The Role of Star-Formation and AGN in Dust Heating of z=0.3-2.8 Galaxies - II. Informing IR AGN fraction estimates through simulations

TL;DR

This study uses hydrodynamic simulations with dust radiative transfer to compare empirical IR AGN fraction estimates in dusty galaxies at z~0.3-2.8, revealing potential underestimations and dependencies on templates and extinction.

Contribution

It demonstrates that simulations can qualitatively match empirical IR AGN fractions and highlights the underestimation of AGN roles in star-forming galaxies, emphasizing the importance of host reprocessing and template choice.

Findings

Simulations agree qualitatively with empirical IR AGN fractions when considering host reprocessing.

Empirical methods may underestimate AGN fractions in star-forming galaxy-AGN composites.

6% of star-forming galaxies have AGN fractions >50%, suggesting possible underestimation of AGN prevalence.

Abstract

A key question in extragalactic studies is the determination of the relative roles of stars and AGN in powering dusty galaxies at $z\sim$1-3 where the bulk of star-formation and AGN activity took place. In Paper I, we present a sample of $336$ 24$\mu$m-selected (Ultra)Luminous Infrared Galaxies, (U)LIRGs, at $z \sim 0.3$-$2.8$, where we focus on determining the AGN contribution to the IR luminosity. Here, we use hydrodynamic simulations with dust radiative transfer of isolated and merging galaxies, to investigate how well the simulations reproduce our empirical IR AGN fraction estimates and determine how IR AGN fractions relate to the UV-mm AGN fraction. We find that: 1) IR AGN fraction estimates based on simulations are in qualitative agreement with the empirical values when host reprocessing of the AGN light is considered; 2) for star-forming galaxy-AGN composites our empirical methods may be underestimating the role of AGN, as our simulations imply $>$50% AGN fractions, $\sim$3$\times$ higher than previous estimates; 3) 6% of our empirically classified "SFG" have AGN fractions $\gtrsim$50%. While this is a small percentage of SFGs, if confirmed, would imply the true number density of AGN may be underestimated; 4) this comparison depends on the adopted AGN template -- those that neglect the contribution of warm dust lower the empirical fractions by up to 2$\times$; and 5) the IR AGN fraction is only a good proxy for the intrinsic UV-mm AGN fraction when the extinction is high ($A_V\gtrsim 1$ or up to and including coalescence in a merger).