MEASUREMENT OF 21 cm BRIGHTNESS FLUCTUATIONS AT z ∼ 0.8 IN CROSS-CORRELATION

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 H I b H I r = [0.43 ± 0.07(stat.) ± 0.04(sys.)] × 10 −3 , where Ω H I is the neutral hydrogen (H i ) fraction, r is the galaxy–hydrogen correlation coefficient, and b H I 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.