Alternative diagnostic diagrams and the "forgotten" population of weak line galaxies in the SDSS

TL;DR

This study develops alternative diagnostic diagrams that classify weak line galaxies more effectively, increasing the identified population of emission line galaxies and refining the understanding of their distribution in the local universe.

Contribution

It introduces transposed diagnostic diagrams that classify weak line galaxies using fewer emission lines, expanding the galaxy census.

Findings

Up to 50% more ELGs can be classified with new diagrams.

WLGs significantly increase the counts of LINERs and metal-rich star-forming galaxies.

New diagrams retain classification accuracy while requiring fewer emission lines.

Abstract

A numerous population of weak line galaxies (WLGs) is often left out of statistical studies on emission line galaxies (ELGs) due to the absence of an adequate classification scheme, since classical diagnostic diagrams, like [OIII]/Hb vs [NII]/Ha (the BPT diagram), require the measurement of at least 4 emission lines. This paper aims to remedy this situation by transposing the usual divisory lines between Star Forming (SF) and AGN hosts, and between Seyferts and LINERs to diagrams that are more economical in terms of line quality requirements. By doing this, we rescue from the classification limbo a substantial number of sources and modify the global census of ELGs. More specifically: (1) We use the SDSS DR7 to constitute a suitable sample of 280k ELGs, 1/3 of which are WLGs. (2) Galaxies with strong emission lines are classified using the widely applied criteria of Kewley et al (2001), Kauffmann et al (2003), Stasinska et al (2006) and Kewley et al (2006). (3) We transpose these classification schemes to alternative diagrams keeping [NII]/Ha as a horizontal axis, but replacing Hb by a stronger line (Ha or [OII]), or substituting [OIII]/Hb ratio with the equivalent width of Ha. Optimized equations for the transposed divisory lines are provided. (4) We show that nothing significant is lost in the translation, but that the new diagrams allow one to classify up to 50% more ELGs. (5) Introducing WLGs in the census of galaxies in the local Universe increases the proportion of metal-rich SF galaxies and especially LINERs. (abridged)