Dynamical evolution of local gas-rich galaxy mergers from NIR spectroscopy

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.