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, such as [O iii]/Hβ versus [N ii]/Hα (the BPT diagram), require the measurement of at least four emission lines. This paper aims to remedy this situation by transposing the usual divisory lines between star‐forming (SF) galaxies and active galactic nuclei (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 Sloan Digital Sky Survey Data Release 7 to constitute a suitable sample of 280 000 ELGs, one‐third of which are WLGs. (2) Galaxies with strong emission lines are classified using the widely applied criteria of Kewley et al., Kauffmann et al. and Stasińska et al. to distinguish SF galaxies and AGN hosts and Kewley et al. to distinguish Seyferts from LINERs. (3) We transpose these classification schemes to alternative diagrams keeping [N ii]/Hα as a horizontal axis, but replacing Hβ by a stronger line (Hα or [O ii]), or substituting the ionization‐level sensitive [O iii]/Hβ ratio with the equivalent width of Hα(WHα). 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 per cent 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. In the course of this analysis, we were led to make the following points. (i) The Kewley et al. BPT line for galaxy classification is generally ill‐used. (ii) Replacing [O iii]/Hβ by WHα in the classification introduces a change in the philosophy of the distinction between LINERs and Seyferts, but not in its results. Because the WHα versus [N ii]/Hα diagram can be applied to the largest sample of ELGs without loss of discriminating power between Seyferts and LINERs, we recommend its use in further studies. (iii) The dichotomy between Seyferts and LINERs is washed out by WLGs in the BPT plane, but it subsists in other diagnostic diagrams. This suggests that the right wing in the BPT diagram is indeed populated by at least two classes, tentatively identified with bona fide AGN and ‘retired’ galaxies that have stopped forming stars and are ionized by their old stellar populations.

The classification of galaxies as star forming or active is generally done in the ([O III]/Hβ, [N II]/Hα) 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-asymptotic giant branch 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.

We have obtained the mass–metallicity (M–Z) relation at different lookback times for the same set of galaxies from the Sloan Digital Sky Survey, using the stellar metallicities estimated with our spectral synthesis code starlight. We have found that this relation steepens and spans a wider range in both mass and metallicity at higher redshifts. We have modelled the time evolution of stellar metallicity with a closed‐box chemical evolution model, for galaxies of different types and masses. Our results suggest that the M–Z relation for galaxies with present‐day stellar masses down to 1010 M⊙ is mainly driven by the history of star formation and not by inflows or outflows.

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 separetely because one cannot fully classify them onto the standard star-forming, Seyfert, or 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.

a partir de su espectro integrado es el objetivo central de la síntesis de población estelar. Recientes avances en modelos de síntesis evolutiva han dado un nuevo aliento a este viejo campo de investigación. Modernas técnicas de síntesis espectral incorporando estos avances hoy permiten el ajuste del espectro de una galaxia Å-por-Å. Estos ajustes detallados son útiles para un número de estudios, como líneas de emisión, cinemática estelar, y, por supuesto, evolución de galáxias. Aplicaciones de esta abordaje semi-empírica a mega bases de datos nos está enseñando mucho acerca de la vida de galaxias. El código de síntesis espectral STARLIGHT es una de las herramientas la cual permite explotar esa combinación favorable de datos y modelos. Como una ilustración, mostramos como la SFHs varían para galaxias en distintas regiones de un diagrama de diagnóstico clásico. Se detectan fuertes tendencias sistemáticas a lo largo de las secuencias de formación estelar y galaxias activas. Experimentos con nuevas versiones de modelos de síntesis son brevemente descritos. Por fin, anunciamos la disponibilidad pública tanto de STARLIGHT como una base de datos con ajustes espectrales detallados y productos relacionados para más de medio millón de galaxias del SDSS. Esta herramienta permite exploraciones físicamente inspiradas del espacio de parámetros, proporcionando nuevas maneras de navegar por el reino de galaxias.