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29th of April 2020
ASKAP FLASH detection of neutral hydrogen absorption
by Allison et al.
Allison et al. have presented early science results from the First Large Absorption Survey in H I (FLASH), a survey for 21-cm absorption lines in cold neutral hydrogen (HI) gas at cosmological distances using the Australian Square Kilometre Array Pathfinder (ASKAP). The team detected 21-cm absorption at z=0.3562 towards the galaxy NVSS J224500−343030. The absorber is associated with GAMA J22450.05−343031.7, a massive early-type galaxy.

The image above shows an optical image of GAMA J224500.05−343031.7, constructed using data from the VLT Survey Telescope KiloDegree Survey (KiDS). The dashed green circle indicates the position and width of the GAMA spectroscopic aperture. The white contours show the 9.5 GHz continuum from ATCA observations of NVSS J224500−343030 (levels are 5, 10, 20, 30, 40 and 50 mJy/beam). The ATCA restoring beam is shown in the bottom left hand corner. The horizontal bar indicates the physical scale at the redshift of the galaxy. The background galaxy is detected at radio wavelengths, but not optical, whereas the foreground source (detected in absorption in the radio spectrum) is detected at optical wavelengths but not radio. The line of sight to the background radio galaxy passes 17 kilo-parsecs from the centre of the foreground galaxy (in comparison the Earth is about 8 kilo-parsecs, ~25,000 light years from the centre of the Milky Way galaxy). More details are given in the paper, to be published in Monthly Notices of the Royal Astronomical Society.

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