1. Classifying Intermediate-redshift Galaxies in SDSS: Alternative Diagnostic Diagrams.
- Author
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Feuillet, Léa M., Meléndez, Marcio, Kraemer, Steve, Schmitt, Henrique R., Fischer, Travis C., and Reeves, James N.
- Subjects
ACTIVE galactic nuclei ,GALAXIES ,SEYFERT galaxies ,GALACTIC redshift ,IONIZATION energy ,ASTRONOMICAL surveys - Abstract
We select a sample of 1437 active galactic nuclei (AGN) from the catalog of the Sloan Digital Sky Survey galaxy properties from the Portsmouth group by detection of the high-ionization [Ne v ] 3426 Å emission line. We compare the fluxes of [Ne III ] 3869 Å, [O III ] 5007 Å, [O II ] 3726, 3728 Å, and [O i ] 6300 Å to that of [Ne v]. All four lines show a strong linear correlation with [Ne v], although lines from ions with lower ionization potentials have a lower correlation coefficient. We investigate the use of two forbidden line ratio diagnostic diagrams that do not rely on H α in order to classify high-redshift galaxies. These use the [Ne III ]/[O II ] line ratio plotted against [O III ]/[O i ] and [O III ]/[O II ], respectively. We use photoionization modeling to characterize the behavior of the narrow-line region in AGN and star-forming regions and test the validity of our diagnostic diagrams. We also use a luminosity cutoff of log L
[O III ] [erg s−1 ] = 42, which lowers the contamination of the AGN region by star-forming galaxies down to 10% but does not remove green pea and purple grape galaxies from the AGN region. We also investigate the OHNO diagram, which uses [Ne III ]/[O II ] plotted against [O III ]/H β. Using our new diagnostic diagrams, we are able to reliably classify AGN up to a redshift of z ≤ 1.06 and add more than 822 new AGN to the [Ne v]-selected AGN sample. [ABSTRACT FROM AUTHOR]- Published
- 2024
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