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 [∼ (100 Mpc) 3 ] to sample reionization "patchiness", while resolving millions of galaxy sources above ∼ 10 8 M , 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 ratesolver 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 × 10 9 M , decreasing the abundance of faint galaxies around M AB1600 = [−10, −12]. For most of reionization, star formation was dominated by haloes between 10 10 − 10 11 M , so low-mass halo suppression was not reflected by a distinct feature in the global star formation history. Intergalactic filaments display sheathed structures, with hot envelopes surrounding cooler cores, but do not self-shield, unlike regions denser than 100 ρ .

The z ∼ 6 Luminosity Function Fainter than −15 mag from the Hubble Frontier Fields: The Impact of Magnification Uncertainties

“…An illustration of this parameterization is provided in Figure 7. Our use of −16 mag allows us to test for possible curvature in the shape of the LF at 16 mag >-, as predicted by some models (e.g., Jaacks et al 2013;Kuhlen et al 2013;Ocvirk et al 2016). −16 mag is also just faintward of luminosities probed in field studies (i.e., −16.77 mag: Bouwens et al 2015a).…”

“…a Same assumptions as in Table 3. b The −3 mag limit is included here for illustrative value and takes as its inspiration results from O'Shea et al (2015) and Ocvirk et al (2016), which predict sources to such faint magnitudes. Notes.…”

“…8 In addition to the clear scientific importance of the faint-end slope α for computing the total ionizing emissivity from faint galaxies, the observations also allow us to test for a possible flattening or turn-over of the UV LF at low luminosities. Many cosmological hydrodynamic simulations of galaxy formation predict a flattening in the UV LF at approximately−13 or approximately−15magdue to less efficient atomic and molecular hydrogen cooling, respectively (Muñoz & Loeb 2011;Krumholz & Dekel 2012;Jaacks et al 2013;Kuhlen et al 2013;Wise et al 2014;Finlator et al 2016;Gnedin 2016;Liu et al 2016), while other simulations predict a flattening in the rangeapproximately−16 to approximately−13 magdue to the impact of radiative feedback (O'Shea et al 2015;Ocvirk et al 2016). Meanwhile, by combining abundance matching and detailed studies of the color-magnitude diagram of lowluminosity dwarfs in the local universe, evidence for a lowmass turn-over in the LF has been reported at −13 mag (Boylan-Kolchin et al 2015 see also Boylan-Kolchin et al 2014).…”

A Rogues’ Gallery of Andromeda's Dwarf Galaxies. I. A Predominance of Red Horizontal Branches

“…A number of recent simulations suggest that reionization may more effectively prevent the accretion of fresh gas onto a low-mass halo, rather than photoevaporate its existing gas supply, thus allowing low-level star formation to continue beyond the end of reionization (e.g., Oñorbe et al 2015;Fitts et al 2017). Another possibility is that reionization may be non-uniform either owing to the statistical variations in some global background (e.g., patchy reionization) or due to the fact that different local sources (e.g., M31 versus the MW versus Virgo) were responsible for reionization (e.g., Busha et al 2010;Lunnan et al 2012;Ocvirk et al 2013Ocvirk et al , 2016. Such differences could be imprinted on the stellar populations of the faintest satellites.…”

Probing the Intergalactic Medium with Lyα and 21 cm Fluctuations

“…Future probes such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA) 1 will be able to detect power spectra of 21cm fluctuations at high redshifts over up to two decades in scale, mapping most of the sky, as well as constrain the timing and morphology of reionization, the properties of early galaxies, and the early sources of heating (Koopmans et al 2015;Pritchard et al 2015;DeBoer et al 2017). A lot of work has gone into modeling and preparing these detections, using seminumerical simulations, such as 21cmFAST (Mesinger et al 2011) or SimFast21 (Santos et al 2010), and hydrodynamical simulations to explore the parameter space for reionization models; see, for example, Ocvirk et al (2016).…”