Accelerating early massive galaxy formation with primordial black holes (2208.13178v2)

Abstract: Recent observations with JWST have identified several bright galaxy candidates at $z\gtrsim 10$, some of which appear unusually massive (up to $\sim 10^{11}\ \rm M_{\odot}$). Such early formation of massive galaxies is difficult to reconcile with standard $\Lambda\rm CDM$ predictions, demanding very high star formation efficiency (SFE), possibly even in excess of the cosmic baryon mass budget in collapsed structures. With an idealized analysis based on linear perturbation theory and the Press-Schechter formalism, we show that the observed massive galaxy candidates can be explained with lower SFE than required in $\Lambda\rm CDM$, if structure formation is accelerated/seeded by massive ($\gtrsim 10^{9}\ \rm M_{\odot}$) primordial black holes (PBHs) that make a up a small fraction ($\sim 10^{-6}-10^{-3}$) of dark matter, considering existing empirical constraints on PBH parameters. We also discuss the potential observational signatures of PBH cosmologies in the JWST era. More work needs to be done to fully evaluate the viability of such PBH models to explain observations of the high-$z$ Universe.

• The paper demonstrates that a minor fraction (10⁻⁶ to 10⁻³) of dark matter as primordial black holes can seed density fluctuations for early galaxy formation.
• The methodology employs linear perturbation theory with the Press-Schechter formalism to reconcile observations with Lambda-CDM predictions.
• The findings imply that incorporating primordial black holes may resolve discrepancies between JWST high-redshift observations and standard cosmological models.

Accelerating Early Massive Galaxy Formation with Primordial Black Holes

In the context of recent astronomical observations, particularly those garnered from the James Webb Space Telescope (JWST), the paper of early massive galaxy formation has encountered new challenges. The paper "Accelerating early massive galaxy formation with primordial black holes" by Liu and Bromm provides an intriguing exploration into this phenomenon, proposing that primordial black holes (PBHs) may play a critical role in reconciling these observations with cosmological models, specifically the Lambda Cold Dark Matter ($\Lambda$CDM) framework.

Observational Motivation and Theoretical Framework

The identification of surprisingly massive galaxies at high redshifts ($z \gtrsim 10$), some with inferred stellar masses up to $\sim 10^{11}\ \rm M_{\odot}$, poses a significant challenge to $\Lambda$CDM predictions. Traditional models require unrealistically high star formation efficiencies to account for such massive structures early in cosmic history. Liu and Bromm suggest that considering PBHs as a fraction of dark matter could provide an alternative solution by accelerating the structure formation process.

Utilizing linear perturbation theory combined with the Press-Schechter formalism, their analysis indicates that massive PBHs, contributing a small fraction ($\sim 10^{-6}$ to $10^{-3}$) of the dark matter, can enhance the power of density fluctuations, thus facilitating early galaxy formation with lower star formation efficiencies than previously thought necessary.

Key Findings and Numerical Insights

The analysis explores PBH parameters, suggesting that PBHs with masses $\gtrsim 10^{9}\ \rm M_{\odot}$ could explain certain high-redshift galaxy observations. The presence of these PBHs would serve to 'seed' structure formation through increased density fluctuations and isocurvature perturbations beyond the standard adiabatic modes of $\Lambda$CDM. The paper finds that the formation of early massive galaxies could occur under a regime where the fraction of PBHs aligns with astrophysical observations constraining $f_{\rm PBH}$ to be much lower than one.

The ramifications are profound—successful prediction of stellar mass densities at redshifts as high as $z \sim 10$ with significant compatibility with empirical data. The analysis demonstrates compatibility with existing constraints from cosmic microwave background (CMB) observations and other large-scale structure observations when non-Gaussianity assumptions are modestly relaxed.

Implications and Future Directions

The implications of this paper extend both practically and theoretically within cosmology and astrophysics. Practically, PBHs offer a potential resolution to the discrepancy between observed and theoretically predicted mass distributions of high-redshift galaxies. Furthermore, this model may impact the understanding of cosmic reionization and subsequent star formation histories by providing an accelerated timeline for galaxy formation.

Theoretically, this work invites further investigation into PBH cosmologies, encouraging a deeper exploration into non-standard dark matter models and their plausible roles in early universe scenarios. A key area for future research involves the evolution of PBHs and their observational signatures—particularly through gravitational lensing, X-ray and infrared background fluctuations, and upcoming gravitational wave observatories.

Moreover, as large-scale telescopes continue to deliver more detailed data on distant galaxies, coupled with advanced simulation techniques, the role of anomalous dark matter constituents such as PBHs could become clearer. This paper lays groundwork for these future developments by offering a robust argument for the reconsideration of PBHs in our cosmological models.

In summary, Liu and Bromm's paper is an insightful contribution towards understanding early universe phenomena. It opens up new pathways in cosmological research, demonstrating the potential for primordial black holes to act as catalysts in early massive galaxy formation, aligning with recent empirical findings from JWST and informing our theoretical frameworks on cosmic structure formation.