Gravitational lensing is a powerful astrophysical and cosmological probe and is particularly valuable at submillimeter wavelengths for the study of the statistical and individual properties of dusty star-forming galaxies. However, the identification of gravitational lenses is often time-intensive, involving the sifting of large volumes of imaging or spectroscopic data to find few candidates. We used early data from the Herschel Astrophysical Terahertz Large Area Survey to demonstrate that wide-area submillimeter surveys can simply and easily detect strong gravitational lensing events, with close to 100% efficiency.

We present a technique to identify optical counterparts of 250‐μm‐selected sources from the Herschel–ATLAS survey. Of the 6621 250 μm > 32‐mJy sources in our science demonstration catalogue we find that ∼60 per cent 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 per cent of the full 250‐μm 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‐μm‐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 submillimetre 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.

We present high-spatial resolution imaging obtained with the Submillimeter Array (SMA) at 880 µm and the Keck Adaptive Optics (AO) system at K S -band of a gravitationally lensed sub-millimeter galaxy (SMG) at z = 4.243 discovered in the Herschel Astrophysical Terahertz Large Area Survey. The SMA data (angular resolution ≈ 0. ′′ 6) resolve the dust emission into multiple lensed images, while the Keck AO K S -band data (angular resolution ≈ 0. ′′ 1) resolve the lens into a pair of galaxies separated by 0. ′′ 3. We present an optical spectrum of the foreground lens obtained with the Gemini-South telescope that provides a lens redshift of z lens = 0.595 ± 0.005. We develop and apply a new lens modeling technique in the visibility plane that shows that the SMG is magnified by a factor of µ = 4.1±0.2 and has an intrinsic infrared (IR) luminosity of L IR = (2.1±0.2)×10 13 L ⊙ . We measure a half-light radius of the background source of r s = 4.4 ± 0.5 kpc which implies an IR luminosity surface density of Σ IR = (3.4 ± 0.9) × 10 11 L ⊙ kpc 2 , a value that is typical of z > 2 SMGs but significantly lower than IR luminous galaxies at z ∼ 0. The two lens galaxies are compact (r lens ≈ 0.9 kpc) early-types with Einstein radii of θ E1 = 0.57 ± 0.01 and θ E2 = 0.40 ± 0.01 that imply masses of M lens1 = (7.4 ± 0.5) × 10 10 M ⊙ and M lens2 = (3.7 ± 0.3) × 10 10 M ⊙ . The two lensing galaxies are likely about to undergo a dissipationless merger, and the mass and size of the resultant system should be similar to other early-type galaxies at z ∼ 0.6. This work highlights the importance of high spatial resolution imaging in developing models of strongly lensed galaxies discovered by Herschel.

We present new observations from Z-Spec, a broadband 185-305 GHz spectrometer, of five submillimeter bright lensed sources selected from the Herschel-Astrophysical Terahertz Large Area Survey science demonstration phase catalog. We construct a redshift-finding algorithm using combinations of the signal to noise of all the lines falling in the Z-Spec bandpass to determine redshifts with high confidence, even in cases where the signal to noise in individual lines is low. We measure the dust continuum in all sources and secure CO redshifts for four out of five (z ∼ 1.5-3). In one source, SDP.17, we tentatively identify two independent redshifts and a water line, confirmed at z = 2.308. Our sources have properties characteristic of dusty starburst galaxies, with magnification-corrected star formation rates of 10 2−3 M yr −1. Lower limits for the dust masses (∼ a few 10 8 M) and spatial extents (∼1 kpc equivalent radius) are derived from the continuum spectral energy distributions, corresponding to dust temperatures between 54 and 69 K. In the local thermodynamic equilibrium (LTE) approximation, we derive relatively low CO excitation temperatures (100 K) and optical depths (τ 1). Performing a non-LTE excitation analysis using RADEX, we find that the CO lines measured by Z-Spec (from J = 4 → 3 to 10 → 9, depending on the galaxy) localize the best solutions to either a high-temperature/low-density region or a low/temperature/high-density region near the LTE solution, with the optical depth varying accordingly. Observations of additional CO lines, CO(1-0) in particular, are needed to constrain the non-LTE models.