  In this letter, 21 cm intensity maps acquired at the Green Bank Telescope are
cross-correlated with large-scale structure traced by galaxies in the WiggleZ
Dark Energy Survey. The data span the redshift range 0.6 < z < 1 over two
fields totaling ~41 deg. sq. and 190 hours of radio integration time. The
cross-correlation constrains Omega_HI b_HI r = [0.43 \pm 0.07 (stat.) \pm
0.04(sys.)] x 10^-3, where Omega_HI is the neutral hydrogen HI fraction, r is
the galaxy-hydrogen correlation coefficient, and b_HI is the HI bias parameter.
This is the most precise constraint on neutral hydrogen density fluctuations in
a challenging redshift range. Our measurement improves the previous 21 cm
cross-correlation at z ~ 0.8 both in its precision and in the range of scales
probed.
