The Deep Diffuse Extragalactic Radio Sky at 1.75 GHz

Abstract

We present a study of diffuse extragalactic radio emission at $1.75$GHz from part of the ELAIS-S1 field using the Australia Telescope Compact Array. The resulting mosaic is $2.46$deg${}^2$, with a roughly constant noise region of $0.61$deg${}^2$ used for analysis. The image has a beam size of $150\times 60$arcsec and instrumental $⟨{\sigma }_{\mathrm{n}}⟩=(52\pm 5)\mu$Jy beam${}^{-1}$. Using point-source models from the ATLAS survey, we subtract the discrete emission in this field for $S\ge 150\mu$Jy beam${}^{-1}$. Comparison of the source-subtracted probability distribution, or \pd, with the predicted distribution from unsubtracted discrete emission and noise, yields an excess of $(76\pm 23)\mu$Jy beam${}^{-1}$. Taking this as an upper limit on any extended emission we constrain several models of extended source counts, assuming ${\Omega }_{\mathrm{source}}\le 2$arcmin. The best-fitting models yield temperatures of the radio background from extended emission of $T_{\mathrm{b}}=(10\pm 7)$mK, giving an upper limit on the total temperature at $1.75$GHz of $(73\pm 10)$mK. Further modelling shows that our data are inconsistent with the reported excess temperature of ARCADE2 to a source-count limit of $1\mu$Jy. Our new data close a loop-hole in the previous constraints, because of the possibility of extended emission being resolved out at higher resolution. Additionally, we look at a model of cluster halo emission and two WIMP dark matter annihilation source-count models, and discuss general constraints on any predicted counts from such sources. Finally, we report the derived integral count at $1.4$GHz using the deepest discrete count plus our new extended-emission limits, providing numbers that can be used for planning future ultra-deep surveys.

18 pages, 15 figures, 7 tables, Accepted by MNRAS

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