Bibcode
Monreal-Ibero, A.; Arribas, S.; Colina, L.; Rodríguez-Zaurín, J.; Alonso-Herrero, A.; García-Marín, M.
Bibliographical reference
Astronomy and Astrophysics, Volume 517, id.A28
Advertised on:
7
2010
Journal
Citations
100
Refereed citations
95
Description
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
1011-1012 L&sun; 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
vs ⪉ 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 1011.0-1012.3 L&sun; luminosity
range). This preliminary result needs to be investigated further with a
larger sample of ULIRGs.
Based on observations collected at the European Organisation for
Astronomical Research in the Southern Hemisphere, Chile (ESO Programs
076.B-0479(A), 078.B-0072(A) and 081.B-0108(A)).Figures 14-17 are only
available in electronic form at http://www.aanda.org