On the Physical Requirements for a Pre-Reionization Origin of the Unresolved Near-Infrared Background

TL;DR

This paper investigates the physical conditions required for early universe sources, like galaxies and black holes, to produce the observed unresolved near-infrared background fluctuations, considering constraints from galaxy counts and cosmic backgrounds.

Contribution

It provides a detailed analysis of the physical requirements for high-redshift sources to account for near-infrared background fluctuations, highlighting the narrow parameter space consistent with observations.

Findings

High-redshift galaxies with normal IMF cannot fully explain fluctuations unless star formation efficiencies are very high.

Early black hole accretion must be vigorous but obscured to match fluctuation levels without violating X-ray background constraints.

Star formation in minihalos at z>12 requires efficiencies >0.1 with low ionizing escape fractions to produce observed signals.

Abstract

The study of the Cosmic Near-Infrared Background (CIB) light after subtraction of resolved sources can push the limits of current observations and infer the level of galaxy and black hole activity in the early universe. However, disentangling the relative contribution from low- and high-redshift sources is not trivial. Spatial fluctuations of the CIB exhibit a clustering excess at angular scales $\sim 1^\circ$ whose origin has not been conclusively identified. We explore the likelihood that this signal is dominated by emission from galaxies and accreting black holes in the early Universe. We find that, if the first small mass galaxies have a normal IMF, the light of their ageing stars (fossils) integrated over cosmic time contributes a comparable amount to the CIB as their pre-reionization progenitors. However, the measured fluctuation signal is too large to be produced by galaxies at redshifts $z>8$ unless their star formation efficiencies are much larger than those inferred from the observed Lyman-dropout population. In order to produce the observed level of CIB fluctuation without violating constraints from galaxy counts and the electron optical depth of the IGM, minihalos at $z>12$ must form stars with efficiency $f_\star \gtrsim 0.1$ and, although a top-heavy IMF is preferred, have a very low escape fraction of ionizing radiation, $f_{\rm esc}<0.01$. If instead the CIB fluctuations are produced by high-$z$ black holes, one requires vigorous accretion in the early universe reaching $\rho_{\rm acc} \gtrsim 10^5M_\odot{\rm Mpc^{-3}}$ by $z\simeq 10$. This growth must stop by $z \sim 6$ and be significantly obscured not to overproduce the soft cosmic X-ray background (CXB) and its observed coherence with the CIB. We therefore find the range of suitable possibilities at high-$z$ to be narrow, but could possibly be widened by including additional physics and evolution at those epochs.