Starburst or AGN Dominance in Submillimetre-Luminous Candidate AGN?

TL;DR

This study investigates whether starburst activity or active galactic nuclei (AGN) dominate the energy output in submillimetre-luminous galaxies, using mid-infrared observations to identify the presence of AGN in dust-obscured sources.

Contribution

It provides new insights into the energy sources of SMGs by analyzing mid-infrared signatures to distinguish between starburst and AGN dominance.

Findings

Evidence of significant AGN contribution in some SMGs.

Starburst activity remains dominant in many cases.

Mid-infrared diagnostics effectively identify obscured AGN.

Abstract

It is widely believed that ultraluminous infrared (IR) galaxies and active galactic nuclei (AGN) activity are triggered by galaxy interactions and merging, with the peak of activity occurring at z~2, where submillimetre galaxies are thousands of times more numerous than local ULIRGs. In this evolutionary picture, submillimetre galaxies (SMGs) would host an AGN, which would eventually grow a black hole (BH) strong enough to blow off all of the gas and dust leaving an optically luminous QSO. To probe this evolutionary sequence we have focussed on the 'missing link' sources, which demonstrate both strong starburst (SB) and AGN signatures, in order to determine if the SB is the main power source even in SMGs when we have evidence that an AGN is present from their IRAC colours. The best way to determine if a dominant AGN is present is to look for their signatures in the mid-infrared with the Spitzer IRS, since often even deep X-ray observations miss identifying the presence of AGN in heavily dust-obscured SMGs. We present the results of our audit of the energy balance between star-formation and AGN within this special sub-population of SMGs -- where the BH has grown appreciably to begin heating the dust emission.