FLASH pilot survey: detections of associated 21 cm H i absorption in GAMA galaxies at 0.42 < z < 1.00

Abstract

We present the results of a search for associated 21 cm H i absorption at redshift 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 H i (FLASH). From a sample of 326 radio sources with 855.5 MHz peak flux density above 10 mJy we detected two associated H i absorption systems, in SDSS J090331+010847 at z = 0.522 and SDSS J113622+004852 at z = 0.563. Both galaxies are massive (stellar mass >1011 M⊙) and have optical spectra characteristic of luminous red galaxies, though SED fitting implies that SDSS J113622+004852 contains a dust-obscured starburst with SFR ∼69 M⊙ yr−1. The H i absorption lines have a high optical depth, with τpk of 1.77 ± 0.16 for SDSS J090331+010847 (the highest value for any z > 0.1 associated system found to date) and 0.14 ± 0.01 for SDSS J113622+004852. In the redshift range probed by our ASKAP observations, the detection rate for associated H i absorption lines (with τpk > 0.1 and at least 3σ significance) is $2.9_{-2.6}^{+9.7}$ per cent. Although the current sample is small, this rate 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. We also searched for OH absorption lines at 0.67 < z < 1.34, but no detection was made in the 145 radio sources searched.

FAST Discovery of a Fast Neutral Hydrogen Outflow

“…While we can directly measure some parameters, we have to assume some other parameters. We assumed that the covering factor is unit, which is an upper limit and often adopted (e.g., Aditya & Kanekar 2018a, 2018bSu et al 2022). We further assumed that Ω is the π that is often used (e.g., Morganti et al 2005;Aditya & Kanekar 2018b), and r * is half the projected size of the VLBI X-band emission (Murthy et al 2021), ∼25 pc.…”

“…Redshifted H I 21 cm absorption provides us with unique insights into the neutral gas in and around galaxies throughout the Universe (e.g., Morganti et al 2015;Allison et al 2022). Depending on whether the absorbing neutral hydrogen gas is along the line of sight or within the host galaxy of the background radio continuum source, H I absorption is classified as intervening (e.g., Kanekar et al 2009Kanekar et al , 2014Curran 2021;Gupta et al 2021a;Mahony et al 2022) or associated (e.g., Allison et al 2012Allison et al , 2014Curran et al 2016;Maccagni et al 2017;Gupta et al 2021b;Murthy et al 2021;Su et al 2022).…”