An ALMA survey of Sub - millimetre Galaxies in the Extended Chandra Deep Field South : Detection of [ C ii

We present ALMA 870-μm (345GHz) observations of two sub-millimetre galaxies (SMGs) drawn from an ALMA study of the 126 sub-millimeter sources from the LABOCA Extended Chandra Deep Field South Survey (LESS). The ALMA data identify the counterparts to these previously unidentified sub-millimeter sources and serendipitously detect bright emission lines in their spectra which we show are most likely to be [Cii] 157.74μm emission yielding redshifts of z=4.42 and z=4.44. This blind detection rate within the 7.5-GHz bandpass of ALMA is consistent with the previously derived photometric redshift distribution of SMGs and suggests a modest, but not dominant (< ∼ 25%), tail of 870-μm selected SMGs at z > ∼ 4. We find that the ratio of L[CII] /LFIR in these SMGs is much higher than seen for similarly far-infraredluminous galaxies at z ∼ 0, which is attributed to the more extended gas reservoirs in these high-redshift ULIRGs. Indeed, in one system we show that the [Cii] emission shows hints of extended emission on > ∼ 3 kpc scales. Finally, we use the volume probed by our ALMA survey to show that the bright end of the [Cii] luminosity function evolves strongly between z=0 and z ∼ 4.4, reflecting the increased ISM cooling in galaxies as a result of their higher star-formation rates. These observations demonstrate that even with short integrations, ALMA is able to detect the dominant fine structure cooling lines from high-redshift ULIRGs, measure their energetics, spatially resolved properties and trace their evolution with redshift.