Herschel-ATLAS: towards a sample of ~1000 strongly-lensed galaxies
  While the selection of strongly lensed galaxies with 500{\mu}m flux density
S_500>100 mJy has proven to be rather straightforward (Negrello et al. 2010),
for many applications it is important to analyze samples larger than the ones
obtained when confining ourselves to such a bright limit. Moreover, only by
probing to fainter flux densities is possible to exploit strong lensing to
investigate the bulk of the high-z star-forming galaxy population. We describe
HALOS (the Herschel-ATLAS Lensed Objects Selection), a method for efficiently
selecting fainter candidate strongly lensed galaxies, reaching a surface
density of ~1.5-2 deg^-2, i.e. a factor of about 4 to 6 higher than that at the
100 mJy flux limit. HALOS will allow the selection of up to ~1000 candidate
strongly lensed galaxies (with amplifications \mu>2) over the full H-ATLAS
survey area. Applying HALOS to the H-ATLAS Science Demonstration Phase field
(~14.4 deg^2) we find 31 candidate strongly lensed galaxies, whose candidate
lenses are identified in the VIKING near-infrared catalog. Using the available
information on candidate sources and candidate lenses we tentatively estimate a
~72% purity of the sample. The redshift distribution of the candidate lensed
sources is close to that reported for most previous surveys for lensed
galaxies, while that of candidate lenses extends to substantially higher
redshifts than found in the other surveys. The counts of candidate strongly
lensed galaxies are also in good agreement with model predictions (Lapi et al.
2011). Even though a key ingredient of the method is the deep near-infrared
VIKING photometry, we show that H-ATLAS data alone allow the selection of a
similarly deep sample of candidate strongly lensed galaxies with an efficiency
close to 50%; a slightly lower surface density (~1.45 deg^-2) can be reached
with a ~70% efficiency.
