FLASH Pilot Survey: Detections of associated 21 cm HI absorption in GAMA galaxies at 0.42 < z <1.00

Renzhi Su, Elaine M. Sadler, James R. Allison, Elizabeth K. Mahony, Vanessa A. Moss, Matthew T. Whiting, Hyein Yoon, J. N. H. S. Aditya, Sabine Bellstedt, Aaron S. G. Robotham, Lilian Garratt-Smithson, Minfeng Gu, Baerbel S. Koribalski, Roberto Soria, Simon Weng

Published 2022-08-09Version 1

We present the results of a search for associated 21 cm HI 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 HI (FLASH). From a sample of 326 radio sources with 855.5 MHz peak flux density above 10 mJy we detected two associated HI absorption systems, in SDSS J090331+010847 at z= 0.522 and SDSS J113622+004852 at z= 0.563. Both galaxies are massive (stellar mass > 10$^{11}$ M$_{sun}$) 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$_{sun}$ yr$^{-1}$. The HI absorption lines have a high optical depth, with $\tau_{pk}$ of 1.77 $\pm$ 0.16 for SDSS J090331+010847 (the highest value for any z > 0.1 associated system found to date) and 0.14 $\pm$ 0.01 for SDSS J113622+004852. In the redshift range probed by our ASKAP observations, the detection rate for associated HI absorption lines (with $\tau_{pk}$ > 0.1 and at least 3$\sigma$ significance) is 2.9 (+9.7 -2.6) percent. 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 HI 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.