A seven square degrees survey for galaxy-scale gravitational lenses with
  the HST imaging archive
  We present the results of a visual search for galaxy-scale gravitational
lenses in nearly 7 square degrees of Hubble Space Telescope (HST) images. The
dataset comprises the whole imaging data ever taken with the Advanced Camera
for Surveys (ACS) in the filter F814W (I-band) up to August 31st, 2011, i.e.
6.03 square degrees excluding the field of the Cosmic Evolution Survey (COSMOS)
which has been the subject of a separate visual search. In addition, we have
searched for lenses in the whole Wide Field Camera 3 (WFC3) near-IR imaging
dataset in all filters (1.01 square degrees) up to the same date. Our primary
goal is to provide a sample of lenses with a broad range of different
morphologies and lens-source brightness contrast in order estimate a lower
limit to the number of galaxy-scale strong lenses in the future Euclid survey
in its VIS band. Our criteria to select lenses are purely morphological as we
do not use any colour or redshift information.The final candidate selection is
very conservative hence leading to a nearly pure but incomplete sample. We find
49 new lens candidates: 40 in the ACS images and 9 in the WFC3 images. Out of
these, 16 candidates are secure lenses owing to their striking morphology, 21
more are very good candidates, and 12 more have morphologies compatible with
gravitational lensing but also compatible with other astrophysical objects. It
is therefore insensitive to cosmic variance and allows to estimate the number
of galaxy-scale strong lenses on the sky for a putative survey depth, which is
the main result of the present work. Because of the incompleteness of the
sample, the estimated lensing rates should be taken as lower limits. Using
these, we anticipate that a 15 000 square degrees space survey such as Euclid
will find at least 60 000 galaxy-scale strong lenses down to a limiting AB
magnitude of I = 24.5 (10-sigma) or I = 25.8 (3-sigma).
