SUBMILLIMETER-HCN DIAGRAM FOR ENERGY DIAGNOSTICS IN THE CENTERS OF GALAXIES
Journal article, 2016
Compiling data from literature and the Atacama Large Millimeter/submillimeter Array 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 that 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 greater than or similar to 10 are a plausible explanation for the submillimeter HCN. enhancement. However, a counterargument of a systematically higher gas density in AGNs than in SB galaxies can also be a plausible scenario. Although we cannot fully distinguish. these two scenarios at this moment owing to an insufficient amount of multi-transition, multi-species data, the former scenario is indicative of abnormal chemical composition in AGNs. Regarding the actual mechanism to realize the composition, we suggest that it is difficult with conventional gas-phase X-ray-dominated region 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 in high-temperature environments and/or mechanically heated regions to further understand the high line ratios in AGNs.
ISM: molecules
galaxies: ISM
galaxies: active
Author
T. Izumi
University of Tokyo
K. Kohno
Research Center for the Early Universe
University of Tokyo
Susanne Aalto
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
D. Espada
National Astronomical Observatory of Japan
The Graduate University for Advanced Studies (SOKENDAI)
Atacama Large Millimeter-submillimeter Array (ALMA)
K. Fathi
Stockholm Observatory
N. Harada
Academia Sinica
B. Hatsukade
National Astronomical Observatory of Japan
P. Y. Hsieh
Academia Sinica
National Central University
M. Imanishi
National Astronomical Observatory of Japan
The Graduate University for Advanced Studies (SOKENDAI)
M. Krips
Institut de Radioastronomie Millimétrique (IRAM)
S. Martin
European Southern Observatory Santiago
Atacama Large Millimeter-submillimeter Array (ALMA)
Institut de Radioastronomie Millimétrique (IRAM)
S. Matsushita
Academia Sinica
D. S. Meier
New Mexico Institute of Mining and Technology
N. Nakai
University of Tsukuba
K. Nakanishi
National Astronomical Observatory of Japan
The Graduate University for Advanced Studies (SOKENDAI)
Atacama Large Millimeter-submillimeter Array (ALMA)
E. Schinnerer
Max Planck Society
K. Sheth
National Aeronautics and Space Administration (NASA)
Y. Terashima
Ehime University
J. L. Turner
University of California
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 818 1 42Subject Categories
Astronomy, Astrophysics and Cosmology
Roots
Basic sciences
DOI
10.3847/0004-637x/818/1/42