An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: High resolution 870um source counts

We report the first counts of faint submillimetre galaxies (SMG) in the 870-um band derived from arcsecond resolution observations with the Atacama Large Millimeter Array (ALMA). We have used ALMA to map a sample of 122 870-um-selected submillimetre sources drawn from the (0.5x0.5)deg^2 LABOCA Extended Chandra Deep Field South Submillimetre Survey (LESS). These ALMA maps have an average depth of sigma(870um)~0.4mJy, some ~3x deeper than the original LABOCA survey and critically the angular resolution is more than an order of magnitude higher, FWHM of ~1.5" compared to ~19" for the LABOCA discovery map. This combination of sensitivity and resolution allows us to precisely pin-point the SMGs contributing to the submillimetre sources from the LABOCA map, free from the effects of confusion. We show that our ALMA-derived SMG counts broadly agree with the submillimetre source counts from previous, lower-resolution single-dish surveys, demonstrating that the bulk of the submillimetre sources are not caused by blending of unresolved SMGs. The difficulty which well-constrained theoretical models have in reproducing the high-surface densities of SMGs, thus remains. However, our observations do show that all of the very brightest sources in the LESS sample, S(870um)>12mJy, comprise emission from multiple, fainter SMGs, each with 870-um fluxes of 1 galaxies with gas masses in excess of ~5x10^10 M_Sun is