ALMACAL VII: First Interferometric Number Counts at 650 $\mu$m

Abstract

Measurements of the cosmic far-infrared background (CIB) indicate that emission from many extragalactic phenomena, including star formation and black hole accretion, in the Universe can be obscured by dust. Resolving the CIB to study the population of galaxies in which this activity takes place is a major goal of submillimetre astronomy. Here, we present interferometric 650$\mu$m submillimetre number counts. Using the Band 8 data from the ALMACAL survey, we have analysed 81 ALMA calibrator fields together covering a total area of 5.5~arcmin$^2$. The typical central rms in these fields is $\sim 100 \mu$Jy~beam$^{-1}$ with the deepest maps reaching $\sigma = 47 \mu$Jy~beam$^{-1}$ at sub-arcsec resolution. Multi-wavelength coverage from ALMACAL allows us to exclude contamination from jets associated with the calibrators. However, residual contamination by jets and lensing remain a possibility. Using a signal-to-noise threshold of $4.5\sigma$, we find 21 dusty, star-forming galaxies with 650$\mu$m flux densities of $\geq 0.7 $mJy. At the detection limit we resolve $\simeq 100$ per cent of the CIB at 650$\mu$m, a significant improvement compared to low resolution studies at similar wavelength. We have therefore identified all the sources contributing to the EBL at 650 microns and predict that the contribution from objects with flux 0.7<mJy will be small.