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 hr of radio integration time. The cross-correlation constrains Ω HI b HI r = [0.43 ± 0.07(stat.) ± 0.04(sys.)] × 10 −3 , where Ω HI is the neutral hydrogen (H I) fraction, r is the galaxy-hydrogen correlation coefficient, and b HI is the H I 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.
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