| 18th of January 2024 |
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| The FLASH pilot survey for HI absorption |
| by Aditya et al. |
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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. |