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

TL;DR

This study uses ALMA observations to detect [C II] emission in high-redshift sub-millimeter galaxies, revealing their properties, extended gas reservoirs, and evolution of the [C II] luminosity function from z=0 to z=4.4.

Contribution

First detection of [C II] emission in z>4 SMGs, showing their extended gas reservoirs and evolution of [C II] luminosity function with redshift.

Findings

Detected [C II] emission at z=4.4 in SMGs.

High L_CII/L_FIR ratio indicates extended gas reservoirs.

Strong evolution of [C II] luminosity function from z=0 to z=4.4.

Abstract

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.