We present Atacama Large Millimeter Array (ALMA) 870-μm (345-GHz)
observations of two submillimetre galaxies (SMGs) drawn from an ALMA
study of the 126 submillimetre sources from the LABOCA Extended Chandra
Deep Field-South Survey (LESS). The ALMA data identify the counterparts
to these previously unidentified submillimetre sources and
serendipitously detect bright emission lines in their spectra which we
show are most likely to be [CII] 157.74 μm emission yielding
redshifts of z = 4.42 and 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 per cent), tail of 870-μm selected SMGs at z
≳ 4. We find that the ratio of L[C II]/LFIR
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 ultraluminous
infrared galaxies (ULIRGs). Indeed, in one system we show that the [C
II] 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 [C II] luminosity function evolves strongly between z
= 0 and ˜4.4, reflecting the increased interstellar medium 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 spatially resolved properties and
trace their evolution with redshift.