Determining the Roles of AGN Feedback or Buried Star Formation as Sources of Mid-IR Emission from Post-Starburst Galaxies

Abstract

Galaxies generally come in two types: star-forming spiral and quiescent (i.e. nonstar-forming) elliptical galaxies. This bimodality suggests a mechanism that quenches star formation in star-forming galaxies. Post-starburst galaxies are galaxies that have recently undergone a period of intense star formation followed by rapid quenching, allowing us to observe this transition. The central supermassive black hole of these galaxies may play a role in quenching their star formation—the black hole can grow by rapidly accreting matter, becoming an active galactic nucleus (AGN) and releasing enough energy to outshine the host galaxy. The released energy heats up and expels the surrounding gas, halting star formation in a process known as AGN feedback. We seek to find evidence of excess AGN activity in post-starburst galaxies. We analyze nine galaxies with high mid-IR luminosities from the SQuIGG⃗L E sample of post-starburst galaxies of z = 0.5 − 0.9. However, AGN and buried star formation both create strong mid-IR emission from the thermal emission of heated dust. Thus, we wish to determine whether an obscured AGN or buried star formation powers the observed mid-IR luminosity. We model the galaxies’ spectra by fitting them to optical SDSS/IR WISE data, performing one set of fits where only buried star formation contributes to the mid-IR emission and one where star formation and an obscured AGN contribute. We incorporate new data from the Chandra X-ray Observatory — AGN are strong X-ray sources and we expect to detect them unless they are heavily obscured by dust and gas. We find that some galaxies’ fits reproduce the observed mid-IR data equally well with and without an AGN, while other galaxies’ fits strongly prefer an AGN, requiring a very high star formation rate (∼ 300 M⊙/yr) without one. We do not have any significant X-ray detections, which suggests that galaxies in the former case may host buried star formation at ∼ 30 M⊙/yr. For latter galaxies, we are presented with two scenarios: extremely obscured AGNs or star formation. Future work will involve collecting higher-resolution IR data to resolve this tension.