Cosmic Reionization On Computers: The Galaxy–Halo Connection between 5 ≤ z ≤ 10

Abstract

We explore the connection between the stellar component of galaxies and their host halos during the epoch of reionization (5 ≤ z ≤ 10) using the CROC (Cosmic Reionization on Computers) simulations. We compare simulated galaxies with observations and find that CROC underpredicts the abundance of luminous galaxies when compared to observed UV luminosity functions, and analogously the most massive galaxies when compared to observed stellar mass functions. We can trace the deficit of star formation to high redshifts, where the slope of the star formation rate to stellar mass relation is consistent with observations, but the normalization is systematically low. This results in a star formation rate density and stellar mass density that are systematically offset from observations. However, the less luminous or lower stellar mass objects have luminosities and stellar masses that agree fairly well with observational data. We explore the stellar-to-halo mass ratio (SHMR), a key quantity that is difficult to measure at high redshifts and that models do not consistently predict. In CROC, the SHMR decreases with redshift, a trend opposite to some abundance-matching studies. These discrepancies uncover where future effort should be focused in order to improve the fidelity of modeling cosmic reionization. We also compare the CROC galaxy bias with observational measurements using Lyman-break galaxy samples, finding reasonable consistency.

The THESAN project: properties of the intergalactic medium and its connection to Reionization-era galaxies

“…Among the proposed sources of the mismatch in the simulations, we mention a too low mass for the smallest galaxies contributing significantly to the ionizing photons budget, an earlier reionization history, and a large stellar escape fraction. Additionally, the stellar-to-halo-mass relation in CROC is very flat at halo masses M halo 10 10 M , in disagreement with observations and theoretical models (see figure 9 in Paper I and Zhu et al 2020), suggesting some issues with the simulated galaxy population.…”

Galaxy Stellar Mass Functions from z ∼ 10 to z ∼ 6 using the Deepest Spitzer/Infrared Array Camera Data: No Significant Evolution in the Stellar-to-halo Mass Ratio of Galaxies in the First Gigayear of Cosmic Time

“…Our estimates are consistent with those of Stefanon et al (2017) at < 1σ and of Finkelstein et al (2015a) at 1σ, both of which are based on abundance matching techniques; however, they are factor 3 − 4× higher (corresponding to a ∼ 3σ difference) than those of Harikane et al (2016Harikane et al ( , 2018, which were derived from clustering measurements. In the same panels we also present SHMR from three recent models which use different assumptions on the evolution of the SHMR with cosmic time: Tacchella et al (2018) assumed the SHMR to be approximately constant above z ∼ 4; Moster et al (2018) linked the star-formation rate to the halo accretion rate through a redshift-dependent parametric baryon conversion efficiency; finally, Behroozi et al (2019) did not introduce any correlation between the evolution of the dark matter halos and (baryonic) galaxy assembly, finding a SHMR increasing with redshift (see also Behroozi et al 2013, but see Zhu et al 2020). Figure 13 shows that our measurements are generally in good agreement with the predictions of Tacchella et al (2018) over the full redshift range probed here, and with those of Behroozi et al (2019) at z ∼ 8−10, further supporting a non-evolving SHMR in the early Universe.…”

Galaxy Stellar Mass Functions from z~10 to z~6 using the Deepest Spitzer/IRAC Data: No Significant Evolution in the Stellar-to-Halo Mass Ratio of Galaxies in the First Gyr of Cosmic Time

“…Our estimates are consistent with those of Stefanon et al (2017) at < 1σ and of Finkelstein et al (2015a) at 1σ, both of which are based on abundance matching techniques; however, they are factor 3 − 4× higher (corresponding to a ∼ 3σ difference) than those of Harikane et al (2016Harikane et al ( , 2018, which were derived from clustering measurements. In the same panels we also present SHMR from three recent models which use different assumptions on the evolution of the SHMR with cosmic time: Tacchella et al (2018) assumed the SHMR to be approximately constant above z ∼ 4; Moster et al (2018) linked the star-formation rate to the halo accretion rate through a redshift-dependent parametric baryon conversion efficiency; finally, Behroozi et al (2019) did not introduce any correlation between the evolution of the dark matter halos and (baryonic) galaxy assembly, finding a SHMR increasing with redshift (see also Behroozi et al 2013, but see Zhu et al 2020). Figure 13 shows that our measurements are generally in good agreement with the predictions of Tacchella et al (2018) over the full redshift range probed here, and with those of Behroozi et al (2019) at z ∼ 8−10, further supporting a non-evolving SHMR in the early Universe.…”