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11th of January 2024
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. Absorption can be detected against a background radio source which could be powered by an active galactic nucleus (AGN). The signal strength is dependent on both the HI column density and the strength of the background radio source, and hence, against bright radio sources the absorption strength is independent of redshift.

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 image above shows the detection of strong associated HI 21-cm absorption towards MRC 0531-237, with a peak absorption fraction of 0.35. The dotted lines are Gaussian fits to the absorption. The vertical line represents the AGN redshift of z=0.851. The grey shaded region represents 5𝜎 noise level.

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