2018ApJ...864L...6P

The M_•–σ relation establishes a connection between central black holes (BHs) and their host spheroids. Supported by observations at M_•≳10⁵M_☉, there is limited data on its validity at lower masses. Employing a semi-analytical model to simulate the combined evolution of BHs and their host galaxies, we predict the observational consequences of assuming a bimodality in the accretion efficiency of BHs, with low-mass BHs (M_•≲10⁵M_☉) accreting inefficiently. We predict a departure from the M_•–σ relation at a transitional BH mass ∼10⁵M_☉, with lower-mass BHs unable to reach the mass dictated by the relation and becoming disconnected from the evolution of the host galaxy. This prediction is an alternative to previous works suggesting a flattening of the relation at ∼10⁵–10⁶M_☉. Furthermore, we predict a deficit of BHs shining at bolometric luminosities ∼10⁴²ergs^–1. Joined with a detection bias, this could partly explain the scarce number of intermediate-mass BHs detected. Conversely, we predict an increase in source density at lower bolometric luminosities, < 10⁴²ergs^–1. Because our predictions assume a bimodal population of high-redshift BH seeds, future observations of fainter BHs will be fundamental for constraining the nature of these seeds.