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27th 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 hydrogen (HI) gas at cosmological distances using the Australian Square Kilometre Array Pathfinder (ASKAP). FLASH is probing how the neutral interstellar medium in galaxies has evolved over the history of the Universe. The team searched for HI absorption towards 1253 radio sources in the GAMA 23 field, covering redshifts between z = 0.34 and 0.79 over a sky area of approximately 50 square degrees. In a purely blind search no detections of 21-cm absorbers were made above the reliability threshold. However, by cross matching their sample of radio sources with optical spectroscopic identifications of galaxies in the GAMA 23 field, they were able to detect 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 ASKAP-12 mosaic of the 900 MHz radio continuum in the GAMA 23 field as a grey scale image. (The Galaxy And Mass Assembly survey is a detailed multi-wavelength survey in five survey fields, with the GAMA 23 field centred on a Right Ascension of 23h.) The small circles indicate the 1253 radio sources which were examined for HI 21-cm absorption. The colour scale denotes the median RMS noise per 18.5 kHz channel as a fraction of the continuum; darker circles indicate higher sensitivity to 21-cm absorption. The small black square indicates the position of NVSS J224500−343030, towards which HI 21-cm absorption was detected. The larger black boundary denotes the extent of the GAMA 23 field. More details are given in the paper, to be published in Monthly Notices of the Royal Astronomical Society.

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