VLT/VIMOS integral field spectroscopy of luminous and ultraluminous infrared galaxies: 2D kinematic properties^⋆

¹ Centro de Astrobiología, Departamento de Astrofísica, CSIC-INTA, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: bellochie@cab.inta-csic.es
² Instituto de Física Teórica, Universidad Autónoma de Madrid, 28049 Madrid, Spain

Received: 29 December 2012
Accepted: 21 June 2013

Abstract

Context. (Ultra) Luminous infrared galaxies [(U)LIRGs] host the most extreme star-forming events in the present universe and are places where a significant fraction of the past star formation beyond z ~ 1 has occurred. The kinematic characterization of this population is important to constrain the processes that govern such events.

Aims. We present and discuss the 2D kinematic properties of the ionized gas (Hα) in sample local (U)LIRGs, for which relatively high linear resolution and signal-to-noise (S/N) ratio can be obtained.

Methods. We have obtained Very Large Telescope VIMOS optical integral field spectroscopy (IFS) for 38 local (z < 0.1) (U)LIRGs (31 LIRGs and 7 ULIRGs, 51 individual galaxies). This sample covers well the less studied LIRG luminosity range, and it includes the morphological types corresponding to the different phases along the merging process (i.e., isolated disks, interacting and merging systems).

Results. The vast majority of objects have two main kinematically distinct components. One component (i.e., narrow or systemic) extends over the whole line-emitting region and is characterized by small-to-intermediate velocity dispersions (i.e., σ from 30 to 160 km s^-1). The second component (broad) has a larger velocity dispersion (up to 320 km s^-1); it is mainly found in the inner regions and is generally blueshifted with respect to the systemic component. The largest extensions and extreme kinematic properties are observed in interacting and merging systems, and they are likely associated with nuclear outflows. The systemic component traces the overall velocity field, showing a large variety of kinematic 2D structures, from very regular velocity patterns typical of pure rotating disks (29%) to kinematically perturbed disks (47%) and highly disrupted and complex velocity fields (24%). Thus, most of the objects (76%) are dominated by rotation. We find that rotation is more relevant in LIRGs than in ULIRGs. There is a clear correlation between the different phases of the merging process and the mean kinematic properties inferred from the velocity maps. In particular, isolated disks, interacting galaxies, and merging systems define a sequence of increasing mean velocity dispersion and decreasing velocity field amplitude, characterized by average dynamical ratios (v*_shear/σ_mean) of 4.7, 3.0 and 1.8, respectively. We also find that the σ_central/σ_mean vs. σ_mean plane is an excellent discriminating plane between disks and interacting/merging systems: disks show a mean ratio a factor of 2 larger than those characterizing the other two classes. The LIRGs classified as isolated disks have similar velocity amplitudes but larger mean velocity dispersions (44 vs. 24 km s^-1) than local spirals, implying a larger turbulence and thicker disks. Interacting systems and mergers have values closer to those of low velocity dispersion elliptical/lenticular galaxies (E/SOs). The subclass of (U)LIRGs classified as mergers have kinematical properties similar to those shown by the Lyman break analogs (LBAs), although the dynamical mass of LBAs is five times lower on average. Therefore, despite the difference in mass and dust content, the kinematics of these two local populations appears to have significantly noncircular motions. These motions may be induced by the tidal forces, producing dynamically hot systems. The dynamical masses range from ~0.04 m_⋆ to 1.4 m_⋆ (i.e., m_⋆ = 1.4    ×    10¹¹  M_⊙), with ULIRGs being more massive (i.e., ~0.5 ± 0.2 m_⋆) than LIRGs by, on average, a factor of about 2. The mass ratio of individual pre-coalescence galaxies is <2.5 for most of the systems, confirming that most (U)LIRG mergers involve sub-m_⋆ galaxies of similar mass.