The Near‐Infrared Background: Interplanetary Dust or Primordial Stars?

Abstract

The intensity of the diffuse $1-4 m sky emission from which solar system and Galactic foregrounds have been subtracted is in excess of that expected from energy released by galaxies and stars that formed during the z P 5 redshift interval.The spectral signature of this excess near-infrared background light (NIRBL) component is almost identical to that of reflected sunlight from the interplanetary dust cloud and could therefore be the result of the incomplete subtraction of this foreground emission component from the diffuse sky maps. Alternatively, this emission component could be extragalactic. Its spectral signature is consistent with that of redshifted continuum and recombination line emission from H ii regions formed by the first generation of very massive stars. In this paper we analyze the implications of this spectral component for the formation rate of these Population III stars, the redshift interval during which they formed, the reionization of the universe, and evolution of collapsed halo masses. Assuming that these Population III stars are massive objects radiating at the Eddington luminosity and ending their lives by directly collapsing into black holes, we find that to reproduce the intensity and spectral shape of the NIRBL requires a peak star formation rate of $2.5 M yr À1 Mpc À3 , with a (1 þ z) À2 dependence on redshift, until the epoch ends at redshifts z % 7-9. It requires a comoving luminosity density of about 2:7 ; 10 11 L Mpc À3 , corresponding to a total energy input of 670-820 keV per baryon, and that about 10% of the total number of baryons in the universe be converted to Population III stars. All these numbers are higher by about a factor of 4-10 than those derived from models in which Population III stars form at a rate that is proportional to the collapse rate of halos in a cold dark matter dominated universe. Furthermore, an extragalactic origin for the NIRBL leads to physically unrealistic absorption-corrected spectra of distant TeV blazars. All these results suggest that Population III stars contribute only a fraction of the NIRBL intensity, with zodiacal light, star-forming galaxies, and /or nonnuclear sources giving rise to the remaining fraction. Further 0.1-10 m observations of the diffuse sky and the zodiacal cloud are therefore crucial for resolving the true spectrum and origin of the NIRBL.

A low level of extragalactic background light as revealed by γ-rays from blazars

“…The H.E.S.S. upper limits agree instead with the most recent theoretical calculations 23 of the EBL, as well as with recent theoretical arguments 24,25 against high EBL fluxes due to Pop III stars.…”

“…The exclusion of the the IRTS data points between 1.4 and 2.2 µm (dashed line) is not enough to avoid very hard spectra (Γ int ≈ 0.2): significantly lower NIR fluxes are needed, as described in the main text. We note that ref [25] argues that the shape of the NIR excess between 1 and 4 µm is almost identical to the spectrum of the zodiacal light, and could therefore be of local instead of extragalactic origin. Fig.…”

Searching for an extragalactic background light attenuation signature in the Fermi/LAT 1st year catalog data

“…Madau & Silk 2005) and experimental (e.g. Dwek et al 2005;Aharonian et al 2006;Thompson et al 2007) grounds. Therefore, it is possible to (i) sample parts of the intrinsic spectrum emitted at the source and; (ii) to observe sources up to higher redshift compared to VHE observations 1 .…”

First stars and the extragalactic background light: how recentγ-ray observations constrain the early universe

“…The nature of this excess is still under debate. Dwek et al (2005a) find that it is likely a foreground artifact from zodiacal light and not of extragalactic origin 5 . A possible PopIII origin of the NIRBE has been investigated by many authors (Santos et al 2002;Dwek et al 2005a;Madau & Silk 2005;Salvaterra & Ferrara 2006;Fernandez & Komatsu 2006).…”