WHAN Diagram

The WHAN diagram (Cid Fernandes et al. 2011) combines the equivalent width of Hα (WHa) and the [NII]/Ha line ratio in order to classify the ionization source(s) in galaxies. Compared to the other optical line diagnostics, it can also classify weak emission line galaxies that lack some of the other diagnostic lines (such as Hb and/or [OIII]5007).

From the abstract of Cid Fernandes et al. (2011):

Five classes of galaxies are identified within the WHAN diagram:

• • • pure star-forming galaxies: log([NII]/Hα) < -0.4 and W_Hα > 3 Å;

    • strong AGN (i.e. Seyferts): log([NII]/Hα) > -0.4 and W_Hα > 6 Å;

    • weak AGN: log([NII]/Hα) > -0.4 and W_Hα between 3 and 6 Å;

    • retired galaxies (i.e. fake AGN): W_Hα < 3 Å;

    • passive galaxies (actually, lineless galaxies): W_Hα and W_{[N ii]} < 0.5 Å.

where "retired galaxies" display LINER signatures thought to originate from processes other than active accretion around a black hole (ionization provided by hot evolved stars) and are separated from objects with LINER signatures indicating the presence of a true (albeit weak) AGN.