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

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, 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.

Witnessing the transformation of a quasar host galaxy at z = 1.6

“…As our preferred explanation for this ensemble of observed properties, we argue that we are witnessing a quasar in the act of transforming its host galaxy through the elimination of the remainder of its cold gas, by an act of violent, radio-mode feedback, to eventually become a 'retired' galaxy (e.g., Stasińska et al 2008;Cid Fernandes et al 2010). This would correspond to a late stage in the life of the AGN, after star formation and accretion onto the SMBH have depleted the host galaxy's reservoir of cold gas, and when feedback activity has eliminated much of the remaining cold gas.…”

OMEGA – OSIRIS Mapping of Emission-line Galaxies in A901/2 – I. Survey description, data analysis, and star formation and AGN activity in the highest density regions

“…Our increased spectral resolution over standard narrowband Hα imaging surveys (see, e.g., Kennicutt 1992, Gallego et al 1997) has the advantages of avoiding contamination from the [Nii] line in the star formation estimates and enabling AGN to be identified (Cid Fernandes et al 2010). Finally, while integral field spectrograph (IFS) surveys, such as CALIFA and MaNGA, also provide spatial information, their fields of view are much smaller than that of OSIRIS.…”

“…13 we show the [Nii] / Hα vs. WHα (WHAN; Cid Fernandes et al 2010, Cid Fernandes et al 2011) diagram for our ELGs. Here we use the RPSF aperture, as this provides the cleanest measurement of the nuclear emission of the galaxy.…”

“…The first of these forces the flux of [Nii] λ6583 (which, physically, can never be negative) to be positive; the second assumes that the Hα line lies within the spectral range and the third sets an upper limit for the [Nii] λ6583 flux such that it cannot be larger than 3 times the flux of Hα (which is the maximum value of [Nii] / Hα observed in the WHAN diagram Cid Fernandes et al 2010), including the possibility of up to 3Å equivalent-width of absorption in Hα.…”

“…We have targeted the Hα (λ0 = 6563Å) and [Nii] (λ0 = 6583Å) lines. The former is the best optical tracer of SFR (Kennicutt 1998), whereas the combination of the two lines provides one of the most reliable optical AGN diagnostics available (Cid Fernandes et al 2010).…”