Infrared Telescope in Space Observations of the Near-Infrared Extragalactic Background Light

We have searched for near-infrared extragalactic background light (EBL) in the data from the Near-Infrared Spectrometer (NIRS) on the Infrared Telescope in Space (IRTS). After subtracting the contribution of faint stars and the zodiacal component based on modeling, a significant isotropic emission is obtained in the wavelength bands from 1.4 to 4.0 μm. The spectrum is stellar-like but shows a spectral jump from the optical EBL. The emission obtained is isotropic over the observed sky, and the in-band flux amounts to ~35 nW m-2 sr-1, which is too bright to be explained by the integrated light from faint galaxies. Analyses of COBE DIRBE data, after removal of starlight, show essentially the same result within the uncertainty in the zodiacal light model, which implies that the isotropic emission observed by IRTS NIRS is of extragalactic origin. Significant fluctuations in sky brightness were also detected that cannot be explained by fluctuations due to faint stars, zodiacal components, and normal galaxies. The excess fluctuation amounts to ~1/4 of the excess emission over the integrated light of galaxies and is consistent with fluctuations observed by COBE DIRBE. A two-point correlation analysis shows that IRTS NIRS data have an angular scale of fluctuations of a few degrees. The spectrum and brightness of the observed excess EBL emission could be explained by the redshifted UV radiation from the first generation of massive stars (Population III stars), which caused the reionization of the universe. Recent Wilkinson Microwave Anisotropy Probe (WMAP) observations of the cosmic microwave background (CMB) polarization have indicated that reionization occurred at z ~ 17 or earlier, while the spectral jump around 1 μm in the observed excess EBL suggests that the Population III star formation terminated at z ~ 9. The observed fluctuations, however, are considerably larger than the theoretical predictions for the Population III stars.