Multiwavelength observations of cirrus clouds in the North Celestial Loop: a study of the OH emission

Abstract

New 1665- and 1667-MHz OH observations were made with the Green Bank Telescope at an angular resolution of 7 arcmin of 108 locations over two diffuse gas clouds at high Galactic latitude. A linear correlation is observed between the IRAS 100 μm infrared intensity, which traces all the gas, and the OH column density, indicating that the amount of OH increases with the visual extinction above a threshold value of Av ≈ 0.5. N(OH) increases monotonically with N(H I) in up to N(OH) ≈ 0.25-0.3 x 10 14 cm -2 . At greater values of N(OH) the H I column density saturates at N(H I) ≈ 5 x 10 20 cm -2 , suggesting that there is molecular gas not traced by H I. No correlation is found between the 12 CO integrated intensity and the OH column density, indicating that OH might be a better tracer of H 2 than CO in those diffuse regions. An H I column density of at least ≈2.2-2.9 x 10 20 cm -2 is needed for the formation of OH, but in regions where the cloud is less shielded from the interstellar radiation field the value rises to N(H I) = 3.3 x 10 20 cm -2 .