VLT-VIMOS integral field spectroscopy of luminous and ultraluminous infrared galaxies

Abstract

Context. Luminous infrared galaxies (LIRGs) are an important class of objects in the low-z universe bridging the gap between normal spirals and the strongly interacting and starbursting ultraluminous infrared galaxies (ULIRGs). Since a large fraction of the stars in the Universe have been formed in these objects, LIRGs are also relevant in a high-z context. Studies of the two-dimensional physical properties of LIRGs are still lacking. Aims. We aim to understand the nature and origin of the ionization mechanisms operating in the extra-nuclear regions of LIRGs as a function of the interaction phase and infrared luminosity. Methods. This study uses optical integral field spectroscopy (IFS) data obtained with VIMOS. Our analysis is based on over 25 300 spectra of 32 LIRGs covering all types of morphologies (isolated galaxies, interacting pairs, and advanced mergers), and the entire 10 11 −10 12 L infrared luminosity range. Results. We found strong evidence for shock ionization, with a clear trend with the dynamical status of the system. Specifically, we quantified the variation with interaction phase of several line ratios indicative of the excitation degree. While the [N ii]λ6584/Hα ratio does not show any significant change, the [S ii]λλ6717,6731/Hα and [O i]λ6300/Hα ratios are higher for more advanced interaction stages. Velocity dispersions are higher than in normal spirals and increase with the interaction class (medians of 37, 46, and 51 km s −1 for class 0−2, respectively). We constrained the main mechanisms causing the ionization in the extra-nuclear regions (typically for distances ranging from ∼0.2−2.1 kpc to ∼0.9−13.2 kpc) using diagnostic diagrams. Isolated systems are mainly consistent with ionization caused by young stars. Large fractions of the extra-nuclear regions in interacting pairs and more advanced mergers are consistent with ionization caused by shocks of v s < ∼ 200 km s −1 . This is supported by the relation between the excitation degree and the velocity dispersion of the ionized gas, which we interpret as evidence for shock ionization in interacting galaxies and advanced mergers but not in isolated galaxies. This relation does not show any dependence with the infrared luminosity (i.e. the level of star formation). All this indicates that tidal forces play a key role in the origin of the ionizing shocks in the extra-nuclear regions. We also showed for the first time what appears to be a common log([O i]λ6300/Hα) − log(σ) relation for the extranuclear ionized gas in interacting (U)LIRGs (i.e. covering the entire 10 11.0 −10 12.3 L luminosity range). This preliminary result needs to be investigated further with a larger sample of ULIRGs.

Shutting down or powering up a (U)LIRG? Merger components in distinctly different evolutionary states in IRAS 19115−2124 (the Bird)

“…pure thermal emission. Measurements such as these pin-point why global velocity dispersions in LIRGs correlate with the ionisation level (Monreal-Ibero et al 2010), as do the σ values of the main apertures in our case (Table 4) -large σ indicate outflows which are shock heated.…”

Understanding the two-dimensional ionization structure in luminous infrared galaxies

“…These results come as a surprise as all LIRGs but two (NGC 5135 and NGC 7130) In addition, according to the optical emission line diagnostics, the excitation of the extended emission is characteristic of star formation and LINERs (Monreal-Ibero et al 2006. This extended LINER emission in LIRGs is explained by the presence of large scale shocks (Monreal-Ibero et al 2010) and composite AGN-starbursts (Kewley et al 2001;Yuan et al 2010), and likely not due to the existence of a weak AGN. In the following, we further investigate these apparent inconsistencies by studying in more detail two LIRGs that are identified as prototypes of excitation dominated by star formation (IC 4687), by a Seyfert 2 nucleus (NGC 7130), and a third LIRG (NGC 5135) classified as composite (AGN, and star-forming clumps of different ages and masses).…”

Integral field spectroscopy based Hαsizes of local luminous and ultraluminous infrared galaxies

“…After combining four dither pointings, the total FoV is about 30 × 30 arcsec 2 , with a spaxel scale of 0.67 arcsec (square). Details about the observations, data reduction, and calibration can be found in Monreal-Ibero et al (2010) and Rodríguez-Zaurín et al (2011, hereafter RZ11).…”