2002ApJ...575..871R

H₂CO observations in the 1₁₀-1₁₁ transition (6 cm) obtained with the Very Large Array in the direction of the supernova remnant Cas A are presented. Absorption images are obtained with a resolution of 6" and a sensitivity of ≲0.003. Molecular structures in the Perseus arm are observed between -34 and -49 km.s^–1. The distribution of the molecular gas is significantly clumpy. Seventy-two clumps could be identified, with sizes of 0.17±0.05 pc, densities between 0.3 and 14x10⁴ cm^–3, and masses below 10 M_☉. In agreement with previous studies, most of the clumps, if not all, appear to be dispersing. The high H₂ column density to the west of Cas A accounts for the lack of optical emission in this region. Moreover, the optical extinction, estimated to be 8 mag toward the expansion center of Cas A, might explain why the supernova explosion was not observed in the 17th century. A major objective of the present survey is to search for signs of a possible interaction between Cas A and a western cloud. A striking correlation between the continuum emission and the H₂CO absorption distribution, and slight spectral broadenings toward the west, are the only evidence that may suggest that Cas A is interacting with an external cloud. However, the evidence is not enough to draw any definitive conclusion.