Emission line taxonomy and the nature of AGN-looking galaxies in the SDSS

TL;DR

This paper reviews emission line classification schemes for SDSS galaxies, highlighting that many LINER-like systems are powered by old stars rather than black holes, and discusses the implications for galaxy taxonomy.

Contribution

It introduces alternative classification schemes including weak emission line galaxies and demonstrates that many LINER-like systems are retired galaxies powered by old stars.

Findings

Most LINER-like systems are powered by old stars, not black holes.

Retired galaxies should be included in emission line taxonomy.

Star formation histories vary across different galaxy classes.

Abstract

Massive spectroscopic surveys like the SDSS have revolutionized the way we study AGN and their relations to the galaxies they live in. A first step in any such study is to define samples of different types of AGN on the basis of emission line ratios. This deceivingly simple step involves decisions on which classification scheme to use and data quality censorship. Galaxies with weak emission lines are often left aside or dealt with separately because one cannot fully classify them onto the standard Star-Forming, Seyfert of LINER categories. This contribution summarizes alternative classification schemes which include this very numerous population. We then study how star-formation histories and physical properties of the hosts vary from class to class, and present compelling evidence that the emission lines in the majority of LINER-like systems in the SDSS are not powered by black-hole accretion. The data are fully consistent with them being galaxies whose old stars provide all the ionizing power needed to explain their line ratios and luminosities. Such retired galaxies deserve a place in the emission line taxonomy.