A Hard X-Ray Test of HCN Enhancements As a Tracer of Embedded Black Hole Growth

April 2020 • 2020ApJ...893..149P

Authors • 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.

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 (≲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 ( ${N}_{{\rm{H}}}\gt {10}^{24}$ ${\mathrm{cm}}^{-2}$ ) 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 ${L}_{\mathrm{AGN}}$ / ${L}_{\mathrm{IR}}$ 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 ongoing supermassive black hole accretion.

Links

• PREPRINT http://arxiv.org/abs/2004.02884
• NED https://ned.ipac.caltech.edu/uri/NED::InRefcode/2020ApJ...893..149P
• ELECTR https://doi.org/10.3847%2F1538-4357%2Fab8015
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2020ApJ...893..149P
• PDF https://stacks.iop.org/0004-637X/893/149/pdf
• DATA http://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/biblink.pl?code=2020ApJ...893..149P
• DATA https://cda.harvard.edu/chaser?obsid=10264,15052