The Herschel-ATLAS Data Release 1 Paper II: Multi-wavelength
  counterparts to submillimetre sources
  This paper is the second in a pair of articles presenting data release 1
(DR1) of the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS), the
largest single open-time key project carried out with the Herschel Space
Observatory. The H-ATLAS is a wide-area imaging survey carried out in five
photometric bands at 100, 160, 250, 350 and 500$\mu$m covering a total area of
600deg$^2$. In this paper we describe the identification of optical
counterparts to submillimetre sources in DR1, comprising an area of 161 deg$^2$
over three equatorial fields of roughly 12$^\circ$x4.5$^\circ$ centred at
9$^h$, 12$^h$ and 14.5$^h$ respectively. Of all the H-ATLAS fields, the
equatorial regions benefit from the greatest overlap with current
multi-wavelength surveys spanning ultraviolet (UV) to mid-infrared regimes, as
well as extensive spectroscopic coverage. We use a likelihood-ratio technique
to identify SDSS counterparts at r<22.4 for 250-$\mu$m-selected sources
detected at $\geq$ 4$\sigma$ ($\approx$28mJy). We find `reliable' counterparts
(reliability R$\geq$0.8) for 44,835 sources (39 per cent), with an estimated
completeness of 73.0 per cent and contamination rate of 4.7 per cent. Using
redshifts and multi-wavelength photometry from GAMA and other public
catalogues, we show that H-ATLAS-selected galaxies at $z<0.5$ span a wide range
of optical colours, total infrared (IR) luminosities, and IR/UV ratios, with no
strong disposition towards mid-IR-classified AGN in comparison with optical
selection. The data described herein, together with all maps and catalogues
described in the companion paper (Valiante et al. 2016), are available from the
H-ATLAS website at www.h-atlas.org.
