Galaxies with extreme infrared and Fe II emission. 1: Markarian 231: The signature of a young infrared QSO

Abstract

We investigate the ultraluminous IR Galaxy/QSO Mrk 231 by means of long-slit optical spectroscopy, high spatial resolution broad-band optical imaging and UV IUE spectra. The spectrum shows an extreme Fe II optical emission (Fe II_OPT_/Hβ ~ 8), broad Balmer and Na ID lines, weak high-excitation lines, double-peaked optical narrow emission lines with velocity differences of about 1000 km s^-1^, a steep UV spectrum, and a weak Lyα line. These spectral features are explained "mainly" by the presence of nuclear and circumnuclear starbursts. The high spatial resolution broad-band images show details of two interesting blue circumnuclear subregions, in particular: (1) a blue region 2"-5" west of the nucleus; and (2) a blue arc ("horseshoe") at ~3.5" S. In "region I" (circumnuclear star-forming region located at ~2"-5" to the west from the nucleus) we 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_ ~ 7800 km s^-1^), the strongest of the three broad absorption-line systems. Moreover, in this region we also detect the probable presence of this BAL1 system (V_NaID_ = 7840 +/- 120 km s^-1^). Consequently, Mrk 231 is the first candidate where a direct link, at least kinematical, between a star-formation process and the BAL phenomenon is observed. We discuss physical, kinematic and morphological evidence of a strong nuclear and circumnuclear starburst with superwind/superbubble and supernova of Type II), in Mrk 231. These results and studies are consistent with a scenario where this ultraluminous IR galaxy has a composite nature in the nuclear region, which is the consequence of the final phases of an ongoing merger process. The starburst is the dominant source of nuclear energy and the nonthermal active galactic nuclei remains strongly obscured. We also discuss the extreme properties of Mrk 231 as the probable characteristics/signature typical of a young IR QSO.