We report on deep near-infrared observations obtained with the Wide
Field Camera-3 (WFC3) onboard the Hubble Space Telescope (HST) of the
first five confirmed gravitational lensing events discovered by the
Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). We succeed
in disentangling the background galaxy from the lens to gain separate
photometry of the two components. The HST data allow us to significantly
improve on previous constraints of the mass in stars of the lensed
galaxy and to perform accurate lens modelling of these systems, as
described in the accompanying paper by Dye et al. We fit the spectral
energy distributions of the background sources from near-IR to
millimetre wavelengths and use the magnification factors estimated by
Dye et al. to derive the intrinsic properties of the lensed galaxies. We
find these galaxies to have star-formations rates (SFR) ˜ 400-2000
M⊙ yr-1, with ˜(6-25) ×
1010 M⊙ of their baryonic mass already turned
into stars. At these rates of star formation, all remaining molecular
gas will be exhausted in less than ˜100 Myr, reaching a final mass
in stars of a few 1011 M⊙. These galaxies are
thus proto-ellipticals caught during their major episode of star
formation, and observed at the peak epoch (z ˜ 1.5-3) of the
cosmic star formation history of the Universe.