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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 516; no. 2
• Main Authors: Su, Renzhi, Sadler, Elaine M., Allison, James R., Mahony, Elizabeth K., Moss, Vanessa A., Whiting, Matthew T., Yoon, Hyein, Aditya, J. N. H. S., Bellstedt, Sabine, Robotham, Aaron S. G., Garratt-Smithson, Lilian, Gu, Minfeng, Koribalski, Bärbel S., Soria, Roberto, Weng, Simon
• Format: Journal Article
• Language: English
• Published: United States Oxford University Press 18-08-2022
• Subjects: Astronomy & Astrophysics
• ISSN: 0035-8711; 1365-2966

We present the results of a search for associated 21 cm H 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 Australian SKA Pathfinder (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 $\gt 10^{11}\, \mathrm{M}_\odot$) and have optical spectra characteristic of luminous red galaxies, though spectral energy distribution fitting implies that SDSS J113622+004852 contains a dust-obscured starburst with star formation rate ~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 $2.9_{-2.6}^{+9.7}$ 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 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.