The SCUBA-2 Cosmology Legacy Survey: galaxies in the deep 850-micron survey, and the star-forming `main sequence'

TL;DR

This study uses deep 850-micron SCUBA-2 imaging to analyze galaxy properties, confirming that sub-mm galaxies are part of the high-mass star-forming main sequence at z > 1.5 and providing insights into their star-formation activity.

Contribution

It presents the deepest 850-micron survey data to date, establishing the redshift distribution and stellar masses of sub-mm galaxies, and confirms their position on the star-forming main sequence at high redshift.

Findings

Sub-mm galaxies are on the high-mass end of the star-forming main sequence at z > 1.5.

No significant flattening of the main sequence at high masses at these redshifts.

Average specific SFR increases slowly with redshift, linearly related to the age of the Universe.

Abstract

We investigate the properties of the galaxies selected from the deepest 850-micron survey undertaken to date with SCUBA-2 on the JCMT. This deep 850-micron imaging was taken in parallel with deep 450-micron imaging in the very best observing conditions as part of the SCUBA-2 Cosmology Legacy Survey. A total of 106 sources were uncovered at 850 microns from ~150, sq. arcmin in the centre of the COSMOS/UltraVISTA/CANDELS field, imaged to a typical rms depth of ~0.25 mJy. We utilise the wealth of available deep multi-frequency data to establish the complete redshift distribution for this sample, yielding <z> = 2.38 +- 0.09, a mean redshift comparable with that derived for all but the brightest previous sub-mm samples. We have also been able to establish the stellar masses of the majority of the galaxy identifications, enabling us to explore their location on the star-formation-rate:stellar-mass (SFR:M*) plane. Crucially, our new deep sample reaches flux densities equivalent to SFR ~ 100 Msun/yr, enabling us to confirm that sub-mm galaxies form the high-mass end of the `main sequence' (MS) of star-forming galaxies at z > 1.5 (with a mean specific SFR of sSFR = 2.25 +- 0.19 /Gyr at z ~ 2.5). Our results are consistent with no significant flattening of the MS towards high masses at these redshifts, suggesting that reports of such flattening possibly arise from under-estimates of dust-enshrouded star-formation activity in massive star-forming galaxies. However, our findings add to the growing evidence that average sSFR rises only slowly at high redshift, resulting in log(sSFR) being an apparently simple linear function of the age of the Universe.