The deep diffuse extragalactic radio sky at 1.75 GHz

We present a study of diffuse extragalactic radio emission at 1.75 GHz from part of the ELAIS-S1 (European Large Area Infrared Space Observatory Survey - South 1) field using the Australia Telescope Compact Array. The resulting mosaic is 2.46 deg2, with a roughly constant noise region of 0.61 deg2 used for analysis. The image has a beam size of 150 arcse x— 60 arcsec and instrumental 〈σn = (52 ± 5) μJy beam-1. Using point-source models from the Australia Telescope Large Area Survey, we subtract the discrete emission in this field for S ≥ 150 μJy beam-1. Comparison of the source-subtracted probability distribution, or P(D), with the predicted distribution from unsubtracted discrete emission and noise, yields an excess of (76 ± 23) μJy beam-1. Taking this as an upper limit on any extended emission, we constrain several models of extended source counts, assuming Ωsource ≤ 2 arcmin. The best-fitting models yield temperatures of the radio background from extended emission of Tb = (10 ± 7) mK, giving an upper limit on the total temperature at 1.75 GHz of (73 ± 10) mK. Further modelling shows that our data are inconsistent with the reported excess temperature of ARCADE2 to a source-count limit of 1 μJy. 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 dark matter particle 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 ultradeep surveys.