Classifying High Redshift Galaxies in SDSS: Alternative Diagnostic Diagrams

 

Léa Feuillet
Graduate Research Assistant
Physics Department
The Catholic University of America

Wed, September 13, 2023 - 4:00 PM

The ability to distinguish between active galactic nuclei (AGN) and star-forming galaxies is essential to obtaining a pure sample of AGN. Multiple kinds of studies rely upon knowing galaxy populations, such as galaxy evolution or the ionization of the intergalactic medium. Diagnostic diagrams use ratios of emission lines to distinguish AGN from star-forming galaxies in large samples. As the limiting factor in common diagnostic diagram is the use of the Hα emission-line, we find alternative methods that do not rely on Hα and may thus be used at higher redshift.

We select a sample of 805 AGN from the catalog of the Sloan Digital Sky Survey (SDSS) galaxy properties from the Portsmouth group by detection of the high- ionization [Ne V] 3426 Å emission line. We compare the luminosities of [Ne III] 3869 Å, [O III] 5007 Å, [O II] 3726 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 (FLR) diagnostic diagrams that do not rely on Hα 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 photo-ionization 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[OIII] = 42 to lower the contamination of the AGN region by star-forming galaxies down to 9%. This method also allows us to isolate Green Pea and Purple Grape galaxies from the star-forming sample. We also suggest the use of a partial FLR diagram with [Ne III]/[O II] plotted against [O III]/Hβ. Using our new diagnostic diagram, we can reliably classify AGN up to a redshift of z ≤ 1.06 and add more than 472 new AGN to the [Ne V]-selected AGN sample.

Refreshments served at 3:45 PM

If you have any questions about the Colloquium Series  or would like to make a donation please contact the Physics Department, cua-physics@cua.edu or (202) 319-5315.