Spectral classification of emission-line galaxies from the Sloan Digital Sky Survey. II. A supplementary diagnostic for AGNs using the Dn(4000) index

• Published in: arXiv.org 2011-05
• Main Authors: Marocco, Julien, Hache, Emeric, Lamareille, Fabrice
• Format: Article
• Language: English
• Subjects: Classification; Diagnostic systems; Emission; Galaxies; Infrared spectra; Near infrared radiation; Seyfert galaxies; Sky surveys (astronomy); Spectral classification; Star & galaxy formation; Star formation
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1105.0488

In this paper we present a classification of emission-line galaxies at intermediate and high redshifts (0.52.5 for near-infrared spectra), using the Dn(4000) index as a supplementary diagnostic. Our goal is to complement the diagnostic based only on emission-line ratios from the blue part of the spectra, which suffer from some limitations for the classification of Seyfert 2 and composite galaxies. We used a sample of 89 379 galaxies with a good signal-to-noise ratio from the Sloan Digital Sky Survey (data release 7). Using the classification scheme presented in Paper I, we classified these galaxies with a diagnostic diagram involving the [Oiii]5007 /Hbeta and [Oii]3726+3729 /Hbeta emission-line ratios. Then we derived a supplementary diagnostic involving Dn(4000) to improve this classification, in the regions where objects of different types are mixed. To show the validity of our spectral classification we established success-rate and contamination charts, then we compared our results to those obtained with the reference classification that was scheme obtained also using Halpha, [Nii]6584, and [Sii]6717+6731 emission lines. We show that our supplementary classification based on the Dn(4000) index allows to separate unambiguously star-forming galaxies from Seyfert 2 in the region where they were mixed in Paper I. It also significantly reduces the region where star-forming galaxies are mixed with composites.