CHARACTERIZING THE COSMIC NEAR-INFRARED AND X-RAY BACKGROUND FLUCTUATIONS

Abstract

The cosmic infrared background (CIB) carries an abundance of information about the star formation and galaxy growth in the Universe. Due to significant and complex foregrounds from our Galaxy, the optimal way to study the unresolved background is to actually study its fluctuations, especially at large angular scales where they reflect the clustering of unresolved populations. I measured the CIB fluctuations reaching the largest angular scale to date for such a study (1 deg), using new COSMOS data from the Spitzer Large Area Survey with Hyper-Suprime-Cam. The auto-power spectra are consistent among various epochs and are significantly correlated at the two IRAC wavelengths (3.6 & 4.5 μm), implying cosmological origin of the excess power. The CIB and Cosmic X-ray Background (CXB) cross-correlation can not only provide observational constrains on the theoretical modeling of the CIB fluctuations but also stands as a unique way to study the formation of the early black holes. Using the Chandra COSMOS Legacy data, I found that the [0.5-2] keV band is correlated with 3.6 and 4.5 μm at 4 significance level. The combined infrared (3.6 + 4.5) μm data are correlated with the X-ray data in [0.5-2], [2-7] and [0.5-7] keV bands at 5.6, 4.4 and 6.6 sigma level, respectively. The coherence between the CIB and CXB fluctuations suggests significant contribution to the CIB fluctuations from accreting black holes that is much higher than among any known populations. I further analyzed the X-ray SED of the cross-power signal, by stacking with other surveys and assuming no significant variation from the infrared. I found that the SED’s shape is consistent with the emission predicted for high-mass black hole seeds (such as direct collapse black holes) and absorbed, high-z AGNs. In the absence of a higher energy resolution in the mean cross power spectra, however, a direct differentiation between high redshift black hole models is challenging with this methodology. Future projects and missions (e.g., LIBRAE) will offer more direct observational information and enable new methods to address the nature of the CIB excess fluctuations and the X-ray counterparts.