A pilot search for extragalactic OH absorption with FAST

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.