Optical diagnostic diagrams are powerful tools to separate different ionizing sources in galaxies.However, the model-constraining power of the most widely used diagrams is very limited and challenging to visualize. In addition, there have always been classification inconsistencies between diagrams based on different line ratios, and ambiguities between regions purely ionized by active galactic nuclei (AGNs) and composite regions. We present a simple reprojection of the 3D line ratio space composed of [N II]λ6583/H α, [S II]λλ6716, 6731/H α, and [O III]λ5007/H β, which reveals its model-constraining power and removes the ambiguity for the true composite objects. It highlights the discrepancy between many theoretical models and the data loci. With this reprojection, we can put strong constraints on the photoionization models and the secondary nitrogen abundance prescription. We find that a single nitrogen prescription cannot fit both the star-forming locus and AGN locus simultaneously, with the latter requiring higher N/O ratios. The true composite regions stand separately from both models. We can compute the fractional AGN contributions for the composite regions, and define demarcations with specific upper limits on contamination from AGN or star formation. When the discrepancy about nitrogen prescriptions gets resolved in the future, it would also be possible to make robust metallicity measurements for composite regions and AGNs.
|Number of pages||16|
|Journal||Monthly Notices of the Royal Astronomical Society|
|State||Published - 2020|
Bibliographical noteFunding Information:
Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the US Department of Energy Office of Science, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS website is www.sdss.org.
The authors wish to thank David Law for useful comments on a draft of this paper. We acknowledge support by NSF AST-1715898 and NASA grant 80NSSC20K0436 subaward S000353.
© 2020 The Author(s).
- Star formation
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science