Can retired galaxies mimic active galaxies? Clues from the Sloan Digital Sky Survey

TL;DR

This paper investigates whether many galaxies classified as active are actually 'retired' galaxies with no ongoing star formation, suggesting that apparent nuclear activity may be overestimated due to misclassification.

Contribution

It introduces a method combining stellar population analysis and photoionization models to distinguish retired galaxies from truly active ones in SDSS data.

Findings

A significant fraction of LINER galaxies may be retired galaxies with ionization from old stars.

Observational selection effects influence the apparent shape of the galaxy distribution in diagnostic diagrams.

The prevalence of nuclear activity in galaxies might be overestimated due to misclassification of retired galaxies.

Abstract

The classification of galaxies as star forming or active is generally done in the ([O III]/Hbeta, [N II]/Halpha) plane. The Sloan Digital Sky Survey (SDSS) has revealed that, in this plane, the distribution of galaxies looks like the two wings of a seagull. Galaxies in the right wing are referred to as Seyfert/LINERs, leading to the idea that non-stellar activity in galaxies is a very common phenomenon. Here, we argue that a large fraction of the systems in the right wing could actually be galaxies which stopped forming stars. The ionization in these "retired" galaxies would be produced by hot post-AGB stars and white dwarfs. Our argumentation is based on a stellar population analysis of the galaxies via our STARLIGHT code and on photoionization models using the Lyman continuum radiation predicted for this population. The proportion of LINER galaxies that can be explained in such a way is however uncertain. We further show how observational selection effects account for the shape of the right wing. Our study suggests that nuclear activity may not be as common as thought. If retired galaxies do explain a large part of the seagull's right wing, some of the work concerning nuclear activity in galaxies, as inferred from SDSS data, will have to be revised.