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

We investigate the origin of the X-ray emission in low-luminosity active galactic nuclei (LLAGNs). Yuan and Cui predicted that the X-ray emission should originate from jets rather than an advection-dominated accretion flow (ADAF) when the X-ray luminosity L _X of the source is below a critical value of L _X,crit ≈ 10^-6 L _Edd. This prediction implies that the X-ray spectrum in such sources should be fitted by jets rather than ADAFs. Furthermore, below L _X,crit the correlation between radio (L _R) and X-ray (L _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 et al. and become steeper. The Merloni et al. correlation is described by logL _R = 0.6logL _X + 0.78logM + 7.33, while the predicted correlation is logL _R = 1.23logL _X + 0.25logM - 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 and Cui prediction. For the 22 LLAGNs with L _X < L _X,crit, the fundamental plane correlation is described by logL _R = 1.22logL _X + 0.23logM - 12.46, also in excellent agreement with the prediction.