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



Infrared galaxies


G. Privon

University of Florida

C. Ricci

Beijing University of Technology

George Mason University

Universidad Diego Portales

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

Universidad Diego Portales

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

Space Science Institute

Pontificia Universidad Catolica de Chile

Instituto Milenio de Astrofísica

A. S. Evans

University of Virginia

National Radio Astronomy Observatory

P. Garg

University of Florida

R. Herrero-Illana

Institute of Space Sciences (ICE) - CSIC

European Southern Observatory Santiago

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

Foundation for Research and Technology Hellas (FORTH)

Panepistimio Kritis

S. Stierwalt

Occidental College

California Institute of Technology (Caltech)

M.A. Pérez-Torres

CSIC - Instituto de Astrofísica de Andalucía (IAA)

Astrophysical Journal

0004-637X (ISSN) 1538-4357 (eISSN)

Vol. 893 2 149

Subject Categories

Subatomic Physics

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics



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