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Galaxy Classification in eBOSS: The Search for BPT Alternatives

Presentation #105.02 in the session SDSS-IVever: Continuing Science from SDSS-IV Surveys.

Published onJun 19, 2024
Galaxy Classification in eBOSS: The Search for BPT Alternatives

As observational datasets grow in volume and extend to higher redshifts, there is a need for reliable methods to categorize galaxies and identify whether their dominant ionization mechanism is star formation or an Active Galactic Nucleus (AGN). Emission lines in galaxy spectra serve as crucial diagnostic tools for understanding and constraining ionization mechanisms. At low redshift it has been traditional to use the Baldwin, Phillips, and Terlevich (BPT) diagram ([OIII]/Hβ vs [NII]/Hα). However, this method becomes unusable when Hα is redshifted from the observable spectra. This study explores alternative diagnostic diagrams, including the Mass Excitation (MEx) diagram, the Blue Diagram, and the new Triple Equivalent Width (TrEW) diagram. The MEx diagram, which utilizes stellar mass in place of [NII]/Hα, encounters challenges due to strong non-linear redshift evolution. The Blue Diagram, which utilizes the ratio of [OII] to Hβ as the x-axis, provides a pure AGN sample but has large regions where multiple ionization mechanisms overlap. The TrEW diagram utilizes the addition of the equivalent widths [OII] 3726, 3729 and Hβ to offer a promising alternative with minimal redshift evolution. This research analyzes these diagrams using Extended Baryon Oscillation Spectroscopic Survey (eBOSS) data, addressing challenges associated with redshift evolution out to z = 1. Statistical analyses, accuracy, and purity assessments, using [NeV] as an AGN indicator, are conducted for each diagnostic, revealing the strengths and limitations of each method. Preliminary results indicate that while the BPT remains a gold standard, the TrEW diagram emerges as a robust alternative, displaying high accuracy for AGN categorization and maintaining purity across various redshifts.

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