Hydroxyl (OH) is expected to be abundant in diffuse interstellar molecular gas because it forms along with H2 under similar conditions and forms within a similar extinction range. We have analyzed absorption measurements of OH at 1665 MHz and 1667 MHz toward 44 extragalactic continuum sources, together with the J = 1-0 transitions of 12CO, 13CO, and C18O, and the J = 2-1 transition of 12CO. The excitation temperatures of OH were found to follow a modified lognormal distribution the peak of which is close to the temperature of the Galactic emission background (CMB+synchrotron). In fact, 90% of the OH has excitation temperatures within 2 K of the Galactic background at the same location, providing a plausible explanation for the apparent difficulty of mapping this abundant molecule in emission. The opacities of OH were found to be small and to peak around 0.01. For gas at intermediate extinctions (A V ∼ 0.05-2 mag), the detection rate of OH with a detection limit N(OH) ≃ 1012 cm-2 is approximately independent of A V. We conclude that OH is abundant in the diffuse molecular gas and OH absorption is a good tracer of "dark molecular gas (DMG)." The measured fraction of DMG depends on the assumed detection threshold of the CO data set. The next generation of highly sensitive low-frequency radio telescopes, such as FAST and SKA, will make feasible the systematic inventory of diffuse molecular gas through decomposing, in velocity, the molecular (e.g., OH and CH) absorption profiles toward background continuum sources with numbers exceeding what is currently available by orders of magnitude.
|Journal||Astrophysical Journal, Supplement Series|
|State||Published - Mar 2018|
Bibliographical noteFunding Information:
This work is supported by the National Key R&D Program of China 2017YFA0402600 and International
Partnership Program of Chinese Academy of Sciences No. 114A11KYSB20160008. D.L. acknowledges support from the “CAS Interdisciplinary Innovation Team” program. J.R.D. is the recipient of an Australian Research Council DECRA Fellowship (project number DE170101086). This work was carried out in part at the Jet Propulsion Laboratory, which is operated for NASA by the California Institute of Technology. L.B. acknowledges support from CONICYT Project PFB06. CO data were observed with the Delingha 13.7 m telescope of the Qinghai Station of Purple Mountain Observatory (PMODLH), the Caltech Submillimeter Observatory (CSO), and the IRAM 30 m telescope. The authors appreciate all the staff members of the PMODLH, CSO, and the IRAM 30 m Observatory for their help during the observations. We thank Lei Qian and Lei Zhu for their help in CSO observations.
© 2018. The American Astronomical Society. All rights reserved..
- ISM: clouds
- ISM: molecules
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science