A pilot search for extragalactic OH absorption with FAST. Issue 3 (1st October 2020)

• Record Type: Journal Article
• Authors: Zheng, Zheng; Li, Di; Sadler, Elaine M; Allison, James R; Tang, Ningyu
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 499:Issue 3(2020)
• Page Start: 3085
• Page End: 3093
• Publication Date: 2020-10-01
• Subjects: ISM: abundances -- galaxies: evolution -- galaxies: ISM -- quasars: absorption lines -- radio lines: galaxies -- radio lines: ISM; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa3033
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: OH absorption is currently the only viable way to detect OH molecules in non-masing galaxies at cosmological distances. There have been only six such detections at z > 0.05 to date and so it is hard to put a statistically robust constraint on OH column densities in distant galaxies. We carried out a pilot OH absorption survey towards eight associated and one intervening H i 21-cm absorbers using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We were able to constrain the OH abundance relative to H i ([OH]/[H i ]) to be lower than 10 −6 ∼ 10 −8 for redshifts z∈ [0.1919, 0.2241]. Although no individual detection was made, stacking three associated absorbers free of RFI provides a sensitive OH column density 3σ upper-limit $\sim 1.57 \times 10^{14} (T_x^{\rm OH}/10\, \mathrm{ K})(1/f_\mathrm{ c}^{\rm OH})\mathrm{ cm}^{-2}$, which corresponds to a [OH]/[H i ] < 5.45 × 10 −8 . Combining with archival data, we show that associated absorbers have a slightly lower OH abundance than intervening absorbers. Our results are consistent with a trend of decreasing OH abundance with decreasing redshift.