Revisiting the "Fundamental Plane" of Black Hole Activity at Extremely Low Luminosities

Abstract

We investigate the origin of the X-ray emission in low-luminosity AGNs (LLAGNs). Yuan & Cui (2005) predicted that the X-ray emission should originate from jets rather than from an advection-dominated accretion flow (ADAF) when the X-ray luminosity $L_{\mathrm{X}}$ of the source is below a critical value of $L_{\mathrm{X},\mathrm{crit}}\approx 10^{-6}{L}_{\mathrm{Edd}}$. This prediction implies that the X-ray spectrum in such sources should be fitted by jets rather than ADAFs. Furthermore, below $L_{\mathrm{X},\mathrm{crit}}$ the correlation between radio ($L_{\mathrm{R}}$) and X-ray ($L_{\mathrm{X}}$) luminosities and the black hole mass ($M$)--the so-called fundamental plane of black hole activity--should deviate from the general correlation obtained by Merloni, Heinz & Di Matteo (2003) and become steeper. The Merloni et al. correlation is described by $\log L_{\mathrm{R}}=0.6\log L_{\mathrm{X}}+0.78\log M+7.33$, while the predicted correlation is $\log L_{\mathrm{R}}=1.23\log L_{\mathrm{X}}+0.25\log M-13.45$. We collect data from the literature to check the validity of these two expectations. We find that among the 16 LLAGNs with good X-ray and radio spectra, 13 are consistent with the Yuan & Cui prediction. For the 22 LLAGNs with $L_{\mathrm{X}}<L_{\mathrm{X},\mathrm{crit}}$, the fundamental plane correlation is described by $\log L_{\mathrm{R}}=1.22\log L_{\mathrm{X}}+0.23\log M-12.46$, also in excellent agreement with the prediction.

24 pages, 2 tables, 3 figures; accepted by ApJ