A Comparison between Anomalous 6-cm H${}_2$CO Absorption and CO(1-0) Emission in the L1204/S140

Abstract

We report observations of the dust cloud L1204 with the Onsala 25-m telescope in the 6 cm (1${}_{11}-1_{10}$) transition of \htco. The observed region includes the S140 H$\alpha$ arc. This spectral line is seen here in absorption against the cosmic microwave background, indicating the presence of widespread warm molecular gas at intermediate densities. Overall, the distributions of H${}_2$CO and CO (taken from the literature) are fairly similar, though significant differences exist at small scales. Most notably, while the CO peak is nearly coincident with the S140 H$\alpha$ arc, the maximum H${}_2$CO absorption is clearly separated from it by a full 10${}^{\prime }$ beam ($\sim$ 3 pc). We argue that these differences result from differing abundances and excitation requirements. The CO(1-0) line is more optically thick and more biased towards warm gas than the H${}_2$CO 6 cm line. On the other hand, formaldehyde is more easily photodissociated and is, therefore, a poorer tracer of the molecular gas located immediately behind Photon Dominated Regions.

arXiv comment9 pages 6 figures ApJ:accepted

References in corpus (5)

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• A model for atomic and molecular interstellar gas: The Meudon PDR code
• Dense Gas in the Milky Way
• Photon Dominated Regions in Low UV Fields: A Study of The Peripheral Region of L1204/S140
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