Through the looking GLASS: HST spectroscopy of faint galaxies lensed by
  the Frontier Fields cluster MACS0717.5+3745
  The Grism Lens-Amplified Survey from Space (GLASS) is a Hubble Space
Telescope (HST) Large Program, which will obtain 140 orbits of grism
spectroscopy of the core and infall regions of 10 galaxy clusters, selected to
be among the very best cosmic telescopes. Extensive HST imaging is available
from many sources including the CLASH and Frontier Fields programs. We
introduce the survey by analyzing spectra of faint multiply-imaged galaxies and
$z\gtrsim6$ galaxy candidates obtained from the first seven orbits out of
fourteen targeting the core of the Frontier Fields cluster MACS0717.5+3745.
Using the G102 and G141 grisms to cover the wavelength range 0.8-1.7$\mu$m, we
confirm 4 strongly lensed systems by detecting emission lines in each of the
images. For the 9 $z\gtrsim6$ galaxy candidates clear from contamination, we do
not detect any emission lines down to a seven-orbit 1$\sigma$ noise level of
$\sim$5$\times$10$^{-18}$erg s$^{-1}$cm$^{-2}$. Taking lensing magnification
into account, our flux sensitivity reaches $\sim$0.2-5$\times$10$^{-18}$erg
s$^{-1}$cm$^{-2}$. These limits over an uninterrupted wavelength range rule out
the possibility that the high-$z$ galaxy candidates are instead strong line
emitters at lower redshift. These results show that by means of careful
modeling of the background - and with the assistance of lensing magnification -
interesting flux limits can be reached for large numbers of objects, avoiding
pre-selection and the wavelength restrictions inherent to ground-based
multi-slit spectroscopy. These observations confirm the power of slitless HST
spectroscopy even in fields as crowded as a cluster core.
