GALAXIES WITH EXTREME INFRARED AND FE-II EMISSION .1. MARKARIAN-231 -THE SIGNATURE OF A YOUNG INFRARED QSO

We investigate the ultraluminous IR Galaxy/QSO Mrk 231 by means of lon g-slit optical spectroscopy, high spatial resolution broad-band optica l imaging and UV IUE spectra. The spectrum shows an extreme Fe II opti cal emission (Fe II(OPT)/Hbeta almost-equal-to 8), broad Balmer and Na ID lines, weak high-excitation lines, double-peaked optical narrow em ission lines with velocity differences of about 1000 km s-1, a steep U V spectrum, and a weak Lyalpha line. These spectral features are expla ined ''mainly'' by the presence of nuclear and circumnuclear starburst s. The high spatial resolution broad-band images show details of two i nteresting blue circumnuclear subregions, in particular: (1) a blue re gion 2''-5'' west of the nucleus; and (2) a blue arc (''horseshoe'') a t approximately 3''.5 S. In ''region I'' (circumnuclear star-forming r egion located at approximately 2''-5'' to the west from the nucleus) w e detect an emission-line system (E0) with a velocity (V(E0) = 7941 +/ - 80 km s-1) similar to that of the nuclear system BAL1 (V(BAL1) appro ximately 7800 km s-1), the strongest of the three broad absorption-lin e systems. Moreover, in this region we also detect the probable presen ce of this BAL1 system (V(NaID) = 7840 +/- 120 km s-1). Consequently, Mrk 231 is the first candidate where a direct link, at least kinematic al, between a star-formation process and the BAL phenomenon is observe d. We discuss physical, kinematic and morphological evidence of a stro ng nuclear and circumnuclear starburst (with superwind/superbubble and supernova of Type II), in Mrk 231. These results and studies are cons istent with a scenario where this ultraluminous IR galaxy has a compos ite nature in the nuclear region, which is the consequence of the fina l phases of an ongoing merger process. The starburst is the dominant s ource of nuclear energy and the nonthermal active galactic nuclei rema ins strongly obscured. We also discuss the extreme properties of Mrk 2 31 as the probable characteristics/signature typical of a young IR QSO .