Irts Observation of the Near-infrared Extragalactic Background Light

We 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 µm to 4.0 µm. The spectrum is stellar like, but shows a spectral jump from the optical EBL. The obtained emission is isotropic for the observed sky, and the in-band flux amounts to ∼ 35 nWm −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 which can not 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 2 Now out of ISAS/JAXA – 2 – correlation analysis shows that IRTS/NIRS data has an angular scale of fluctuations of a few degrees. Recent WMAP results of CMB polarization indicate that the reionization of the Universe occurred at z ∼ 17 or earlier. The observed near infrared background light could be redshifted UV light from the first generation of massive stars (Population III stars) which caused the reionization of the Universe. The spectral jump around 1 µm over the optical EBL suggests that massive star formation terminated at z ∼ 9. The observed fluctuations, however, are considerably larger than the theoretical predictions and require a new scenario for the Pop.III era.