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Lutz et al. have used the Australia Telescope Compact Array to study
galaxies that contain more than 2.5 times the neutral hydrogen
expected based on their optical properties. The doppler shift of the
21cm line of neutral hydrogen across the galaxy enables the rotation
rate to be determined. This is illustrated for the galaxy ESO 417-G018
in the figure above, where the colour code shows the recession
velocity in kilometres per second. The outer edges of the galaxy are
rotating at 200 km/sec, resulting in an overall spread of over 400
km/sec in the rotation rate of the galaxy. The recessional velocity
of the galaxy, due to the expansion of the Universe, is 4750 km/sec.
Lutz et al. find that these "H I extreme" galaxies are H I-rich
because they can support more H I against gravitational instability
due to their high specific angular momentum. The H I content of these
galaxies might be further increased by gas-rich minor mergers.
The results are published in
"The H IX galaxy survey - II. H I kinematics of H I eXtreme galaxies"
in Monthly Notices of the Royal Astronomical Society.
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