A pilot search for extragalactic OH absorption with FAST

TL;DR

This pilot survey using FAST aimed to detect OH absorption in distant galaxies, setting upper limits on OH abundance and observing a potential decrease in OH abundance with decreasing redshift, despite no individual detections.

Contribution

First systematic search for extragalactic OH absorption with FAST, providing upper limits on OH abundance and insights into its redshift evolution.

Findings

Upper limits on OH abundance relative to HI were established.

Stacked data yielded a sensitive upper limit on OH column density.

Associated absorbers may have lower OH abundance than intervening ones.

Abstract

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.