THE COSMIC NEAR-INFRARED BACKGROUND. II. FLUCTUATIONS

Abstract

The near-infrared background (NIRB) is one of a few methods that can be used to observe the redshifted light from early stars at a redshift of 6 and above, and thus it is imperative to understand the significance of any detection or nondetection of the NIRB. Fluctuations of the NIRB can provide information on the first structures, such as halos and their surrounding ionized regions in the intergalactic medium (IGM). We combine, for the first time, N-body simulations, radiative transfer code, and analytic calculations of luminosity of early structures to predict the angular power spectrum (Cl) of fluctuations in the NIRB. We study in detail the effects of various assumptions about the stellar mass, the initial mass spectrum of stars, the metallicity, the star formation efficiency (f*), the escape fraction of ionizing photons (fesc), and the star formation timescale (tSF), on the amplitude as well as the shape of Cl. The power spectrum of NIRB fluctuations is maximized when f* is the largest (as Cl ∝ f2*) and fesc is the smallest (as more nebular emission is produced within halos). A significant uncertainty in the predicted amplitude of Cl exists due to our lack of knowledge of tSF of these early populations of galaxies, which is equivalent to our lack of knowledge of the mass-to-light ratio of these sources. We do not see a turnover in the NIRB angular power spectrum of the halo contribution, which was claimed to exist in the literature, and explain this as the effect of high levels of nonlinear bias that was ignored in the previous calculations. This is partly due to our choice of the minimum mass of halos contributing to NIRB (∼2 × 109 M☉), and a smaller minimum mass, which has a smaller nonlinear bias, may still exhibit a turnover. Therefore, our results suggest that both the amplitude and shape of the NIRB power spectrum provide important information regarding the nature of sources contributing to the cosmic reionization. The angular power spectrum of the IGM, in most cases, is much smaller than the halo angular power spectrum, except when fesc is close to unity, tSF is longer, or the minimum redshift at which the star formation is occurring is high. In addition, low levels of the observed mean background intensity tend to rule out high values of f* ≳ 0.2.