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18th of January 2024
A figure plotting the ASKAP flux density against frequency, showing an absorption dip in the spectrum
The FLASH pilot survey for HI absorption
by Aditya et al.
Neutral hydrogen (HI) is the most abundant gaseous element in the interstellar medium of galaxies, and acts as a key reservoir for star formation, particularly in its cold phase. Probing the distribution of HI is hence critical for understanding the evolution of galaxies. The HI 21-cm hyperfine transition is an excellent means of tracing neutral gas in galaxies from low to high redshifts, and the neutral gas can be detected through both HI 21-cm emission and absorption. Aditya et al. report an ASKAP search for associated HI 21-cm absorption against bright radio sources from the Molonglo Reference Catalogue (MRC) 1-Jy sample. The search uses pilot survey data from the ASKAP First Large Absorption Survey in HI (FLASH) covering the redshift range 0.42 < z < 1.00. From a sample of 62 MRC 1-Jy radio galaxies and quasars in this redshift range they report three new detections of associated HI 21-cm absorption.

The figure above shows the detection of a narrow HI 21-cm absorption line towards MRC 2216-281, where the line peak has a redshift of z=0.626. The grey shaded region represents 5𝜎 noise level. The AGN redshift previously reported in the literature, of z=0.657 ± 0.050, has a large uncertainty which means that it is not possible to assess whether the detected absorption is associated with the MRC galaxy, or if it is arising from a lower-redshift gas cloud that is intervening our line of sight.

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