The 21cm neutral hydrogen (HI) spectral line can be detected from
galaxies in the nearby universe, but is too faint to detect from
individual galaxies at even moderate redshifts. However,
three-dimensional intensity mapping allows the study of large-scale
structure directly by detecting the aggregate emission from many
galaxies in a large volume. Li et al. report results from a neutral
hydrogen (HI) intensity mapping survey conducted with a Phased Array
Feed (PAF) on the Parkes telescope in 2016.
The initial results presented by Li et al. target a narrow redshift range of 0.73 < z < 0.78 where
the effect of radio frequency interference (RFI) was less problematic.
The image above shows the
the rms noise level in the gridded data cubes for each field over the frequency range
considered in the paper. The frequency channels in red were masked as they were affected by RFI.
The frequency channels
in blue correspond to the final data cube.
reports the detection of a cross-correlation signal between the
HI intensity maps and WiggleZ redshift data.
A future Parkes
cryogenic PAF is expected to detect the cross-correlation signal
with higher accuracy than previously possible and allow measurement of the cosmic HI density up
to redshifts of ~1.