Galaxy Protoclusters as Drivers of Cosmic Star-Formation History in the First 2 Gyr (1705.01634v2)

Abstract: Present-day clusters are massive halos containing mostly quiescent galaxies, while distant protoclusters are extended structures containing numerous star-forming galaxies. We investigate the implications of this fundamental change in a cosmological context using a set of N-body simulations and semi-analytic models. We find that the fraction of the cosmic volume occupied by all (proto)clusters increases by nearly three orders of magnitude from z=0 to z=7. We show that (proto)cluster galaxies are an important, and even dominant population at high redshift, as their expected contribution to the cosmic star-formation rate density rises (from 1% at z=0) to 20% at z=2 and 50% at z=10. Protoclusters thus provide a significant fraction of the cosmic ionizing photons, and may have been crucial in driving the timing and topology of cosmic reionization. Internally, the average history of cluster formation can be described by three distinct phases: at z~10-5, galaxy growth in protoclusters proceeded in an inside-out manner, with centrally dominant halos that are among the most active regions in the Universe; at z~5-1.5, rapid star formation occurred within the entire 10-20 Mpc structures, forming most of their present-day stellar mass; at z<~1.5, violent gravitational collapse drove these stellar contents into single cluster halos, largely erasing the details of cluster galaxy formation due to relaxation and virialization. Our results motivate observations of distant protoclusters in order to understand the rapid, extended stellar growth during Cosmic Noon, and their connection to reionization during Cosmic Dawn.

Overview of "Galaxy Protoclusters as Drivers of Cosmic Star-Formation History in the First 2 Gyr"

The paper by Yi-Kuan Chiang and collaborators presents an insightful analysis of the role of galaxy protoclusters in the cosmic star-formation history across the first two billion years of the Universe. This research employs advanced $N$-body simulations and semi-analytic models to explore how these early galactic structures significantly influenced the cosmic star-formation rate density (CSFRD).

Key Findings and Methodology

The paper reveals that the volume fraction of the Universe occupied by protoclusters increases substantially with redshift, from nearly negligible at $z=0$ to about $5\%$ at $z=7$. Notably, the contribution of protocluster galaxies to the CSFRD rises dramatically from $1\%$ at $z=0$ to $50\%$ at $z=10$. These findings suggest that protocluster galaxies were critically important in the early Universe, potentially monopolizing the production of cosmic ionizing photons and affecting the timeline and topology of cosmic reionization.

The analysis relies on two semi-analytic models (SAMs), one from Henriques et al. (H15) and the other from Guo et al. (G13), each applied to the Millennium Simulation with cosmological scaling matched to Planck and WMAP7 parameters, respectively. The models account for environmental effects in dense galactic regions, and the results have been scaled for comparison with Planck cluster abundances.

Cosmic Impact of Protoclusters

The protoclusters undergo three distinct phases delineated in the paper:

1. Inside-Out Growth (z~10 to 5): Initially, galaxy growth occurs from inside out, with central halos forming as massive star-forming regions.
2. Extended Star Formation (z~5 to 1.5): A dramatic increase in star formation occurs, creating most of the stellar mass within clusters as we observe today, spreading over 10-20 Mpc scales.
3. Gravitational Collapse (z < 1.5): Galaxies experience gravitational collapse into singular cluster halos, undergoing relaxation and virialization, effectively erasing initial formation details.

These stages reflect the transition of galaxies from active, star-forming entities to gravitationally bound clusters. Protoclusters' dominant role in ionizing photon production suggests significant implications for the reionization period, aligning with observations of high-redshift galaxy overdensities and massive early structures.

Implications and Future Directions

This paper highlights the importance of protoclusters in understanding the high-redshift Universe's structure formation. While current models provide essential insights, their predictions necessitate further observational validation. Future missions and surveys, such as JWST and WFIRST, as well as large field spectrographs, can potentially uncover more about the protocluster environments, offering robust tests of these statistical predictions.

The findings stimulate further inquiry into star formation efficiency and feedback mechanisms within high-density galactic environments. Moreover, protoclusters could serve as cosmological probes for studying variations in the initial mass function over different environments and epochs.

In conclusion, Chiang et al.'s research lays critical groundwork for comprehending protoclusters' role in galaxy evolution and Universe reionization, paving the way for diverse future investigations in astrophysical and cosmological contexts.