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Session: Extragalactic Astronomy and Astrophysics

Name: Mr. Charalampos Daoutis (IA-FORTH & Univ. of Crete)
Coauthors: Kyritsis Elias (Institute of Astrophysics - FORTH, Univ. of Crete)
Zezas Andreas (University of Crete - I.A FORTH)
Kouroumpatzakis Konstantinos (Astronomical Institute, Czech Academy of Sciences)
Bonfini Paolo (University of Crete)
Type: Poster
Title: A holistic approach in optical galactic activity diagnostics
Abstract:

The activity classification of galaxies is an important but difficult topic in observational astrophysics. Today, well-known galactic activity classification methods are based primarily on the use of characteristic optical emission-line ratios. However, the use of emission lines does not allow a unifying optical diagnostic for passive and active galaxies. To address this limitation, we introduce a new activity diagnostic based on the equivalent with (EW) of the Hα, OIII[λ5007], and NII[λ6584] lines and the D4000 continuum break. Using these data we build a machine-learning model that is able to discriminate between the different excitation mechanisms in galaxies. Besides the fact that this diagnostic allows us to include the class of passive galaxies, the use of the D4000 allows us to discriminate between composite-like spectra resulting from mixed activity galaxies (i.e., star-forming and AGN) or from older stellar populations (e.g., post-AGB stars). Finally, we find that this new diagnostic can be reduced to a 2-dimensional plot of the D4000 against the log(EW(OIII[λ5007])2), while maintaining good accuracy. This offers a more direct and detailed characterization of the galactic activity than any other existing method. Since this diagnostic is based on equivalent widths: it does not require the removal of the stellar continuum, it can be applied to a wider number of galaxies, and it includes all possible types of galactic activity under one unified scheme.