CO Absorption Observations in the Inner Galaxy

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Session 26 -- Interstellar Medium and Radiative Shocks
Oral presentation, Monday, 9, 1995, 10:00am - 11:30am

[26.02] CO Absorption Observations in the Inner Galaxy

Henry A. (Chip) Kobulnicky, John M. Dickey (University of Minnesota)

We present $^{12}CO (1-0)$ absorption spectra obtained with the Owens Valley Millimeter Interferometer toward two low-latitude background continuum sources in the inner Galaxy. Along these lines of sight, we find that all the observed absorption features have corresponding CO emission features in our single-dish spectra. This correspondance between emission and absorption stands in contrast to other lines of sight where CO absorption features are observed at outer-Galaxy velocities without counterparts in emission (Lequeux, Allen \& Guilloteau, 1993, A\&A, 280, L23). We also present interferometer observations of absorption in the OH 1667 MHz transition along the same lines of sight. We find OH absorption features that correspond to the CO absorption lines, but with only a fraction (10\%) of the optical depth.

These observations add to the small, but growing, body of literature on CO absorption which suggests a substantial population of sub-thermally excited molecular gas in the outer Galaxy which is not detected in emission surveys. Our data presented here show no evidence for such a cold population of molecular gas in the inner Galaxy.

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