Abstract

This study aimed to trace how the stellar kinematics evolve when two gas-rich galaxies merge. To study the evolution of such mergers, Very-Large-Telescope observations were performed for the best-suited galaxy population in the local Universe, the Ultraluminous Infrared Galaxies (ULIRGs). The prodigious infrared emission in ULIRGs originates from the intense star formation that occurs when large amounts gas are driven to the center of the merging system. We acquired near-infrared H- and K-band spectroscopic data of 54 ULIRGs at various merger phases; approximately half of the sources in our sample have two distinct nuclei. For each source, we derived the stellar kinematics from the high-resolution spectra by studying the profile of the stellar CO absorption lines. The shape of the profile along various slit positions enables us to measure the rotational velocity, V_rot, and velocity dispersion, sigma, and to compute the stellar and black hole (BH) masses. We find that ULIRGs are mostly triggered by mergers of roughly equal-mass galaxies. Their mean velocity dispersion equals 150+- 33 km/s. A possible trend of dynamical heating of the galaxies as the merger evolves is observed at marginal statistical levels. The kinematic, structural, and photometric properties of ULIRGs indicate that they are dispersion-dominated systems and that they mainly result in the formation of 10^10-10^11 solar-mass elliptical galaxies (Es). Their locus on the fundamental plane of Es indicates that their end products are typically inconsistent with giant Es. The BH masses in ULIRGs are of the order 10^7- 10^8 solar. To investigate whether ULIRGs go through quasar (QSO) phases during their evolution, we have acquired similar data for 12 local Palomar-Green (PG) QSOs. The mean bulge dispersion of the PG QSOs in our sample equals 186 +-24 km/s. The measurement of the stellar dispersion in QSOs enables us to place them on significant observational diagrams, such as the local BH mass and host-galaxy bulge relation and the fundamental plane of Es. On the latter, PG QSOs are located between the regions occupied by moderate-mass and giant Es. Their bulge and BH masses are on the order of 10^11 and 5*10^7-10^8 solar masses respectively. PG QSOs seem to be triggered by gas-rich mergers, and therefore likely formed in an analogous manner to ULIRGs. However, other local QSOs with supermassive black holes of 5*10^8-10^9 solar masses that reside in massive spheroids have a different formation mechanism.