2021ApJS..257...47T

We present high-sensitivity CH 9 cm ON/OFF observations toward 18 extragalactic continuum sources that have been detected with OH 18 cm absorption in the Millennium survey with the Arecibo telescope. CH emission was detected toward 6 of the 18 sources. The excitation temperature of CH has been derived directly through analyzing all detected ON and OFF velocity components. The excitation temperature of CH 3335 MHz transition ranges from -54.5 to -0.4 K and roughly follows a log-normal distribution peaking within [-5, 0] K, which implies overestimation by 20% to more than 10 times during calculating CH column density by assuming the conventional value of -60 or -10 K. Furthermore, the column density of CH would be underestimated by a factor of 1.32 ± 0.03 when adopting local thermal equilibrium assumption instead of using the CH three hyperfine transitions. We found a correlation between the column density of CH and OH following log N(CH) = (1.80 ± 0.49) log N(OH) -11.59 ± 6.87. The linear correlation between the column density of CH and H₂ is consistent with that derived from visible wavelengths studies, confirming that CH is one of the best tracers of H₂components in diffuse molecular gas.