An accurate measurement of the anisotropies and mean level of the cosmic infrared background at 100 µm and 160 µm

Abstract

Context. The measurement of the anisotropies in the cosmic infrared background (CIB) is a powerful means of studying the evolution of galaxies and large-scale structures. These anisotropies have been measured by a number of experiments, from the far-infrared (AKARI 90 μm) to the millimeter (Planck and the South Pole Telescope at ~2 mm). One of the main impediments to an accurate measurement on large scales (≲ 1 degree) is the contamination of the foreground signal by Galactic dust emission. Aims. Our goal is to show that we can remove the Galactic cirrus contamination using Hi data, and thus accurately measure the clustering of starburst galaxies in the CIB. Methods. We use observations of the so-called extragalactic ELAIS N1 field at far-infrared (100 and 160 μm) and radio (21 cm) wavelengths. We compute the correlation between dust emission, traced by far-infrared observations, and Hi gas traced by 21 cm observations, and derive dust emissivities that enable us to subtract the cirrus emission from the far-infrared maps. We then derive the power spectrum of the CIB anisotropies, as well as its mean level at 100 μm and 160 μm. Results. We compute dust emissivities for each of the Hi-velocity components (local, intermediate, and high velocity). Using IRIS/IRAS data at 100 μm, we demonstrate that we can use the measured emissivities to determine and remove the cirrus contribution to the power spectrum of the CIB on large angular scales where the cirrus contribution dominates. We then apply this method to Spitzer/MIPS data for 160 μm. We measure correlated anisotropies at 160 μm, and for the first time at 100 μm. We also combine the Hi data and Spitzer total power mode absolute measurements to determine the CIB mean level at 160 μm. We find B_(160) = 0.77 ± 0.04 ± 0.12 MJy/sr, where the first error is statistical and the second one systematic. Combining this measurement with the B_(100)/B_(160) color of the correlated anisotropies, we also derive the CIB mean at 100 μm, B_(100) = 0.24 ± 0.08 ± 0.04 MJy/sr. This measurement is in line with values obtained with recent models of infrared galaxy evolution and Herschel/PACS data, but is much smaller than the previous DIRBE measurements. In contrast to Matsuura and collaborators, we do not find any evidence of a new galaxy population at high redshift or unknown diffuse emission. Part of this discrepancy is likely to be explained by their use of an incorrect template for the Galactic cirrus emission. Conclusions. The use of high-angular resolution Hi data is mandatory to accurately differentiate the cirrus from the CIB emission. The 100 μm IRAS map (and thus the map developed by Schlegel and collaborators) in such extragalactic fields is highly contaminated by the CIB anisotropies and hence cannot be used as a Galactic cirrus tracer.

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