Abstract

The interstellar hydride hydroxyl (OH) is a potential tracer of CO-dark molecular gas. We present new high-sensitivity absorption line observations of the four ground state hyperfine-splitting transitions of OH at 18 cm toward four Galactic and extragalactic continuum sources as follow-up to the THOR survey. We compared these to deep observations of the [C II] 158 mu m line at 1.9 THz obtained with the upGREAT instrument on SOFIA, observations of the neutral atomic hydrogen (H I) 21 cm line with the VLA, and CO (J = 2-1) lines obtained with the APEX PI230 receiver at the APEX 12 m sub-mm telescope. We trace OH over a large range of molecular hydrogen column densities of 7.9 x 10¹⁹ cm^-2 to 4.7 x 10²² cm^-2, and derive OH abundances with respect to molecular and total hydrogen column densities of XOH,H2 = NOH/NH2 = 1.2-0.2^+0.3 x 10^-7 and XOH,H = NOH/NH = 4.8-0.8^+0.9 x 10^-8, respectively. Increased sensitivity and spectral resolution allowed us to detect weak and narrow features with the lowest column density detected at NOH = 3.7 x 10¹³ cm^-2. The increase in sensitivity is a factor of five in direct comparison at the resolution the OH observations in the THOR survey (1.5 km s^-1). We identify only one OH absorption component out of 23 without CO counterpart, yet several with intermediate molecular gas fractions (fmol <= 0.8). A potential association of [C II] 158 mu m emission with an OH absorption component is seen toward one sightline. Our results confirm that OH absorption traces molecular gas across diffuse and dense environments of the interstellar medium. At the sensitivity limits of the present observations our detection of only one CO-dark molecular gas feature appears to be tracing only the upper end of the distribution of CO-dark OH features found by previous studies. We conclude that if OH absorption was to be used as a CO-dark molecular gas tracer, deeper observations or stronger background targets are necessary to unveil its full potential as a CO-dark molecular gas tracer, and yet it is not an exclusive tracer of CO-dark molecular gas. For OH hyperfine-splitting transitions in the vicinity of photodissociation regions in W43-South, we detect a spectral and spatial offset between the peak of the inversion of the OH 1612 MHz line and the absorption of the OH 1720 MHz line on the one hand, and the absorption of the OH main lines on the other hand, which provides additional constraints on the interpretation of the OH 18 cm line signatures typical of HII regions.