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 six and above. Fluctuations of the NIRB can provide information on the first structures, such as halos and their surrounding ionized regions in the 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 (C_l) of fluctuations in the NIRB. We study the effects of various assumptions about the stellar mass, the initial mass spectrum of stars, metallicity, the star formation efficiency (f_*), the escape fraction of ionizing photons (f_esc), and the star formation timescale (t_SF), on the amplitude as well as the shape of C_l. The power spectrum of NIRB fluctuations is maximized when f_* is the largest (as C_l ~ (f_*)^2) and f_esc is the smallest. A significant uncertainty in the predicted amplitude of C_l exists due to our lack of knowledge of t_SF of these galaxies, which is equivalent to our lack of knowledge of the mass-to-light ratio. We do not see a turnover in the NIRB angular power spectrum of the halo contribution and explain this as the effect of high levels of non-linear bias. This is partly due to our choice of the minimum mass of halos contributing to NIRB, and a smaller minimum mass, which has a smaller non-linear bias, may still exhibit a turn over. Therefore, 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. In addition, low levels of the observed mean background intensity tend to rule out high values of f_* > 0.2.