The Intrinsic Eddington Ratio Distribution of Active Galactic Nuclei in Star-forming Galaxies from the Sloan Digital Sky Survey

TL;DR

This study investigates the true distribution of black hole accretion rates in active galactic nuclei within star-forming galaxies, proposing that the intrinsic distribution is a broad power law, reconciling previous observational discrepancies.

Contribution

The paper introduces an improved method for extracting intrinsic Eddington ratio distributions from optical data, accounting for star formation contamination and supporting a universal power law model.

Findings

Intrinsic Eddington ratio distribution is consistent with a broad power law.

Optical diagnostics can probe lower Eddington ratios by mitigating star formation effects.

The distribution aligns with X-ray observations and is similar across galaxy types.

Abstract

An important question in extragalactic astronomy concerns the distribution of black hole accretion rates of active galactic nuclei (AGN). Based on observations at X-ray wavelengths, the observed Eddington ratio distribution appears as a power law, while optical studies have often yielded a lognormal distribution. There is increasing evidence that these observed discrepancies may be due to contamination by star formation and other selection effects. Using a sample of galaxies from the Sloan Digital Sky Survey Data Release 7, we test if an intrinsic Eddington ratio distribution that takes the form of a Schechter function is consistent with previous work that suggests that young galaxies in optical surveys have an observed lognormal Eddington ratio distribution. We simulate the optical emission line properties of a population of galaxies and AGN using a broad instantaneous luminosity distribution described by a Schechter function near the Eddington limit. This simulated AGN population is then compared to observed galaxies via the positions on an emission line excitation diagram and Eddington ratio distributions. We present an improved method for extracting the AGN distribution using BPT diagnostics that allows us to probe over one order of magnitude lower in Eddington ratio counteracting the effects of dilution by star formation. We conclude that for optically selected AGN in young galaxies, the intrinsic Eddington ratio distribution is consistent with a possibly universal, broad power law with an exponential cutoff, as this distribution is observed in old optically selected galaxies and in X-rays.