  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 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 [C II]157.74um 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-um selected
SMGs at z>4. We find that the ratio of L_CII/L_FIR in these SMGs is much higher
than seen for similarly far-infrared-luminous 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 [C II] emission shows hints of extended
emission on >3kpc scales. Finally, we use the volume probed by our ALMA survey
to show that the bright end of the [C II] 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 and
trace their evolution with redshift.
