1 Introduction

The characteristics of galaxies that emit a substantial amount of their bolometric luminosity in the far infared (FIR) has been a matter of debate since their discovery (e.g., Rieke & Low 1972; Joseph & Wright 1985; Soifer et al. 1987). Particular interest is focussed on the class of ultra-luminous infrared galaxies (ULIRGs), i.e. galaxies with quasar-like infrared luminosities of $L_{\rm IR} \ge 10^{12}~L_{\odot}$(e.g., Sanders & Mirabel 1996; Kennicutt 1996; Genzel et al. 1998; Rigopoulou et al. 1999; Scoville et al. 2000). Nuclear starbursts and/or AGN activity, dynamically triggered by gravitational interactions, are thought to be the energy sources in the cores of ULIRGs. Absorption and re-emission in the FIR is expected to be a consequence of thermalization of radiation by large masses of dust grains.

It has been demonstrated by a large number of studies that a high fraction of luminous and ultraluminous infrared galaxies show morphological peculiarities, such as tidal debris and double nuclei, indicative of gravitaional perturbations. For instance, all ULIRGs of the original IRAS bright galaxy survey (BGS; Sanders et al. 1988a) show indications of strong gravitational interaction or merging. Published values for the interacting/merging fraction among the ULIRGs from various studies cover the range from 50% to about 100% (e.g., Aurière et al. 1996; Clements et al. 1996; Murphy et al. 1996; Duc et al. 1997; Borne et al. 2000). Murphy et al. have analysed near-infrared (NIR) and visual images for 46 luminous infrared galaxies with $L_{\rm IR} > 8.5\times 10^{11}~L_{\odot}$ from the IRAS 2 Jy sample. After combining their sample with the BGS, they find that 95% of the galaxies in the combined sample show evidence for current or past interactions. There are only three ULIRGs in the Murphy et al. sample that do not, to the limits of the images given there, show indications of interactions. Among them is IRAS03158+4227, one of the most luminous ULIRGs ( $\log\,L_{\rm IR}/L_{\odot} = 12.55,\ f_{60}$/ f₁₀₀ = 0.95). This seems astonishing since the fraction of perturbed systems among the FIR-bright galaxies is known to increase with $L_{\rm IR}$ (e.g., Sanders & Mirabel 1996, and references therein).

On the numerical side, simulations clearly show that the major morphological features observed in many peculiar galaxies are explained as being due to tidal forces during galaxy encounters (e.g., Toomre & Toomre 1972; Barnes & Hernquist 1992; Bekki & Noguchi 1994; Spoke 1997; Mihos et al. 1998). Self-consistent models of tidally disturbed galaxies indicate high gas concentrations in the centres due to strong and sudden gaseous inflow (Negroponte & White 1983; Noguchi 1991; Barnes & Hernquist 1991, 1996; Mihos & Hernquist 1996). The models predict that merger-driven gas-dynamics and associated star formation may result in spectacular starbursts (e.g. Noguchi & Ishibashi 1986; Mihos & Hernquist 1996), although there is considerable uncertainty about the treatment of star formation and of the feedback from young stars in the simulations. Galaxy merger and the ULIRG phenomenon were tied together in the models by Mihos & Hernquist (1996).

ULIRGs are widely claimed to represent the final stages of merging galaxies. Murphy et al. (1996) give projected linear separations of less than a few kpc for a large fraction of their sample. In the case of IRAS03158+4227, Murphy et al. noticed a nearby, resolved component which they described as "not apparently interacting with the primary galaxy''. IRAS03158+4227 was therefore not included by these authors in their sample of double-nuclei systems and was subsequently considered a single system where the nuclei separation must be smaller than the available resolution of $0\hbox{$.\!\!^{\prime\prime}$ }8$. The projected linear distance to the nucleus of the galaxy mentioned by Murphy et al. amounts to 18'', corresponding to a projected linear separation of 47 kpc (throughout this paper we adopt an Einstein- de Sitter comology with H₀ = 75 kms^-1Mpc^-1). There are only a few 2 Jy ULIRGs with nuclear distances larger than 10 kpc and only one other system (IRAS14394+5332) with a separation of about 50 kpc.

In this paper, we present deep optical imaging, high-resolution (adaptive optics) NIR imaging, and optical spectroscopy of the remarkable ULIRG IRAS03158+4227. The observational data (imaging and spectroscopy) are described in Sect. 2. The results are presented in Sect. 3 and are discussed in the context of simulations in Sect. 4. Finally, conclusions are given in Sect. 5.