  HST is commonly thought of as an optical-IR imaging or UV-spectroscopy
observatory. However, the advent of WFC3-IR made it possible to do slitless
infrared spectroscopic surveys over an area significant for galaxy evolution
studies (~0.15 deg^2). Slitless infrared spectroscopy is uniquely possible from
space due to the reduced background. Redshift surveys with WFC3-IR offer probes
of the astrophysics of the galaxy population at z=1-3 from line features, and
the true redshift and spatial distribution of galaxies, that cannot be done
with photometric surveys alone. While HST slitless spectroscopy is low spectral
resolution, its high multiplex advantage makes it competitive with future
ground based IR spectrographs, its flux calibration is stable, and its high
spatial resolution allows measuring the spatial extent of emission lines, which
only HST can do currently for large numbers of objects. A deeper slitless IR
spectroscopic survey over hundreds of arcmin^2 (eg one or more GOODS fields) is
one of the remaining niches for large galaxy evolution studies with HST, and
would produce a sample of thousands of spectroscopically confirmed galaxies at
1<z<3 to H=25 and beyond, of great interest to a large community of
investigators. Finally, although JWST multislit spectroscopy will outstrip HST
in resolution and sensitivity, I believe it is critical to have a spectroscopic
sample in hand before JWST flies. This applies scientifically, to be prepared
for the questions we want to answer with JWST, and observationally, because
JWST's lifetime is limited and a classic problem in targeted spectroscopy has
been the turn-around time for designing surveys and for deciding which classes
of objects to target. This white paper is released publicly to stimulate open
discussion of future large HST programs.
