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

Abstract

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 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.

ALMA HCN AND HCO+ J = 3 − 2 OBSERVATIONS OF OPTICAL SEYFERT AND LUMINOUS INFRARED GALAXIES: CONFIRMATION OF ELEVATED HCN-TO-HCO+ FLUX RATIOS IN AGNS

“…Interpretations of the observed HCN-to-HCO + flux ratios are less ambiguous than the flux comparison among molecules with largely different dipole moments (e.g., HCN vs. CO). Using ALMA, a similar enhancement of observed HCN-to-HCO + flux ratios was also found at J=4-3 for LIRGs that are infrareddiagnosed to be AGN-important (Imanishi & Nakanishi 2013a,b, 2014Iono et al 2013;Garcia-Burillo et al 2014;Izumi et al 2015Izumi et al , 2016. These observations suggest that elevated HCN-to-HCO + flux ratios could be used to identify AGNs, including deeply buried (= obscured in virtually all directions) ones.…”

Disentangling the Circumnuclear Environs of Centaurus A. III. An Inner Molecular Ring, Nuclear Shocks, and the CO to Warm H2 Interface

“…However, the X-ray luminosity of Cen A is in between other low luminosity AGNs such as NGC 1097 (e.g. Nemmen et al 2006;Izumi et al 2013), and NGC 7469, and in these two cases the obtained R HCN/HCO + ratios towards their nuclei are > 1 (Izumi et al 2015). Cosmic-ray ionization rates might be higher in Cen A producing even further an over-density of HCO + over HCN.…”

“…Kohno et al 2001;Krips et al 2008;Izumi et al 2016), but this does not hold for the nuclear regions of Cen A even though it is a well known AGN. This is further discussed in § 7.…”

“…Recently the favored scenario for the enhancement of HCN around AGNs in observational data is high temperature chemistry likely due to mechanical heating (Harada et al 2010;Izumi et al 2013;Matsushita et al 2015). The increasing number of sources detected in these species seem to show in average an enhancement of the HCN/HCO + ratio in the presence of an AGN (Privon et al 2015;Izumi et al 2016;Imanishi et al 2016). However, the scatter is large among the ratios observed in both galaxies hosting AGNs and starbursts (Privon et al 2015), which makes it necessary to study individual cases at high resolution to understand the origin of such dispersion.…”

“…However, the scatter is large among the ratios observed in both galaxies hosting AGNs and starbursts (Privon et al 2015), which makes it necessary to study individual cases at high resolution to understand the origin of such dispersion. In addition, recent high resolution observations with ALMA have shown that this enhancement may occur in the surroundings and not towards the AGN itself (García-Burillo et al 2014;Martín et al 2015;Izumi et al 2016).…”