A Characteristic Division Between the Fueling of Quasars and Seyferts: Five Simple Tests

TL;DR

This paper investigates the different fueling mechanisms of quasars and Seyferts, proposing a division based on galaxy mergers and bulge growth, supported by observational tests across multiple galaxy properties.

Contribution

It introduces five simple tests to distinguish between merger-driven quasar fueling and secular processes in Seyferts, highlighting a characteristic division around the luminosity threshold.

Findings

Evidence for a transition to bulge-dominated hosts at the luminosity divide.

Differences in bulge types suggest different fueling mechanisms.

Clustering and redshift trends support merger-driven quasar activity.

Abstract

Given the existence of the M_BH-sigma relation, models of self-regulated black hole (BH) growth require both a fuel supply and growth of the host bulge to deepen the potential, or else the system will either starve or self-regulate without sustained activity. This suggests that bright quasars must be triggered in major mergers: a large fraction of the galaxy must be converted to new bulge mass in a dynamical time or less. Low-luminosity AGN, in contrast, require little bulge growth and small gas supplies, and could be triggered in more common non-merger events. This predicts a transition to merger-induced fueling around the traditional quasar-Seyfert luminosity divide (growth of BH masses above/below 10^7 M_sun). We compile observations to test several predictions of such a division, including: (1) A transition to bulge-dominated hosts. (2) A transition between 'pseudobulges' and 'classical' bulges hosting the remnant BHs: pseudobulges are formed in secular processes and minor mergers, whereas classical bulges are relics of major mergers. (3) An increase in the amplitude of small-scale clustering where mergers are more efficient. (4) Different redshift evolution, with gas-rich merger rates rising to redshifts z>2 while secular processes are relatively constant in time. (5) An increasing prominence of post-starburst features in more luminous systems. Our compilation of observations provides tentative evidence for the predicted division around the Seyfert-quasar threshold. We discuss how future observations can improve these constraints and break degeneracies between different fueling models.