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 × 1014 cm-2. At greater values of N(OH) the HI column density saturates at N(H I) ≈ 5 × 1020 cm-2, suggesting that there is molecular gas not traced by HI. No correlation is found between the 12CO integrated intensity and the OH column density, indicating that OH might be a better tracer of H2 than CO in those diffuse regions. An HI column density of at least ≈2.2-2.9 × 1020 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 × 1020 cm-2. © 2010 The Authors. Journal compilation. © 2010 RAS.