Can pregalactic stars or black holes generate an IR background?

Abstract

Matsumoto et al. report¹ that they have detected an IR background in the waveband 2–5 µm which has a density Ω_R ∼ 10⁻⁴h⁻² in units of the critical density (ρ_crit = 5 × 10⁻³⁰h² g cm⁻³ with H₀ = 50h km s⁻¹ Mpc⁻¹) and an approximately black-body spectrum with a temperature of about 1,500 K. They claim that the intensity is too high to be explained by zodiacal light, interplanetary dust, low mass halo stars or galactic emission, and suggest that it is derived from a generation of pregalactic (population III) stars. We argue here that this is possible only if the stars began forming at a redshift exceeding 40 with a density parameter Ω_* ∼ 1 and a mass in the range 10²–10⁵ M_⊙. With such a high density, they could avoid over-enriching the background medium with heavy elements only if they collapsed to black holes after their nuclear burning phase². These holes may also have contributed to the IR background, provided they formed optically thick accretion disks. However, we argue that holes could generate the entire background only if they have a density parameter Ω_B∼0.1 and a mass in the range 10^6–10⁸M_⊙, in which case they would be too large to have stellar precursors.