Herschel-ATLAS: counterparts from the UV--NIR in the science demonstration phase catalogue

TL;DR

This paper develops a method to identify optical counterparts of Herschel 250 micron sources, estimating redshifts and analyzing their distribution, which enhances understanding of the galaxy populations detected in submillimeter surveys.

Contribution

The study introduces a likelihood ratio technique for reliable optical counterpart identification of Herschel-ATLAS sources and provides redshift estimates for these galaxies.

Findings

Approximately 37% of Herschel sources have reliable optical counterparts.

Redshift distribution peaks at z ~ 0.25 for identified galaxies.

Evidence suggests a bimodal redshift distribution with some sources at z > 1.

Abstract

We present a technique to identify optical counterparts of 250 um-selected sources from the Herschel-ATLAS survey. Of the 6621 250 um > 32 mJy sources in our science demonstration catalogue we find that ~60 percent have counterparts brighter than r=22.4 mag in the Sloan Digital Sky Survey. Applying a likelihood ratio technique we are able to identify 2423 of the counterparts with a reliability R > 0.8. This is approximately 37 percent of the full 250 micron catalogue. We have estimated photometric redshifts for each of these 2423 reliable counterparts, while 1099 also have spectroscopic redshifts collated from several different sources, including the GAMA survey. We estimate the completeness of identifying counterparts as a function of redshift, and present evidence that 250 um-selected Herschel-ATLAS galaxies have a bimodal redshift distribution. Those with reliable optical identifications have a redshift distribution peaking at z ~ 0.25 +/- 0.05, while sub-mm colours suggest that a significant fraction with no counterpart above the r-band limit have z > 1. We also suggest a method for selecting populations of strongly-lensed high redshift galaxies. Our identifications are matched to UV--NIR photometry from the GAMA survey, and these data are available as part of the Herschel-ATLAS public data release.