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We present the first results of a survey of the [CII]158um emission line in241 luminous infrared galaxies (LIRGs) comprising the Great ObservatoriesAll-sky Survey (GOALS) sample, obtained with the PACS instrument on boardHerschel. The [CII] luminosities of the LIRGs in GOALS range from ~10^7 to2x10^9 Lsun. We find that LIRGs show a tight correlation of [CII]/FIR withfar-IR flux density ratios, with a strong negative trend spanning from ~10^-2to 10^-4, as the average temperature of dust increases. We find correlationsbetween the [CII]/FIR ratio and the strength of the 9.7um silicate absorptionfeature as well as with the luminosity surface density of the mid-IR emittingregion (Sigma_MIR), suggesting that warmer, more compact starbursts havesubstantially smaller [CII]/FIR ratios. Pure star-forming (SF) LIRGs have amean [CII]/FIR ~ 4x10^-3, while galaxies with low 6.2um PAH equivalent widths(EWs), indicative of the presence of active galactic nuclei (AGN), span thefull range in [CII]/FIR. However, we show that even when only pure SF galaxiesare considered, the [CII]/FIR ratio drops by an order of magnitude, from 10^-2to 10^-3, with Sigma_MIR and Sigma_IR, implying that the [CII] luminosity isnot a good indicator of the star formation rate (SFR) for most LIRGs, for itdoes not scale linearly with the warm dust emission. Moreover, even in LIRGs inwhich we detect an AGN in the mid-IR, the majority (2/3) of galaxies show[CII]/FIR >= 10^-3 typical of high 6.2um PAH EW sources, suggesting that mostAGNs do not contribute significantly to the far-IR emission. We provide anempirical relation between the [CII]/FIR and the specific SFR (SSFR) for SFLIRGs. Finally, we present predictions for the starburst size based on theobserved [CII] and far-IR luminosities which should be useful for comparingwith results from future surveys of high-redshift galaxies with ALMA and CCAT.

Explaining the [CII]158um Deficit in Luminous Infrared Galaxies - First Results from a Herschel/PACS Study of the GOALS Sample