| 2nd of September 2022 |
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| ASKAP FLASH detections of neutral hydrogen absorption in GAMA galaxies |
| by Su et al. |
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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. |