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 6 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 8 associated and 1 intervening HI 21-cm absorbers
using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We were
able to constrain the OH abundance relative to HI ([OH]/[HI]) to be lower than
10^-6 ~ 10^-8 for redshifts z within [0.1919, 0.2241]. Although no individual
detection was made, stacking three associated absorbers free of RFI provides a
sensitive OH column density 3-sigma upper-limit ~ 1.57 x 10^14 (Tx/10K)(1/fc)
cm^-2, which corresponds to a [OH]/[HI] < 5.45 x 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.
