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2nd of September 2022
ASKAP FLASH detections of neutral hydrogen absorption in GAMA galaxies
by Su et al.
Hydrogen is the most common element in the Universe and neutral hydrogen provides the main fuel for star formation in galaxies, which is known to evolve across cosmic time. Observations of the distribution, mass and kinematics of neutral hydrogen in galaxies help to advance our understanding of their assembly and evolution. In its neutral atomic state, hydrogen can be traced in the radio band (either in emission or in absorption) through a hyperfine transition at a rest-frame frequency of 1420.4 MHz, corresponding to a wavelength of 21cm. The 21 cm line emission is faint, which makes the detections difficult beyond the local Universe. In contrast, the detection sensitivity for HI absorption is independent of redshift and depends only on the brightness of the background radio source used as a probe. HI absorption searches can detect both intervening lines (which trace neutral gas in and around the general population of distant galaxies) and associated lines (which allow us to trace the kinematics of neutral gas within radio-loud AGN).

Su et al. present the results of a search for associated 21 cm HI absorption at redshifts 0.42 < 𝑧 < 1.00 in radio-loud galaxies from three Galaxy And Mass Assembly (GAMA) survey fields. These observations were carried out as part of a pilot survey for the ASKAP First Large Absorption Survey in HI (FLASH). From a sample of 326 radio sources with 855.5 MHz peak flux density above 10 mJy, they detected two associated HI absorption systems, both of which are massive (> 100 billion solar masses) and have optical spectra characteristic of luminous red galaxies. The detection rate for associated HI absorption lines is consistent with a trend, seen in other studies, for a lower detection rate of associated 21 cm H i absorption systems at higher redshift. The plot above shows a portion of the ASKAP spectrum towards SDSS J090331+010847. The black line is the continuum subtracted flux density (left Y-axis), or the fraction of the continuum flux density (right Y-axis), as a function of frequency (lower X-axis) or redshift (upper X-axis). The grey region denotes 5 times the rms noise. An absorption line is visible at 933.352 MHz, which is consistent with the GAMA spectroscopic redshift of SDSS J090331+010847.

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