An Eddington ratio-driven origin for the ${\rm L}_{\rm X}-{\rm M}_{*}$ relation in quiescent and star forming active galaxies

TL;DR

This study investigates how the mean X-ray luminosity to stellar mass relation in active galaxies is driven by the Eddington ratio, revealing its evolution over cosmic time and its dependence on galaxy activity phases.

Contribution

The paper introduces semi-empirical mock catalogs to identify the primary parameters shaping the Lx-M* relation, emphasizing the role of the Eddington ratio over duty cycle effects.

Findings

The normalization of the Lx-M* relation depends mainly on the mean Eddington ratio.

The mean Eddington ratio decreases with cosmic time and galaxy stellar mass.

Models with lower M_BH-M_* relations fit the observed data better.

Abstract

A mild correlation exists in active galaxies between the mean black hole accretion, as traced by the mean X-ray luminosity $\left<{\rm L}_{\rm X}\right>$, and the host galaxy stellar mass M$_*$, characterised by a normalisation steadily decreasing with cosmic time and lower in more quiescent galaxies. We create comprehensive semi-empirical mock catalogues of active black holes to pin down which parameters control the shape and evolution of the $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation of X-ray detected active galaxies. We find that the normalisation of the $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation is largely independent of the fraction of active galaxies (the duty cycle), but strongly dependent on the mean Eddington ratio, when adopting a constant underlying M$_{\rm BH}-{\rm M}_*$ relation as suggested by observational studies. The data point to a decreasing mean Eddington ratio with cosmic time and with galaxy stellar mass at fixed redshift. Our data can be reproduced by black holes and galaxies evolving on similar M$_{\rm BH}-{\rm M}_*$ relations but progressively decreasing their average Eddington ratios, mean X-ray luminosities, and specific star formation rates, when moving from the starburst to the quiescent phase. Models consistent with the observed $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation and independent measurements of the mean Eddington ratios, are characterised by M$_{\rm BH}-{\rm M}_*$ relations lower than those derived from dynamically measured local black holes. Our results point to the $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation as a powerful diagnostic to: 1) probe black hole-galaxy scaling relations and the level of accretion onto black holes; 2) efficiently break the degeneracies between duty cycles and accretion rates in cosmological models of black holes.