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Dickey et al. have used the Australian Square Kilometre Array
Pathfinder to measure 21-cm absorption spectra toward continuum
background sources, in order to study the cool phase of the neutral
atomic gas in our Galaxy.
The figures above show a comparison with spectra from the Southern
Galactic Plane Survey (SGPS), undertaken with the ATCA and Parkes 64m telescope, Murriyang.
The blue shading indicates the 1-sigma noise level in the SGPS data, which is
typically larger than that of the GASKAP spectra by a factor of three
to ten. The two panels are spectra toward bright continuum
sources, so that discrepencies between the two surveys are due to
emission fluctuations causing confusion that manifests as positive or
negative “pseudo-absorption”.
The GASKAP spectra are rich with absorption features at negative velocities,
corresponding to structures in the cool neutral medium of
the inner Galaxy. Particularly prominent is the line at a veolcity of −30 km/s
that is due to cold clouds in the Scutum-Centaurus Arm at distance of ~2.8kpc
from the solar system and at Galactocentric radius of ~5.8 kpc.
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