Optical Imaging of Very Luminous Infrared Galaxy Systems: Photometric Properties and Late Evolution

A sample of 19 low-redshift (0.03 < z < 0.07), very luminous infrared galaxy [VLIRG: 1011 L⊙ < L(8–1000 μm) < 1012 L⊙] systems (30 galaxies) has been imaged in B, V, and I using ALFOSC with the Nordic Optical Telescope. These objects cover a luminosity range that is key to linking the most luminous infrared galaxies with the population of galaxies at large. As previous morphological studies have reported, most of these objects exhibit features similar to those found in ultraluminous infrared galaxies (ULIRGs), which suggests that they are also undergoing strong interactions or mergers. We have obtained photometry for all of these VLIRG systems, the individual galaxies (when detached), and their nuclei, and the relative behavior of these classes has been studied in optical color-magnitude diagrams. The observed colors and magnitudes for both the systems and the nuclei lie parallel to the reddening vector, with most of the nuclei having redder colors than the galaxy disks. Typically, the nuclei compose 10% of the total flux of the system in B and 13% in I. The photometric properties of the sample are also compared with previously studied samples of ULIRGs. The mean observed optical colors and magnitudes agree well with those of cool ULIRGs. The properties of the nuclei also agree with those of warm ULIRGs, although the latter show a much larger scatter in both luminosity and color. Therefore, the mean observed photometric properties of VLIRG and ULIRG samples, considered as a whole, are indistinguishable at optical wavelengths. This suggests that not only ULIRGs, but also the more numerous population of VLIRGs, have similar rest-frame optical photometric properties to the submillimeter galaxies (SMGs), reinforcing the connection between low-z LIRGs and high-z SMGs. When the nuclei of the young and old interacting systems (classified according to a scheme based on morphological features) are considered separately, some differences between the VLIRG and the ULIRG samples are found. In particular, although the young VLIRGs and ULIRGs seem to share similar properties, the old VLIRGs are less luminous and redder than old ULIRG systems. If confirmed with larger samples, this behavior suggests that the late-stage evolution is different for VLIRGs and ULIRGs. Specifically, as suggested from spectroscopic data, the present photometric observations support the idea that the activity during the late phases of VLIRG evolution is dominated by starbursts, while a higher proportion of ULIRGs could evolve into a QSO type of object.