Glimmering in the dark: modeling the low-mass end of the $M_{\bullet}-\sigma$ relation and of the quasar luminosity function

TL;DR

This paper models the low-mass end of the black hole mass-velocity dispersion relation and quasar luminosity function, predicting deviations at low masses and implications for black hole seed populations.

Contribution

It introduces a semi-analytical model with bimodal accretion efficiencies to explain low-mass black hole behavior and their observational signatures.

Findings

Predicted departure from the $M_{\bullet} - \sigma$ relation at $\sim 10^5 M_{\odot}$

Forecasted a deficit of luminous intermediate-mass black holes

Suggested increased detection of low-luminosity black holes at $<10^{42}$ erg/s

Abstract

The $M_{\bullet} - \sigma$ relation establishes a connection between central black holes (BHs) and their host spheroids. Supported by observations at $M_{\bullet} \gtrsim 10^5 \, M_{\odot}$, 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 ($M_{\bullet} \lesssim 10^5 \, M_{\odot}$) accreting inefficiently. We predict a departure from the $M_{\bullet} - \sigma$ relation at a transitional BH mass $\sim 10^5 \, M_{\odot}$, 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 $\sim 10^5-10^6 \, M_{\odot}$. Furthermore, we predict a deficit of BHs shining at bolometric luminosities $\sim 10^{42} \, \mathrm{erg \, s^{-1}}$. 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, $<10^{42} \, \mathrm{erg \, s^{-1}}$. 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.