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 a...

High-resolution imaging of the molecular outflows in two mergers: IRAS 17208-0014 and NGC 1614

“…Bellocchi et al (2012) find a main component associated with a rotating gas disk and a second (broader) component blueshifted by up to 300 km s −1 relative to the systemic velocity (v sys ) of the galaxy. The latter was interpreted as the signature of a dusty outflow in the ionized gas.…”

“…Overall, the bulk of the molecular outflow is cospatial with the 2 -diameter Paα star-forming ring imaged by Alonso-Herrero et al (2001). However, the Hα emission coming from the blueshifted lobe of the outflow, identified by Bellocchi et al (2012), extends farther out up to r ∼ 5 (1.6 kpc) on the eastern side of the disk. In this region, there is tentative evidence of outflowing CO emission that stems from a number (∼4) of clumps.…”

“…2). The two targets present previous evidence of an outflow in several gas tracers, including the ionized, the atomic, and the molecular ISM (NGC 1614: Bellocchi et al 2012;IRAS 17208-0014: Sturm et al 2011;Rupke & Veilleux 2013;Arribas et al 2014). Both systems are classified as mergers in an advanced stage of interaction, and they host massive episodes of star formation.…”

Warm molecular gas temperature distribution in six local infrared bright Seyfert galaxies

“…Three of the galaxies (NGC 34, IRASF 05189-2524, and UGC 05101) are advanced major mergers. The other three galaxies (NGC 5135,NGC 7130,and NGC 7469) are spirals, although NGC 7130 and NGC 7469 have peculiar morphologies suggesting recent minor interactions (Genzel et al 1995;Bellocchi et al 2012). …”

The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time

“…Kinemetry (Krajnović et al 2006) was originally developed to measure the fine kinematic structure of local elliptical galaxies and is the kinematic extension of photometric moments. It has been applied to a small sample of four local IR-selected merging galaxies by Bellocchi, Arribas, & Colina (2012), who found good consistency with photometric classifications. There is Kinematics of no publication presenting quantitative or qualitative kinematic classification of a large sample of local mergers, so this would be valuable future work for comparison with highredshift, where as we will see in Section 4.5 this has been done of necessity.…”