Cosmic Dawn (CoDa): the First Radiation-Hydrodynamics Simulation of Reionization and Galaxy Formation in the Local Universe

Abstract: Cosmic reionization by starlight from early galaxies affected their evolution, thereby impacting reionization, itself. Star formation suppression, for example, may explain the observed underabundance of Local Group dwarfs relative to N-body predictions for Cold Dark Matter. Reionization modelling requires simulating volumes large enough [~(100Mpc)^3] to sample reionization "patchiness", while resolving millions of galaxy sources above ~10^8 Msun , combining gravitational and gas dynamics with radiative transfer. Modelling the Local Group requires initial cosmological density fluctuations pre-selected to form the well-known structures of the local universe today. Cosmic Dawn ("CoDa") is the first such fully-coupled, radiation-hydrodynamics simulation of reionization of the local universe. Our new hybrid CPU-GPU code, RAMSES-CUDATON, performs hundreds of radiative transfer and ionization rate-solver timesteps on the GPUs for each hydro-gravity timestep on the CPUs. CoDa simulated (91Mpc)^3 with 4096^3 particles and cells, to redshift 4.23, on ORNL supercomputer Titan, utilizing 8192 cores and 8192 GPUs. Global reionization ended slightly later than observed. However, a simple temporal rescaling which brings the evolution of ionized fraction into agreement with observations also reconciles ionizing flux density, cosmic star formation history, CMB electron scattering optical depth and galaxy UV luminosity function with their observed values. Photoionization heating suppressed the star formation of haloes below ~2 x 10^9 Msun , decreasing the abun- dance of faint galaxies around MAB_1600 = [-10,-12]. For most of reionization, star formation was dominated by haloes between 10^10 - 10^11 Msun , so low-mass halo suppression was not reflected by a distinct feature in the global star formation history. (Abridged)

Overview of Cosmic Dawn (CoDa) Simulation

The paper "Cosmic Dawn (CoDa): the First Radiation-Hydrodynamics Simulation of Reionization and Galaxy Formation in the Local Universe" by Pierre Ocvirk et al. presents a comprehensive analysis of early universe simulations employing coupled radiation-hydrodynamics. The study utilizes the CoDa simulation to investigate the Epoch of Reionization (EoR) and the associated processes in galaxy formation, specifically targeting the local universe. The authors employ the innovative RAMSES-CUDATON code, implementing a hybrid CPU-GPU architecture that facilitates large-scale simulation of cosmic phenomena.

Methodology and Simulation Setup

The simulation explores a substantial volume of the universe, specifically a comoving cube of 91 Mpc per side with $4096^3$ particles and cells, executed on the Titan supercomputer. This simulation uses a new hybrid code, RAMSES-CUDATON, to carry out coupled radiation-hydrodynamics-gravity simulations. The dynamics of gas, dark matter, and radiation were advanced with high precision, leveraging GPUs to manage the computational demands of radiative transfer calculations. The simulation operates with a fine mass resolution due to its high particle count and synchronizes hydrodynamical evolutions with those of radiation transport and gravitational interactions.

Results and Key Findings

The CoDa simulation represents a pioneering step in modeling the complex interplay between galaxy formation and reionization, highlighting several critical aspects:

1. End of Reionization: The simulation indicates that global reionization completed slightly later than observational evidence suggests. A temporal rescaling aligns the simulation's results with observed redshift-dependent ionized fractions and galaxy luminosity functions. This suggests that fine-tuning in star formation efficiency parameters may be required.
2. Star Formation Suppression: The study identifies a prominent suppression of star formation in low-mass halos ($< 2 \times 10^9 M_\odot$) during reionization due to feedback from photoionization, resulting in a decreased formation rate of faint galaxies. This revelation contributes significantly to resolving the "missing satellites problem."
3. Impact on the Local Universe: The paper reveals that photoionization can effectively limit gas cooling and star formation efficiency in protogalactic halos, particularly in the Local Group. This insight could also explain the relative scarcity of dwarf galaxies in contemporary observations compared to theoretical models.
4. Cosmic 21cm Background and Large-Scale Structure: A notable aspect of the CoDa simulation is its ability to capture the spatial variability or "patchiness" of reionization, influencing predictions on the cosmic 21cm signal—a critical observable for future radio telescope arrays like SKA.

Implications and Future Directions

The research undertaken in the CoDa project has practical implications for understanding galaxy formation and evolution, as well as the universe's reionization timeline. It suggests potential adjustments in computational cosmology related to radiative transfer and gas dynamics modeling. Furthermore, the implications extend to interpreting observational data, offering a framework to connect past cosmic events with present-day universe configurations.

Future research might expand upon the implications of cosmic variance, explore enhancements in feedback models (including AGN impacts), and refine the mass resolution to better capture the smaller-scale processes involved in star formation and suppression. Additionally, as computational capabilities advance, studies like CoDa can serve as benchmarks for exploring deeper into the high-redshift universe, providing a more cohesive narrative of cosmic dawn.

The CoDa simulation represents a crucial development in computational astrophysics and cosmology, encapsulating a profound understanding of how the early universe unfolded and formed the complex large-scale structures apparent today.