The Search for HI Emission at z $\approx0.4$ in Gravitationally Lensed
  Galaxies with the Green Bank Telescope
  Neutral Hydrogen (HI) provides a very important fuel for star formation, but
is difficult to detect at high redshift due to weak emission, limited
sensitivity of modern instruments, and terrestrial radio frequency interference
(RFI) at low frequencies. We the first attempt to use gravitational lensing to
detect HI line emission from three gravitationally lensed galaxies behind the
cluster Abell 773, two at redshift of 0.398 and one at z=0.487, using the Green
Bank Telescope. We find a 3 sigma upper limit for a galaxy with a rotation
velocity of 200 km/s is M_HI=6.58x10^9 and 1.5x10^10 M_solar at z=0.398 and
z=0.487. The estimated HI masses of the sources at z=0.398 and z=0.487 are a
factor of 3.7 and ~30 times lower than our detection limits at the respective
redshifts. To facilitate these observations we have used sigma clipping to
remove both narrow- and wide-band RFI but retain the signal from the source. We
are able to reduce the noise of the spectrum by ~25% using our routine instead
of discarding observations with too much RFI. The routine is most effective
when ~10 of the integrations or fewer contains RFI. These techniques can be
used to study HI in highly magnified distant galaxies that are otherwise too
faint to detect.
