The Optical Spectroscopic Properties of Seyfert 1 Galaxies Selected from Ultraluminous Infrared Galaxies

Abstract

The ultraluminous infrared galaxies (ULIRGs) are strong interacting or merging galaxies. Statistical study shows that about ten percent of ULIRGs are QSOs or Seyfert 1 galaxies, and high–resolution HST images suggest that they are likely to be at the post merger stage. We investigate the optical spectroscopic properties for a complete sample of 28 QSOs/Seyfert 1s selected mainly from the IRAS galaxy redshift survey QDOT and 1.2 Jy catalogs. The optical spectra were obtained with the 2.16 m optical telescope of the Beijing Astronomical Observatory between October 1998 and the end of 1999. Statistical results for both continuum and emission lines show the following:

1. Most of our sample galaxies are strong or extremely strong FeII emitters with a ratio of FeII/Hβ larger than 2. There also exists an anti-correlation of the ratio of FeII/Hβ with EW(OIII) and FWHM(Hβ).

2. We fit the continuum for all sample galaxies. Aside from a few objects which could be fitted well by a power-law model, the continuum for most sample Seyfert 1 galaxies can be fitted well by a blackbody model or by composites of power–law and blackbody models. For Seyfert 1 galaxies which fit a power–law, E(B-V) is small and in the range of optically selected Seyfert 1 galaxies. However, for Seyfert 1 galaxies fitting a blackbody model, the temperature increases as E(B-V) increases. This is strong observational evidence to support the evolutionary scenario from starbursts to optical QSOs/Seyfert 1s.

3. We also discuss the possible mechanism for strong FeII emission from the point of view of evolution of mergers to AGNs.