Optical spectral classification of southern ultraluminous infrared galaxies

TL;DR

This study classifies the optical spectral types of 115 southern ULIRGs, identifying AGN presence and correlating spectral features with infrared properties, using new empirical criteria for non-H_alpha spectra.

Contribution

Introduces a new empirical criterion for spectral classification of ULIRGs without H_alpha measurements, achieving about 90% reliability in distinguishing AGNs from non-AGNs.

Findings

Approximately 50% of ULIRGs host AGNs.

AGN fraction increases with infrared luminosity and IRAS 25-60 micron colour.

IRAS 60-100 micron colours are similar between AGN and non-AGN ULIRGs.

Abstract

We present a study of the optical spectral properties of 115 ultraluminous infrared galaxies (ULIRGs) in the southern sky. Using the optical spectra obtained at CTIO 4 m and provided by the 2dF Galaxy Redshift Survey and the 6dF Galaxy Survey, we measure emission line widths and fluxes for spectral classification. We determine the spectral types of ULIRGs with H_alpha measurement using the standard diagnostic diagrams. For ULIRGs without H_alpha measurement, we determine their spectral types using the plane of flux ratio between [OIII]_lambda5007 and H_beta versus [OIII] line width based on our new empirical criterion. This criterion is efficient to distinguish active galactic nuclei (AGNs) from non-AGN galaxies with completeness and reliability of about 90 per cent. The sample of 115 ULIRGs is found to consist of 8 broad-line AGNs, 49 narrow-line AGNs, and 58 non-AGNs. The AGN fraction is on average 50 per cent and increases with infrared luminosity and IRAS 25-60 micron colour, consistent with previous studies. The IRAS 25-60 micron colour distributions are significantly different between AGN and non-AGN ULIRGs, while their IRAS 60-100 micron colour distributions are similar.