Abstract:
　  We present new spectroscopic observations of the main far-infrared (FIR) emission lines detected in a sample of nearby Luminous Infrared Galaxies (LIRGs) using the PACS instrument onboard the Herschel Space Telescope. The Great Observatories All-sky LIRG Survey (GOALS) is a complete, flux-limited galaxy sample that comprises the 202 LIRGs systems (z < 0.088) included in the IRAS Revised Bright Galaxy Sample. The GOALS sample has been observed using Chandra, Galex, HST, Spitzer, and AKARI, among other first-class telescopes. Now, as part of a Herschel Open Time Cycle 1 proposal (PI: L. Armus), we have obtained [CII]158-micron and [OI]63-micron emission line spectroscopy for all GOALS systems as well as [OIII]88-micron spectroscopy for half of the sample. We confirm that LIRGs are a critical galaxy population that bridges the gap between lower luminosity star-forming galaxies showing normal [CII] / L_FIR ratios, and the most IR luminous sources (ULIRGs), which display ratios one order of magnitude smaller -- the so-called far infrared "deficit". We also show that the [CII] / IRAC_8-micron ratio is constant for most of the LIRGs independently of their FIR luminosity, implying that polycyclic aromatic hydrocarbons (PAHs), through the emission of photo-electrons, are the main responsible for the heating of the FIR lines.