We present first results from our Very Large Telescope large program to study the dynamical evolution of Ultraluminous Infrared Galaxies ( ULIRGs ) , which are the products of mergers of gas-rich galaxies .
The full data set consists of high resolution , long-slit , H- and K-band spectra of 38 ULIRGs and 12 QSOs ( between 0.042 < z < 0.268 ) .
In this paper , we present the sources that have not fully coalesced , and therefore have two distinct nuclei .
This sub-sample consists of 21 ULIRGs , the nuclear separation of which varies between 1.6 and 23.3 kpc .
From the CO bandheads that appear in our spectra , we extract the stellar velocity dispersion , \sigma , and the rotational velocity , V _ { rot } .
The stellar dispersion equals 142 km s ^ { -1 } on average , while V _ { rot } is often of the same order .
We combine our spectroscopic results with high-resolution infrared ( IR ) imaging data to study the conditions for ULIRG activity in interacting pairs .
We find that the majority of ULIRGs are triggered by almost equal-mass major mergers of 1.5:1 average ratio .
Less frequently , 3:1 encounters are also observed in our sample .
However , less violent mergers of mass ratio > 3:1 typically do not force enough gas into the center to generate ULIRG luminosities .