Statistical properties of ultraluminous IRAS galaxies from an HST imaging survey

We perform photometric measurements on a large HST snapshot imaging survey sample of 97 ultraluminous infrared galaxies (ULIRGs). We select putative n uclei from bright clumps in all the sample targets, mainly based on a quant itative criterion of I-band luminosity as well as the global and local morp hological information. All the sources are then classified into three categ ories with multiple, double, and single nucleus/nuclei. The resultant fract ions of multiple, double, and single nucleus/nuclei ULIRGs are 18%, 39%, an d 43%, respectively. This supports the multiple merger scenario as a possib le origin of ULIRGs, in addition to the commonly accepted pair merger model . Further statistical studies indicate that the AGN fraction increases from multiple (36%) to double (65%) and then to single (80%) nucleus/nuclei ULI RGs. For the single nucleus category, there is a high-luminosity tail in th e luminosity distribution, which corresponds to a Seyfert 1/QSO excess. Thi s indicates that active galactic nuclei tend to appear at final merging sta ge. For multiple/double nuclei galaxies, we also find a high fraction of ve ry close nucleus pairs (e.g., 3/4 for those separated by less than 5 kpc). This strengthens the conclusion that systems at late merging phase preferen tially host ULIRGs.