LUMINOUS INFRARED GALAXIES - AGN OR STARBURSTS

A spectroscopic survey of a sample of more than 200 IRAS galaxies was carried out. Great care was taken in classifying the nuclear spectra of these galaxies as ''H II region-like'' or ''AGN-like'' using a large number of line-ratio diagnostics corrected for the underlying stellar absorption features. We confirm that both the fraction of luminous infrared galaxies (''LIGs'') with AGN-like spectra and the fraction of Seyferts among the AGN-like objects increase with infrared luminosity. The fraction of LINER-like objects, on the other hand, is relatively constant at approximately 25%. The origin of the line emission sometimes is a function of the distance from the nucleus. Based on the emission-line ratios and the strengths of the absorption features, circumnuclear starburst activity appears to be common among LIGs, regardless of their nuclear spectral types. Line width information suggests that large-scale nuclear winds are present in many of these objects and are an efficient way of getting rid of the obscuring material in the nuclear region. Based on their emission-line, absorption-line, and morphological properties, Seyfert LIGs appear to be at a more advanced stage of evolution than H II region-like LIGs.