Submillimeter-HCN Diagram for an Energy Diagnostics in the Centers of
  Galaxies
  Compiling data from literature and the ALMA archive, we show enhanced
HCN(4-3)/HCO$^+$(4-3) and/or HCN(4-3)/CS(7-6) integrated intensity ratios in
circumnuclear molecular gas around active galactic nuclei (AGNs) compared to
those in starburst (SB) galaxies (submillimeter HCN-enhancement). The number of
sample galaxies is significantly increased from our previous work. We expect
this feature could potentially be an extinction-free energy diagnostic tool of
nuclear regions of galaxies. Non-LTE radiative transfer modelings of the above
molecular emission lines involving both collisional and radiative excitation,
as well as a photon trapping effect were conducted to investigate the cause of
the high line ratios in AGNs. As a result, we found that enhanced abundance
ratios of HCN-to-HCO$^+$ and HCN-to-CS in AGNs as compared to SB galaxies by a
factor of a few to even $>$ 10 is a plausible explanation for the submillimeter
HCN-enhancement. However, a counter argument of a systematically higher gas
density in AGNs than in SB galaxies can also be a plausible scenario. Although
we could not fully discriminate these two scenarios at this moment due to
insufficient amount of multi-transition, multi-species data, the former
scenario equivalently claims for abnormal chemical composition in AGNs.
Regarding the actual mechanism to realize the composition, we suggest it is
difficult with conventional gas phase X-ray dominated region (XDR) ionization
models to reproduce the observed high line ratios. We might have to take into
account other mechanisms such as neutral-neutral reactions that are efficiently
activated at high temperature environments and/or mechanically heated regions
to further understand the high line ratios in AGNs.
