THE SEARCH FOR HI EMISSION AT z ≈ 0.4 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 report the first attempt to use gravitational lensing to detect HI line emission from three gravitationally lensed galaxies behind the cluster Abell 773, two at redshifts of 0.398 and one at z = 0.487, using the Green Bank Telescope. We find that a 3 sigma upper limit for a galaxy with a rotation velocity of 200 km s(-1) is M-HI = 6.58 x 10(9) and 1.5 x 10(10) M-circle dot at z = 0.398 and z = 0.487. The estimated HI masses of the sources at z = 0.398 and z = 0.487 are factors of 3.7 and similar to 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 wideband RFI but retain the signal from the source. We are able to reduce the noise of the spectrum by similar to 25% using our routine instead of discarding observations with too much RFI. The routine is most effective when similar to 10% of the integrations or fewer contain RFI. These techniques can be used to study HI in highly magnified distant galaxies that are otherwise too faint to detect.