The AGN contribution to the UV–FIR luminosities of interacting galaxies and its role in identifying the main sequence

Andrés F Ramos Padilla,Mario-A Higuera-G,Jeremy Dietrich,Juan R Martínez-Galarza,Aliza G Beverage,Howard A Smith,Aaron S Weiner,M L N Ashby

doi:10.1093/mnras/staa2813

Abstract

ABSTRACT Emission from active galactic nuclei (AGNs) is known to play an important role in the evolution of many galaxies including luminous and ultraluminous systems (U/LIRGs), as well as merging systems. However, the extent, duration, and exact effects of its influence are still imperfectly understood. To assess the impact of AGNs on interacting systems, we present a spectral energy distribution (SED) analysis of a sample of 189 nearby galaxies. We gather and systematically re-reduce archival broad-band imaging mosaics from the ultraviolet to the far-infrared using data from GALEX, SDSS, 2MASS, IRAS, WISE, Spitzer, and Herschel. We use spectroscopy from Spitzer/IRS to obtain fluxes from fine-structure lines that trace star formation and AGN activity. Utilizing the SED modelling and fitting tool cigale, we derive the physical conditions of the interstellar medium, both in star-forming regions and in nuclear regions dominated by the AGN in these galaxies. We investigate how the star formation rates (SFRs) and the fractional AGN contributions (fAGN) depend on stellar mass, galaxy type, and merger stage. We find that luminous galaxies more massive than about $10^{10} \,\rm {M}_{*}$ are likely to deviate significantly from the conventional galaxy main-sequence relation. Interestingly, infrared AGN luminosity and stellar mass in this set of objects are much tighter than SFR and stellar mass. We find that buried AGNs may occupy a locus between bright starbursts and pure AGNs in the fAGN–[Ne v]/[Ne ii] plane. We identify a modest correlation between fAGN and mergers in their later stages.

Highlights

Interacting systems are crucial to our understanding of galaxy assembly over cosmic time, and of the mechanisms that shape the observed scaling relations
For the galaxies included in this work, which by construction emphasize infrared-luminous systems and mergers at various stages, we show that the active galactic nucleus (AGN) alone can contribute up to ∼ 80% of the infrared luminosity for these systems
Are our major findings: 1. A reliable measure of the fractional contribution of AGN emission to the total luminosity of galaxies is essential in the understanding of galaxy-wide physics, such as star formation evolution and total energy output

Summary

Introduction

Interacting systems are crucial to our understanding of galaxy assembly over cosmic time, and of the mechanisms that shape the observed scaling relations. Systems with luminosities greater than 1011 L (so-called Luminous InfraRed Galaxies, or LIRGs) are typically found in interacting systems, which results in a strong correlation between enhanced SFR and galaxy interaction (Sanders & Mirabel 1996; Su et al 2013). This simple description does not capture the full range of observed behaviour. We created a mask for potential contaminating foreground sources (Milky Way stars) by flagging all pixels with a SNR higher than 3.0 for point sources This step is crucial for accurate background estimation. We speculate that the annuli used for the local background estimation were contaminated by low-level emission from the target galaxies at large radii

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Discussion

Conclusion