Figure 2.3. A diagnostic (or BPT) diagram for emission-line galaxies. The vertical axis is the [O III] 5007 / H flux ratio and the horizontal axis is the [N II] 6583 / H flux ratio. Both ratios are based on lines close in wavelength and are therefore nearly reddening independent. The open circles are for H II regions and similar sources that are clearly ionized by hot stars. The closed circles are narrow-line AGNs (Seyfert 2s and NLRGs) which are ionized by ``harder'' continua (i.e., with a greater fraction of high-energy photons, such as a power-law spectrum); the solid line is an empirical division between these two classes of object. The triangles represent LINERs, which can be distinguished from H II regions by higher values of [N II] 6583 / H, and from Seyfert galaxies by lower values of [O III] 5007 / H, as can also be seen in Figure 2.1. Based on a similar diagram from Osterbrock (1989), p. 346. Figure courtesy of R. W. Pogge, based on data from Veilleux and Osterbrock (1987).