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