The Revisited BPT Diagram from the Self-Consistent Analysis

Galaxies' spectral energy distribution has been explored through the use of spectral synthesis codes, and these techniques have been essential in identifying many aspects of the current galaxy evolution model. Most of the spectral synthesis codes that have been developed so far are solely stellar and assume a negligible nebular contribution to the overall continuum. FADO (Fitting Analysis using Differential Evolution Optimisation) is the first spectral synthesis code to fit self-consistently stellar and nebular components. Diagnostic diagrams are powerful tools for classifying galaxies based on the emission line ratio of collisionally excited lines such as [OIII] λ 5007, [NII] λ 6584, [SII] λ 6716, 6731, [OI] λ 6300, and the Balmer recombination lines such as Hα, Hβ. This paper explores the impact of including nebular components on diagnostic diagrams. We investigated the results of the application of FADO to the Sloan Digital Sky Survey Data Release 8 using the data analysed by MPA-JHU, the Max Planck Institute for Astrophysics, and Johns Hopkins University. We found that in all diagnostic diagrams, the fluxes for FADO are higher than those for MPA-JHU; the difference is significant compared to the error in the flux measurement. FADO overestimates the flux ratio of all three diagnostic diagrams over MPA-JHU, but the overestimation is comparable with the line flux ratio errors. The results indicate that the inclusion of a nebular continuum is very important when fitting the spectral energy distribution as it increases the fluxes of all galaxies. However, there is a mild impact from the inclusion of nebular component analysis in the diagnostic diagrams used to classify the ionisation state of galaxies’ interstellar medium.