First Extragalactic Detection of Far-Infrared CH Rotational Lines from the Herschel Space Observatory

TL;DR

This study reports the first extragalactic detection of far-infrared CH rotational lines in four nearby galaxies using Herschel, revealing different chemical environments and suggesting CH/CO ratio as an AGN indicator.

Contribution

It presents the first detection of extragalactic far-infrared CH lines and analyzes their relation to galaxy activity, highlighting the potential of CH/CO ratio as an AGN diagnostic.

Findings

CH lines are brighter than HCN and HCO+ lines in the spectra.

Low CH/CO ratio in star-forming galaxies indicates diffuse gas chemistry.

High CH/CO ratio in NGC 1068 suggests X-ray driven chemistry from an AGN.

Abstract

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.