Local luminous infrared galaxies. I. Spatially resolved observations with the Spitzer infrared spectrograph

© 2010 The American Astronomical Society. M.P.-S. acknowledges support from the CSIC under grant JAE-Predoc-2007. Support for this work was provided by NASA through contract 1255094 issued by JPL/California Institute of Technology. M.P.-S., A.A.-H., L. C., and T.D.-S. acknowledge support from the Spanish Plan Nacional del Espacio under grant ESP2007-65475-C02-01. A.A.-H. also acknowledges support for this work from the Spanish Ministry of Science and Innovation through Proyecto Intramural Especial under grant number 200850I003. T. D. S. acknowledges support from the Consejo Superior de Investigaciones Científicas under grant I3P-BPD-2004. P.G.P.-G. acknowledges support from the Ramón y Cajal Program financed by the Spanish Government and the European Union, and from the Spanish Programa Nacional de Astronomía y Astrofísica under grants AYA 2006-02358 and AYA 2006-15698-C02-02. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.We present results from the Spitzer Infrared Spectrograph spectral mapping observations of 15 local luminous infrared galaxies (LIRGs). In this paper, we investigate the spatial variations of the mid-IR emission which includes fine structure lines, molecular hydrogen lines, polycyclic aromatic features (PAHs), continuum emission, and the 9.7 μm silicate feature. We also compare the nuclear and integrated spectra. We find that the star formation takes place in extended regions (several kpc) as probed by the PAH emission, as well as the [Ne II]12.81 μm and [Ne III]15.56 μm emissions. The behavior of the integrated PAH emission and 9.7 μm silicate feature is similar to that of local starburst galaxies. We also find that the minima of the [Ne III]15.56 μm/[Ne II]12.81 μm ratio tends to be located at the nuclei and its value is lower than that of H II regions in our LIRGs and nearby galaxies. It is likely that increased densities in the nuclei of LIRGs are responsible for the smaller nuclear [Ne III]15.56 μm/[Ne II]12.81 μm ratios. This includes the possibility that some of the most massive stars in the nuclei are still embedded in ultracompact H II regions. In a large fraction of our sample, the 11.3 μm PAH emission appears more extended than the dust 5.5 μm continuum emission. We find a dependency of the 11.3 μm PAH/7.7 μm PAH and [Ne II]12.81 μm/11.3 μm PAH ratios with the age of the stellar populations. Smaller and larger ratios, respectively, indicate recent star formation. The estimated warm (300 K <T< 1000 K) molecular hydrogen masses are of the order of 10^8 M_☉, which are similar to those found in ULIRGs, local starbursts, and Seyfert galaxies. Finally we find that the [Ne II]12.81 μm velocity fields for most of the LIRGs in our sample are compatible with a rotating disk at ~kpc scales, and they are in a good agreement with Hα velocity fields.Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEMinisterio de Economía y Competitividad (MINECO)Ministerio de Ciencia e Innovación (MICINN)Consejo Superior de Investigaciones Científicas (CSIC)Programa Nacional del Espacio (PNE) / Plan Estatal de Investigación Científica y Técnica y de Innovación , MINECOAyudas para contratos Ramón y Cajal (RYC), MICINNGobierno de EspañaUnión Europea (UE)Programa Nacional de Astronomía y Astrofísica (PNAyA)National Aeronautics and Space Administration (NASA)Proyecto Intramural Especial (PIE), CSICpu