First Extragalactic Detection of Far-Infrared CH Rotational Lines from
  the Herschel Space Observatory
  We present the first extragalactic detections of several CH rotational
transitions in the far-infrared (FIR) in four nearby galaxies: NGC 1068, Arp
220, M 82 and NGC 253 using the \textit{Herschel Space Observatory}. The CH
lines in all four galaxies are a factor of 2 - 4 brighter than the adjacent HCN
and HCO+ J = 6-5 lines (also detected in the same spectra). In the star
formation dominated galaxies, M 82, NGC 253 and Arp 220, the CH/CO abundance
ratio is low (1e-5), implying that the CH is primarily arising in diffuse and
translucent gas where the chemistry is driven by UV radiation as found in the
Milky Way ISM. In NGC 1068, which has a luminous AGN, the CH/CO ratio is an
order of magnitude higher suggesting that CH formation is driven by an X-ray
dominated region. Our XDR models show that both the CH and CO abundances in NGC
1068 can be explained by an XDR-driven chemistry for gas densities and
molecular hydrogen column densities that are well constrained by the CO
observations. We conclude that the CH/CO ratio may a good indicator of the
presence of AGN in galaxies. We also discuss the feasibility of detecting CH in
intermediate- to high-z galaxies with ALMA.
