The importance of stochasticity in determining galaxy emissivities and UV LFs during cosmic dawn and reionization

TL;DR

This paper demonstrates that accounting for stochastic scatter in galaxy properties during cosmic dawn significantly impacts estimates of galaxy emissivities and the timing of reionization, revealing the importance of variability in high-redshift galaxy modeling.

Contribution

The authors develop a semi-empirical model incorporating scatter in key galaxy relations, showing its critical role in accurately predicting emissivities and reionization history.

Findings

Ignoring scatter overestimates the duration of the EoR by up to Δz ~ 2.

Scatter around the SFMS significantly affects emissivity estimates at high redshift.

Scatter in the X-ray luminosity to SFR relation accounts for about half of X-ray emissivity variability.

Abstract

The stochastic nature of star formation and photon propagation in high-redshift galaxies can result in sizable galaxy-to-galaxy scatter in their properties. Ignoring this scatter by assuming mean quantities can bias estimates of their emissivity and corresponding observables. We construct a flexible, semi-empirical model, sampling scatter around the following mean relations: (i) the conditional halo mass function (CHMF); (ii) the stellar-to-halo mass relation (SHMR); (iii) galaxy star formation main sequence (SFMS); (iv) fundamental metallicity relation (FMR); (v) conditional intrinsic luminosity; and (vi) photon escape fraction. In our fiducial model, ignoring scatter in these galaxy properties overestimates the duration of the EoR, delaying its completion by up to $\Delta z$ ~ 2. We quantify the relative importance of each of the above sources of scatter in determining the ionizing, soft-band X-ray and Lyman Werner (LW) emissivities as a function of scale and redshift. We find that scatter around the SFMS is important for all bands, especially at the highest redshifts where the emissivity is dominated by the faintest, most "bursty" galaxies. Ignoring this scatter would underestimate the mean emissivity and its standard deviation computed over 5 cMpc regions by factors of up to $\sim$2-10 at $5< z < 15$. Scatter around the X-ray luminosity to star formation rate relation is important for determining X-ray emissivity, accounting for roughly half of its mean and standard deviation. The importance of scatter in the ionizing escape fraction depends on its functional form, while scatter around the SHMR contributes at the level of ~10-20%. Although scatter does flatten the UV luminosity functions, shifting the bright end by 1-2 magnitudes, the level of scatter in our fiducial model is insufficient to fully explain recent estimates from JWST photometry (consistent with previous studies).