A hard x-ray test of hcn enhancements as a tracer of embedded black hole growth
Journal article, 2020
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 (≲15%) 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 (NH > 1024 cm-2) AGN to their bolometric luminosity. The upper limits on the X-ray flux are presently an order of magnitude below what XDR-driven chemistry models 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 ∼3% 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 J upper levels) and LAGN/LIR f 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 ongoing supermassive black hole accretion. © 2020. The American Astronomical Society. All rights reserved
Dust
Galaxies
Infrared galaxies
Author
G. Privon
University of Florida
C. Ricci
Beijing University of Technology
George Mason University
Diego Portales University
Susanne Aalto
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Serena Viti
University College London (UCL)
Lee Armus
California Institute of Technology (Caltech)
T. Diaz-Santos
Chinese Academy of Sciences
Diego Portales University
E. Gonzalez-Alfonso
University of Alcalá
K. Iwasawa
Catalan Institution for Research and Advanced Studies
University of Barcelona
D. L. Jeff
University of Florida
E. Treister
Pontificia Universidad Catolica de Chile
F. E. Bauer
Millennium Institute of Astrophysics
Pontificia Universidad Catolica de Chile
Space Science Institute
A. S. Evans
National Radio Astronomy Observatory
University of Virginia
P. Garg
University of Florida
R. Herrero-Illana
European Southern Observatory Santiago
Institute of Space Sciences (ICE) - CSIC
J. Mazzarella
California Institute of Technology (Caltech)
K. Larson
California Institute of Technology (Caltech)
L. Blecha
University of Florida
L. Barcos-Muoz
National Radio Astronomy Observatory
V. Charmandaris
University of Crete
Foundation for Research and Technology Hellas (FORTH)
S. Stierwalt
California Institute of Technology (Caltech)
Occidental College
M.A. Pérez-Torres
Institute of Astrophysics of Andalusia (IAA)
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 893 2 149Subject Categories
Subatomic Physics
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
DOI
10.3847/1538-4357/ab8015