2018ApJ...860..134Q

It is widely known that in active galactic nuclei (AGNs) and black hole X-ray binaries (BHXBs), there is a tight correlation among their radio luminosity (L_R), X-ray luminosity (L_X), and BH mass (M_BH), the so-called "fundamental plane" (FP) of BH activity. Yet the supporting data are very limited in the M_BH regime between stellar mass (i.e., BHXBs) and 10^6.5 M_☉ (namely, the lower bound of supermassive BHs in common AGNs). In this work, we developed a new method to measure the 1.4 GHz flux directly from the images of the VLA FIRST survey, and apply it to the type-1 low-mass AGNs in the Dong et al. sample. As a result, we obtained 19 new low-mass AGNs for FP research with both M_BH estimates (M_BH ≃ 10^5.5–6.5 M_☉), reliable X-ray measurements, and (candidate) radio detections, tripling the number of such candidate sources in the literature. Most (if not all) of the low-mass AGNs follow the standard radio/X-ray correlation and the universal FP relation fitted with the combined data set of BHXBs and supermassive AGNs by Gultekin et al.; the consistency in the radio/X-ray correlation slope among those accretion systems supports the picture that the accretion and ejection (jet) processes are quite similar in all accretion systems of different M_BH. In view of the FP relation, we speculate that the radio loudness R (i.e., the luminosity ratio of the jet to the accretion disk) of AGNs depends not only on Eddington ratio, but probably also on M_BH.