The BPT diagrams (named after "Baldwin, Phillips & Telervich") are a set of nebular emission line diagrams used to distinguish the ionization mechanism of nebular gas. The most famous version consists of [NII]6584/Ha versus [OIII]5007/Hb (the BPT-NII diagram; Fig. 5 of Baldwin et al 1981). The next two more commonly-used AGN diagnostics, published by Veilleux & Osterbrock 1987 ( hereafter VO87), are [SII]6717,6731/Ha versus [OIII]5007/Hb (VO87-SII) and [OI]6300/Ha versus [OIII]5007/Hb (VO87-OI). These diagrams were studied in numerous work and the dividing lines have been developed and adapted as a function of the ionization models and/or observations available (e.g., VO87; Osterbrock 1989; Kewley et al 2001; Kauffmann et al 2003; Kewley et al 2006; Stasinska et al 2006; Kewley et al 2013a,b).
Below are listed the demarcations summarized by Kewley et al 2006 for each diagram:
1) BPT-NII:
log([OIII]/Hb) = 0.61 / (log([NII]/Ha) - 0.05) + 1.3 (Kauffmann+03 line)
log([OIII]/Hb) = 0.61 / (log([NII]/Ha) - 0.47) + 1.19 (Kewley+01 line)
2) VO87-SII:
log([OIII]/Hb) = 0.72 / (log([SII]/Ha) - 0.32) + 1.30 (main AGN line)
log([OIII]/Hb) = 1.89 log([SII]/Ha) + 0.76 (LINER/Sy2 line)
3) VO87-OI:
log([OIII]/Hb) = 0.73 / (log([OI]/Ha) + 0.59) + 1.33 (main AGN line)
log([OIII]/Hb) = 1.18 log([OI]/Ha) + 1.30 (LINER/Sy2 line)
The diagnostics can be used either separately or combined, keeping in mind that they behave similarly generally-speaking but also show differences in the details of their respective sensitivity to physical conditions of the ionized gas. Kewley et al. (2006) presented a combined scheme to classifying galaxies based on all three diagnostics, while Juneau et al. (2011) combined only the BPT-NII and VO87-SII diagrams for their analysis.
More recently, Kewley et al (2013a) presented theoretical expectations for evolutionary trends on the BPT-NII diagram based on four scenarios:
(1) Normal ISM conditions and metal-rich AGN NLR at high-z;
(2) Normal ISM conditions and metal-poor AGN NLR at high-z;
(3) Extreme ISM conditions and metal-rich AGN NLR at high-z;
(4) Extreme ISM conditions and metal-poor AGN NLR at high-z.
These four scenarios yield four different predictions of combined location of star-forming and AGN galaxies on the BPT-NII at redshift > 0. They are illustrated in Figure 5 of Kewley et al (2013a) and are confronted to observations in a companion paper (Kewley et al 2013b). In the latter, the authors provide the reader with a modified, redshift-dependent dividing line for the BPT-NII diagram.