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We present ALMA 870-um (345GHz) observations of two sub-millimetre galaxies(SMGs) drawn from an ALMA study of the 126 sub-millimeter sources from theLABOCA Extended Chandra Deep Field South Survey (LESS). The ALMA data identifythe counterparts to these previously unidentified sub-millimeter sources andserendipitously detect bright emission lines in their spectra which we show aremost likely to be [C II]157.74um emission yielding redshifts of z=4.42 andz=4.44. This blind detection rate within the 7.5-GHz bandpass of ALMA isconsistent with the previously derived photometric redshift distribution ofSMGs and suggests a modest, but not dominant (<25%), tail of 870-um selectedSMGs at z>4. We find that the ratio of L_CII/L_FIR in these SMGs is much higherthan seen for similarly far-infrared-luminous galaxies at z~0, which isattributed to the more extended gas reservoirs in these high-redshift ULIRGs.Indeed, in one system we show that the [C II] emission shows hints of extendedemission on >3kpc scales. Finally, we use the volume probed by our ALMA surveyto show that the bright end of the [C II] luminosity function evolves stronglybetween z=0 and z~4.4, reflecting the increased ISM cooling in galaxies as aresult of their higher star-formation rates. These observations demonstratethat even with short integrations, ALMA is able to detect the dominant finestructure cooling lines from high-redshift ULIRGs, measure their energetics andtrace their evolution with redshift.

An ALMA survey of Sub-millimetre Galaxies in the Extended Chandra Deep Field South: Detection of [C II] at z=4.4