| 19th 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 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. |