Studying the kinematic asymmetries of disks and post-coalescence mergers using a new "kinemetry" criterion

Abstract

Context. Ultra luminous and luminous infrared galaxies [(U)LIRGs] are important galaxy populations for studying galaxy evolution, and are likely to have been responsible for a significant fraction of the star formation that occurred prior to z ~ 1. Local (U)LIRGs can be used to study criteria that are suitable for characterizing similar high redshift populations. We are particularly interested in identifying reliable kinematic-based methods capable of distinguishing disks and mergers, as their relative fraction is a key observational input to constrain different evolutionary scenarios. Aims. Our goal is to analyze in detail the kinematics of the ionized gas of a small sample of LIRGs and study criteria that permit us to characterize the evolutionary status of these systems. Methods. We obtained Very Large Telescope VIMOS optical integral field spectroscopy (IFS) data of four LIRGs selected at similar distances (~70 Mpc) to avoid relative resolution effects. Two of these systems had been previously classified as regular isolated disks galaxies and the other two as post-coalescence mergers based on their morphology. The kinemetry method (developed by Krajnović and coworkers) is used to characterize the kinematic properties of these galaxies and discuss new criteria for distinguishing their status. Results. We present and discuss new kinematic maps (i.e., velocity field and velocity dispersion) for these four galaxies. These kinematic data suggest that nuclear outflows exist in all these galaxies, and are particularly intense for the post-coalescence merger systems. The v c/σ c parameter has values between those that are typical of local spiral galaxies (i.e., v c/σ c = 5-15) and those obtained for Lyman break analogs at z ~ 0.2 (i.e., v c/σ c = 0.4-1.8). Our use of one-dimensional parameters, such as v c/σ c or v shear/ Σ, does not allow us to distinguish between the two groups (i.e., disks, post-coalescence systems). However, when the full two-dimensional kinematic information of the IFS data is analyzed by means of kinemetry, their morphological and kinematic classifications are consistent, with disks having lower kinematic asymmetries than post-coalescence mergers. We propose and discuss a new kinematic criterion to differentiate between these two groups. In particular, we introduce a weighting that favors the outer parts of the kinematic maps when computing the total asymmetries. This step is taken because post-coalescence mergers display relatively small kinematic asymmetries in their inner parts as a consequence of the rapid relaxation of gas into a rotating disk, whereas the outer parts continue to be out of equilibrium (i.e., to have larger asymmetries). We find that, in addition to distinguishing post-coalescence mergers from rotating disks, this new criterion has the advantage of being less sensitive to angular resolution effects. According to previous kinemetry-based analyses designed to distinguish disks and mergers at high-z, the present post-coalescence systems would have been classified as disks. This indicates that the separation of disks from mergers depends on the definition of a merger. It also suggests that previous estimates of the merger/disk ratio might have been underestimated, but larger samples are necessary to establish a firmer conclusion. © 2012 ESO.