Optical spectral classification of southern ultraluminous infrared galaxies

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 the Cerro Tololo Inter-American Observatory (CTIO) 4 -m telescope and provided by the Two-degree Field (2dF) Galaxy Redshift Survey and the Six-degree Field (6dF) Galaxy Survey, we measure emission linewidths and fluxes for spectral classification. We determine the spectral types of ULIRGs with Hα measurement using the standard diagnostic diagrams. For ULIRGs without Hα measurement, we determine their spectral types using the plane of flux ratio between [O III]λ5007 and Hβ versus [O III] linewidth 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 eight 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 Infrared Astronomical Satellite (IRAS) 25-60 μm colour, consistent with previous studies. The IRAS 25-60 μm colour distributions are significantly different between AGN and non-AGN ULIRGs, while their IRAS 60-100 μm colour distributions are similar.