Determination of z ∼ 0.8 neutral hydrogen fluctuations using the 21 cm intensity mapping autocorrelation

The large-scale distribution of neutral hydrogen in the Universe will be luminous through its 21 cm emission. Here, for the first time, we use the auto-power spectrum of 21 cm intensity fluctuations to constrain neutral hydrogen fluctuations at z similar to 0.8. Our data were acquired with the Green Bank Telescope and span the redshift range 0.6 < z < 1 over two fields totalling approximate to 41 deg(2) and 190 h of radio integration time. The dominant synchrotron foregrounds exceed the signal by similar to 10(3), but have fewer degrees of freedom and can be removed efficiently. Even in the presence of residual foregrounds, the auto-power can still be interpreted as an upper bound on the 21 cm signal. Our previous measurements of the cross-correlation of 21 cm intensity and the WiggleZ galaxy survey provide a lower bound. Through a Bayesian treatment of signal and foregrounds, we can combine both fields in auto-and cross-power into a measurement of Omega(H I)b(H I) = [0.62(0.15)(+0.23)] x 10(-3) at 68 per cent confidence with 9 per cent systematic calibration uncertainty, where Omega(H I) is the neutral hydrogen (H I) fraction and b(H I) is the HI bias parameter. We describe observational challenges with the present data set and plans to overcome them.