PhOiU UV U. DldSiriSKd

PhOiU UV U. DldSiriSKd

Figure 1.4. Galaxies from the Sloan Digital Sky Survey in the BPT diagram are distributed in two wings: the wings of a flying seagull.

models aimed at reproducing the upper envelope of the left wing of the seagull. Its equation is where y = log([OIll] A5007/HP) and x = log([NIl] A6584/Ha).

Note that, among the three diagnostic diagrams proposed by Veilleux and Osterbrock (1989), [OIII] A5007/HP versus [NII] A6584/Ha is the most efficient, due to the greater N/O in galaxies of high metallicity. Then, heating by an AGN boosts the [NII] line and creates a clear separation of the two wings. One then understands why the [O Ill] A5007/Hp versus [OII] A3727/Hp diagram, which is used as a surrogate of the BPT diagram at redshifts of ^0.2 (Lamareille et al. 2004), is much less efficient at separating AGN hosts from normal star-forming galaxies.

Stasinska et al. (2006) also noted that a classification based on [NII] A6584/Ha only is also feasible, and has the advantage of being applicable to a much larger number of galaxies, since [O III] A5007 and Hp are not needed.

Finally, these authors proposed another classification diagram, plotting Dn(4000)^ as a function of the EW of [OII] A3727 (the DEW diagram). The rationale for this diagram is that, if, in a galaxy with a large Dn (4000), emission lines can be seen, they must be due to some other cause than photoionization by massive stars. The hope is to be able to use this diagram to distinguish AGNs at redshifts higher than 0.2. However, the fact that higher redshifts correspond to younger ages of the stellar populations is an issue that requires further investigation.

To end this section, let us remark that the term "LINER" is nowadays often employed to designate the galaxies that are found to the lower right of the BPT diagram. This is an unfortunate deviation from the original meaning, since it is likely that, in these galaxies, the emission lines come not only from the nucleus, but also from a much larger zone of the galaxy. The original "LINERS" discovered by Heckman would not appear in the BPT diagram due to the low luminosity of the active nucleus, unless there were significant line emission from the rest of the galaxy.

t Dn (4000) is the discontinuity observed at 4000 A in the spectra of galaxies; it increases with stellar metallicity and age.

y = (-30.787 + 1.1358x + 0.27297x2)tanh(5.7409x) - 31.093,

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