``Baldwin, Phillips & Terlevich'' (BPT) diagrams demonstrate how LINERs
can be distinguished from normal H II regions and normal AGNs (Seyferts
and QSOs) on the basis of their [O III]
Figure 2.3. A diagnostic (or BPT) diagram
for emission-line galaxies. The vertical axis is the [O III]
Adapted from B.M. Peterson
An Introduction to Active Galactic Nuclei, Cambridge University Press,
(1997)
BPT diagram
5007 / H
, [N II] 6583 /
H
, and
[S II] 6716, 6731 / H flux ratios. Here it is seen that
the Seyfert 2s
have high values of each ratio. H II regions define a locus of lower
values which does not overlap with the region of parameter space
occupied by the Seyferts. The LINERs can be distinguished from the
Seyfert 2s by their low values of [O III] 5007 / H relative to [N II]
6583 / H, and from the H II regions by
their larger values of [N II]
6583 / H.
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).