Searching for intermediate-mass black holes in galaxies with low-luminosity AGN: A multiple-method approach

Aims. This work is the first stage of a campaign to search for intermediate-mass black holes (IMBHs) in low-luminosity active galactic nuclei (LLAGN) and dwarf galaxies. An additional and equally important aim of this pilot study is to investigate the consistency between the predictions of several popular black hole scaling relations and the fundamental plane (FP) of black-hole activity (FP-BH). Methods. We used well established X-ray and radio luminosity relations in accreting black holes, along with the latest scaling relations between the mass of the central black hole (M_BH) and the properties of its host spheroid, to predict M_BH in seven LLAGN, that were previously reported to be in the IMBH regime. Namely, we used the recently re-evaluated M_BH-M_sph (M_sph: spheroid absolute magnitude at 3.6 μm) scaling relation for spiral galaxies, the M_BH-n_sph (n_sph: major axis Sérsic index of the spheroid component) relation, the M_BH-PA (PA: spiral pitch angle) relation, and a recently re-calibrated version of the FP-BH for weakly accreting BHs, to independently estimate M_BH in all seven galaxies. Results. We find that all LLAGN in our list have low-mass central black holes with log M_BH/M_⊙ ≠6.5 on average, but that they are, most likely, not IMBHs. All four methods used predicted consistent BH masses in the 1σ range. Furthermore, we report that, in contrast to previous classification, galaxy NGC 4470 is bulge-less, and we also cast doubts on the AGN classification of NGC 3507. Conclusions. We find that our latest, state-of-the-art techniques for bulge magnitude & Sérsic index computations and the most recent updates of the M_BH-L_sph, M_BH-n_sph, and M_BH-PA relations and the FP-BH produce consistent results in the low-mass regime. We thus establish a multiple-method approach for predicting BH masses in the regime where their spheres of gravitational influence cannot be spatially resolved. Our approach mitigates against outliers from any one relation and provides a more robust average prediction. We will use our new method to revisit more IMBH candidates in LLAGN.