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arXiv · 1512.03438

Submillimeter-HCN Diagram for an Energy Diagnostics in the Centers of Galaxies

Takuma Izumi·Kotaro Kohno·Susanne Aalto·Daniel Espada·Kambiz Fathi·Nanase Harada·Bunyo Hatsukade·Pei-Ying Hsieh·Masatoshi Imanishi·Melanie Krips·Sergio Martín·Satoki Matsushita·David S. Meier·Naomasa Nakai·Kouichiro Nakanishi·Eva Schinnerer·Kartik Sheth·Yuichi Terashima·Jean L. Turner
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Abstract

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.

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Takuma Izumi, Kotaro Kohno, Susanne Aalto, Daniel Espada, Kambiz Fathi, Nanase Harada, Bunyo Hatsukade, Pei-Ying Hsieh, Masatoshi Imanishi, Melanie Krips, Sergio Martín, Satoki Matsushita, David S. Meier, Naomasa Nakai, Kouichiro Nakanishi, Eva Schinnerer, Kartik Sheth, Yuichi Terashima, Jean L. Turner. 2015-12-10. Submillimeter-HCN Diagram for an Energy Diagnostics in the Centers of Galaxies. https://doi.org/10.3847/0004-637x%2F818%2F1%2F42

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