An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: high-resolution 870 μm source counts

Karim, A. and Swinbank, A. M. and Hodge, J. A. and Smail, I. R. and Walter, F. and Biggs, A. D. and Simpson, J. M. and Danielson, A. L. R. and Alexander, D. M. and Bertoldi, F. and de Breuck, C. and Chapman, S. C. and Coppin, K. E. K. and Dannerbauer, H. and Edge, A. C. and Greve, T. R. and Ivison, R. J. and Knudsen, K. K. and Menten, K. M. and Schinnerer, E. and Wardlow, J. L. and Weiß, A. and van der Werf, P. (2013) An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: high-resolution 870 μm source counts. Monthly Notices of the Royal Astronomical Society, 432 (1). pp. 2-9. ISSN 0035-8711

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Abstract

We report the first counts of faint submillimetre galaxies (SMGs) in the 870-μm band derived from arcsecond-resolution observations with the Atacama Large Millimeter Array (ALMA). We have used ALMA to map a sample of 122 870-μm-selected submillimetre sources drawn from the 0.5°×0.5° the Large Apex BOlometer CAmera (LABOCA) Extended Chandra Deep Field South submillimetre survey (LESS). These ALMA maps have an average depth of σ870 μm ˜ 0.4 mJy, some approximately three times deeper than the original LABOCA survey and critically the angular resolution is more than an order of magnitude higher, FWHM of ˜1.5 arcsec compared to ˜19 arcsec for the LABOCA discovery map. This combination of sensitivity and resolution allows us to precisely pinpoint 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, S870 μm ≳ 12 mJy, comprise emission from multiple, fainter SMGs, each with 870-μm fluxes of ≲9 mJy. This implies a natural limit to the star formation rate in SMGs of ≲103 M⊙ yr-1, which in turn suggests that the space densities of z > 1 galaxies with gas masses in excess of ˜5 × 1010 M⊙ is