학술논문
A Hard X-ray Test of HCN Enhancements as a Tracer of Embedded Black Hole Growth
Document Type
Working Paper
Author
Privon, G. C.; Ricci, C.; Aalto, S.; Viti, S.; Armus, L.; Díaz-Santos, T.; González-Alfonso, E.; Iwasawa, K.; Jeff, D. L.; Treister, E.; Bauer, F.; Evans, A. S.; Garg, P.; Herrero-Illana, R.; Mazzarella, J. M.; Larson, K.; Blecha, L.; Barcos-Muñoz, L.; Charmandaris, V.; Stierwalt, S.; Pérez-Torres, M. A.
Source
ApJ 893 149 (2020)
Subject
Language
Abstract
Enhanced emission from the dense gas tracer HCN (relative to HCO$^+ $) has been proposed as a signature of active galactic nuclei (AGN). In a previous single-dish millimeter line survey we identified galaxies with HCN/HCO$ ^+ $ (1-0) intensity ratios consistent with those of many AGN but whose mid-infrared spectral diagnostics are consistent with little to no ( $\lesssim15\% $) contribution of an AGN to the bolometric luminosity. To search for putative heavily obscured AGN, we present and analyze \nustar hard X-ray (3-79 keV) observations of four such galaxies from the Great Observatories All-sky LIRG Survey. We find no X-ray evidence for AGN in three of the systems and place strong upper limits on the energetic contribution of any heavily obscured ($N_{\rm H}>10^{24}$ cm$^{-2}$) AGN to their bolometric luminosity. The X-ray flux upper limits are presently an order of magnitude below what XDR-driven chemistry model predict are necessary to drive HCN enhancements. In a fourth system we find a hard X-ray excess consistent with the presence of an AGN, but contributing only $\sim3\%$ of the bolometric luminosity. It is also unclear if the AGN is spatially associated with the HCN enhancement. We further explore the relationship between HCN/HCO$^+$ (for several $\mathrm{J}_\mathrm{upper}$ levels) and $L_\mathrm{AGN}/L_\mathrm{IR}$ for a larger sample of systems in the literature. We find no evidence for correlations between the line ratios and the AGN fraction derived from X-rays, indicating that HCN/HCO$^+$ intensity ratios are not driven by the energetic dominance of AGN, nor are they reliable indicators of whether SMBH accretion is ongoing.
Comment: 14 pages, 4 figures. accepted for publication in The Astrophysical Journal
Comment: 14 pages, 4 figures. accepted for publication in The Astrophysical Journal