5$ and $z \approx 9.1$, it implies improbably high star formation efficiencies greater than 0.57. Such high efficiencies are unprecedented in known astrophysical conditions, indicating either potential issues in observational interpretations or the theoretical constraints of the ΛCDM framework.

The implication of these findings is significant. The discovery of galaxies with such high stellar mass densities challenges the existing understanding of cosmic structures, possibly suggesting that high-redshift galaxy formation is more efficient than previously believed. If galaxies can form with such efficiencies, it necessitates a reevaluation of early star formation models or even modifications to existing cosmological models.

Future Perspectives and Speculations

To resolve these discrepancies, it is crucial to validate the observational data through further JWST observations, employing spectroscopy and additional photometric studies to confirm the mass estimates and redshift identifications. Should the existence of these massive galaxies at high redshifts be confirmed, it may prompt a reevaluation of the ΛCDM model, potentially involving revisions to account for enhanced star formation efficiencies or considering alternatives like early structure formation scenarios linked to Early Dark Energy models.

The current paper lays the groundwork for a robust discussion on cosmological parameters and the behavior of baryonic matter at early times. As future JWST data is analyzed, the community will gain a more comprehensive understanding of the implications on galaxy formation theories, which could lead to paradigmatic shifts in our understanding of the early universe and the forces shaping it.