GEPI

Classifying galaxies in terms of their emission line properties is one of those boring but unavoidable first steps when dealing with massive surveys. Yet, decisions taken at this step, however, will strongly shape your results. The most popular method of classifying galaxies is through the [OIII]/Hb vs [NII]/Ha BPT diagram, where galaxies split into Star Forming and AGN branches. By merely requiring 4 emission lines, this method leaves out galaxies where either [OIII] or Hb is not detected, while Ha and [NII] clearly are. This talk focuses on these poor forgotten souls. We show that Weak Line Galaxies comprise a huge population of sources in the SDSS, and present ways of rescuing them from the classification limbo by means of alternative and economical diagnostic diagrams. We find that 1/3 of WLGs are massive metal-rich starbursts while 2/3 are LINER-like.

This leads to the impression that black-hole accretion is even more common than usually thought. Not really... Instead, we argue that a large fraction of LINERs in the SDSS could actually be galaxies which stopped forming stars and whose black holes are not much active either. 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. Retired galaxies pose as fake-AGN, and this confusion implies that some of the work concerning nuclear activity in galaxies, as inferred from SDSS data, will have to be revised.