Determination of z ~ 0.8 neutral hydrogen fluctuations using the 21cm 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 ∼ 0.8. Our data were acquired with the Green Bank Telescope and span the redshift range 0.6 < z < 1 over two fields totalling ≈41 deg² and 190 h of radio integration time. The dominant synchrotron foregrounds exceed the signal by ∼10³, 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 Ω_HI b_HI= [0.62^+0.23_-0.15] × 10^-3 at 68 per cent confidence with 9 per cent systematic calibration uncertainty, where Ω_HI is the neutral hydrogen (H I) fraction and b_HI is the H I bias parameter. We describe observational challenges with the present data set and plans to overcome them.