An Ultraluminous IRAS Galaxy with an Extremely Soft X-Ray Spectrum

Abstract

Redshift surveys of the extragalactic IRAS sources have revealed a class of galaxies, ultraluminous infrared galaxies (ULIRGs), that radiate an enormous amount of energy in the far-infrared (Soifer et al. 1986), typically LIR ≥ 1012L⊙, which is comparable to the luminosities of quasars. The energy source for most ULIRGs is believed to be massive nuclear starbursts triggered by galaxy interactions or mergers (Rigopoulou et al. 1996, Crawford et al. 1996). There is much debate about the evolution of ULIRGs. Sanders et al. (1988) suggested that ULIRGs are forming QSOs. However, it is not clear how merging galaxies evolve to QSOs. Meanwhile, observations and N-body simulations show that mergers between disk galaxies can form ellipticals. Some ULIRGs, such as Arp 220 and NGC 6240, do have elliptical characteristics. What is the evolutionary connection between QSOs and elliptical galaxies? About 10% of ULIRGs are IR QSOs or Seyfert 1 galaxies, on the basis of their optical spectra and appearance. Some of them are hosted in ellipticals (Hutchings et al. 1988, Lipari et al. 1994). Also, most of them are strong Fen emitters. Studying this subsample of ULIRGs in detail may give some clues to understanding the evolutionary process mentioned above.