Evidence for large-scale excesses associated with low HI column densities in the sky $$\\$$I. Dust excess

Abstract: Where dust and gas are uniformly mixed, atomic hydrogen can be traced through the detection of far-infrared (FIR) or UV emission of dust. We considered, for the origin of discrepancies observed between various direct and indirect tracers of gas outside the Galactic plane, possible corrections to the zero levels of the Planck-HFI detectors. We set the zero levels of the Planck High Frequency Instrument (HFI) skymaps as well as the 100 $\mu$m map from COBE/DIRBE and IRAS from the correlation between FIR emission and atomic hydrogen column density excluding regions of lowest gas column density. A modified blackbody model fit to those new zero-subtracted maps led to significantly different maps of the opacity spectral index $\beta$ and temperature $T$ and an overall increase in the optical depth at 353 GHz $\tau_{353}$ of 7.1$\times$10$^{-7}$ compared to the data release 2 Planck map. When comparing $\tau_{353}$ and the HI column density, we observed a uniform spatial distribution of the opacity outside regions with dark neutral gas and CO except in various large-scale regions of low NHI that represent 25% of the sky. In those regions, we observed an average dust column density 45% higher than predictions based on NHI with a maximum of 250% toward the Lockman Hole region. From the average opacity $\sigma_{e 353}$=(8.9$\pm$0.1)$\times$10$^{-27}$ cm$^2$ we deduced a dust-to-gas mass ratio of 0.53$\times$10$^{-2}$. We did not see evidence of dust associated to a Reynolds layer of ionized hydrogen. We measured a far-ultraviolet isotropic intensity of 137$\pm$15 photons s$^{-1}$cm$^{-2}$sr$^{-1}$$A$$^{-1}$ in agreement with extragalactic flux predictions and a near-ultraviolet isotropic intensity of 378$\pm$45 photons s$^{-1}$cm$^{-2}$sr$^{-1}$$A$$^{-1}$ corresponding to twice the predicted flux.