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22nd of June 2018
HI eXtreme galaxies
by Lutz et al.
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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