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Local Luminous Infrared Galaxies. I. Spatially resolved observations with Spitzer/IRS

We present results from the Spitzer/IRS 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 9.7um 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 [NeII] and [NeIII] emissions. The behavior of the integrated PAH emission and 9.7um silicate feature is similar to that of local starburst galaxies. We also find that the minima of the [NeIII]/[NeII] ratio tends to be located at the nuclei and its value is lower than that of HII regions in our LIRGs and nearby galaxies. It is likely that increased densities in the nuclei of LIRGs are responsible for the smaller nuclear [NeIII]/[NeII] ratios. This includes the possibility that some of the most massive stars in the nuclei are still embedded in ultracompact HII regions. In a large fraction of our sample the 11.3um PAH emission appears more extended than the dust 5.5um continuum emission. We find a dependency of the 11.3um PAH/7.7 um PAH and [NeII]/11.3um 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_Sun, which are similar to those found in ULIRGs, local starbursts and Seyfert galaxies. Finally we find that the [NeII] 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 H-alpha velocity fields.