The Impossibly Early Galaxy Problem (1506.01377v2)

Abstract: The current hierarchical merging paradigm and $\Lambda$CDM predict that the $z \sim 4-8$ universe should be a time in which the most massive galaxies are transitioning from their initial halo assembly to the later baryonic evolution seen in star-forming galaxies and quasars. However, no evidence of this transition has been found in many high redshift galaxy surveys including CFHTLS, CANDELS and SPLASH, the first studies to probe the high-mass end at these redshifts. Indeed, if halo mass to stellar mass ratios estimated at lower-redshift continue to $z \sim 6-8$, CANDELS and SPLASH report several orders of magnitude more $M \sim 10^{12-13} M_\odot$ halos than are possible to have formed by those redshifts, implying these massive galaxies formed impossibly early. We consider various systematics in the stellar synthesis models used to estimate physical parameters and possible galaxy formation scenarios in an effort to reconcile observation with theory. Although known uncertainties can greatly reduce the disparity between recent observations and cold dark matter merger simulations, even taking the most conservative view of the observations, there remains considerable tension with current theory.

Overview of "The Impossibly Early Galaxy Problem"

The paper "The Impossibly Early Galaxy Problem" by Steinhardt et al. addresses a significant discrepancy in our understanding of galaxy formation and evolution within the framework of the Lambda Cold Dark Matter ($\Lambda$CDM) model. This model predicts that the universe at redshifts $z \sim 4-8$ should exhibit a transition in the most massive galaxies from initial halo assembly to baryonic evolution observed in star-forming galaxies and quasars. However, observations from high-redshift galaxy surveys like CFHTLS, CANDELS, and SPLASH indicate an unexpectedly high number of massive galaxies, suggesting they were formed "impossibly early".

Main Points

1. Discrepancy in Predicted and Observed Halo Mass Functions:
   • The hierarchical merging paradigm predicts rapid evolution in the density of massive halos at $z>4$. Nonetheless, observations report several orders of magnitude more $M \sim 10^{12-13} M_\odot$ halos than predicted, implying these galaxies formed much earlier than expected.
   • This inconsistency raises questions about both the stellar synthesis models used to estimate physical parameters and existing galaxy formation theories.
2. Analysis of Early Galaxy Properties:
   • High-redshift galaxies, especially those at the high-mass end, appear 'normal' in several respects based on analyses, maintaining consistency with scaling relations and evolutionary trends observed at lower redshifts.
   • The star-forming main sequence properties at $z \sim 6-8$ align with lower-redshift observations, contradicting the expected deviation if stellar synthesis models were fundamentally flawed.
3. Luminosity Functions as Probes of Halo Mass:
   • A persistent mass-to-light ratio at $z \sim 4-8$ defies expectations of sharp evolution. This indicates a potential breakdown in assumptions about how UV luminosities translate to halo masses.
   • Numerical modeling suggests the observed UV luminosity functions should exhibit sharper evolution due to the time delay between halo formation and star formation.
4. Exploration of Systematic Uncertainties:
   • Various factors like stellar evolution, changing IMF, evolving dust corrections, and time delays between halo assembly and star formation were considered as explanations for the discrepancy.
   • A plausible solution would require a new understanding of early star formation processes or the possibility of exotic physics altering halo formation timelines.

Implications and Future Directions

This paper suggests several implications regarding current theoretical models and observational methodologies:

• Future Research and Observations: Extensive focus on identifying and characterizing the earliest, most massive galaxies is essential. This involves expanding wide-area surveys to include sufficient numbers of these objects, with potential input from JWST to refine our stellar synthesis models and redshift measurements.
• Alterations to $\Lambda$CDM: Potential modifications may involve adjustments to current understanding of baryonic processes or introducing new physics influencing early halo formation. This could impact the perceived formation timelines of galaxies and address discrepancies observed in high-redshift galaxy counts.
• Exploration of Alternative Models: The paper paves the way for considering alternative models that may better explain the anomalous assembly of massive galaxies, such as variations in dark matter properties or early star formation efficiencies.

In conclusion, the paper challenges existing paradigms within $\Lambda$CDM models and highlights the need for further research and innovative approaches to understand the universe's early structure formation. These efforts might provide fresh insights into the nature of dark matter, galaxy evolution processes, and the initial conditions of cosmic formation.