Central black hole growth in the intermediate-mass regime

The origin of supermassive black holes (SMBHs) in galaxy centers remains one of the major unsolved mysteries of modern astrophysics. Were SMBH seeds ‘light’ (<100 M_Sun), that is the remnants of first stars, or ‘massive’ (>10⁵ M_Sun) originating from direct collapse of gas clouds in the early Universe? An elusive population of intermediate-mass black holes (IMBHs, 100<M_BH<10⁵ M_Sun) holds an answer to the question of SMBH origin: their very existence would strongly argue for the ‘light seeds’ scenario. In our multi-wavelength observational campaign we confirmed in X-ray 30 active IMBHs in galaxy nuclei, and over a hundred of ‘light-weight’ SMBHs (M_BH<10⁶ M_Sun), re-measured masses of dozens of them using intermediate-resolution optical spectra collected with 6.5-10m class telescopes, and obtained structural properties from Hubble Space Telescope images. We constructed scaling relations between SMBHs and their host galaxies (M_BH-σ and M_BH-M_bulge) in the light-weight regime suggesting that the bulge and SMBH growth is not as tightly connected as for ‘normal’ more massive SMBHs. We identified a sub-population of Eddington-limited IMBHs/LWSMBHs and also an active binary IMBH with the estimated mass ratio of ~10:1, which proves that both BH growth channel play their roles in the low-mass regime. However, the high abundance of high-Eddington sources (~1/3 of the total sample) suggests that the accretion is the dominant growth mechanism.