Probing black hole accretion tracks, scaling relations and radiative efficiencies from stacked X-ray active galactic nuclei

TL;DR

This study combines X-ray data and galaxy growth models to investigate black hole accretion, revealing that selection biases affect observed relations and suggesting typical black hole spins are moderate to rapid.

Contribution

It introduces a novel method linking X-ray active black hole accretion with galaxy growth to assess radiative efficiencies and biases in black hole mass estimates.

Findings

De-biased local Mbh-Mstar relation implies radiative efficiency ~0.15.

Observed inactive black hole relation suggests efficiency ~0.02.

Results support moderate-to-rapid black hole spins.

Abstract

The masses of supermassive black holes at the centres of local galaxies appear to be tightly correlated with the mass and velocity dispersions of their galactic hosts. However, the local Mbh-Mstar relation inferred from dynamically measured inactive black holes is up to an order-of-magnitude higher than some estimates from active black holes, and recent work suggests that this discrepancy arises from selection bias on the sample of dynamical black hole mass measurements. In this work we combine X-ray measurements of the mean black hole accretion luminosity as a function of stellar mass and redshift with empirical models of galaxy stellar mass growth, integrating over time to predict the evolving Mbh-Mstar relation. The implied relation is nearly independent of redshift, indicating that stellar and black hole masses grow, on average, at similar rates. Matching the de-biased local Mbh-Mstar relation requires a mean radiative efficiency ~0.15, in line with theoretical expectations for accretion onto spinning black holes. However, matching the "raw" observed relation for inactive black holes requires a mean radiative efficiency around 0.02, far below theoretical expectations. This result provides independent evidence for selection bias in dynamically estimated black hole masses, a conclusion that is robust to uncertainties in bolometric corrections, obscured active black hole fractions, and kinetic accretion efficiency. For our fiducial assumptions, they favour moderate-to-rapid spins of typical supermassive black holes, to achieve a mean radiative efficiency ~0.12-0.20. Our approach has similarities to the classic Soltan analysis, but by using galaxy-based data instead of integrated quantities we are able to focus on regimes where observational uncertainties are minimized.