2007ApJ...663..824R

We report observations of the dust cloud L1204 with the Onsala 25 m telescope in the 6 cm (1₁₁-1₁₀) transition of H₂CO. The observed region includes the S140 Hα 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 H₂ CO and CO (taken from the literature) are fairly similar, although significant differences exist at small scales. Most notably, while the CO peak is nearly coincident with the S140 Hα arc, the maximum H₂CO absorption is clearly separated from it by a full 10' beam (∼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 toward warm gas than the H₂ 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.