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19th 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 therefore 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 image above shows the HI 21-cm absorption spectrum toward the Compact Steep Spectrum source MRC 2156-245. The vertical dashed lines represent the redshifts determined from five different transitions in optical spectra, and the vertical dotted line at 0 km/s represents the reference for the average redshift, z = 0.862. The associated absorption feature can be seen near a velocity of 1000 km/s compared to the systemic recessional velocity of the source. "Associated" HI 21-cm absorption, defined to be within 3000 km/s of the galaxy redshift, is thought to arise from within the host galaxy. The grey shaded region represents 5𝜎 noise level -- while no individual spectral channel exceeds the 5𝜎 significance level, the grouping of neighbouring channels clearly signifies an absorption feature.

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