An upper limit on the OH abundance in the intercloud medium.
A search for weak OH emission at 1665 and 1667 MHz was carried out in three high-latitude directions containing no compact clouds of H I or dust. No OH emission was detected down to low limits, and the comparison of our upper limits with the known H I and dust-column densities reinforces the suggestion that OH tends to be concentrated to regions of high dust or gas density.
Knapp, G. R.↗