A Hard X-ray Test of HCN Enhancements as a Tracer of Embedded Black Hole Growth

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 ( $\lesssim 15\%$) 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 ($N_{\mathrm{H}}>10^{24}$ 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 $\sim 3\%$ 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 ${\mathrm{J}}_{\mathrm{upper}}$ 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.

14 pages, 4 figures. accepted for publication in The Astrophysical Journal

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