We present the first results of a survey of the [CII]158um emission line in 241 luminous infrared galaxies (LIRGs) comprising the Great Observatories All-sky Survey (GOALS) sample, obtained with the PACS instrument on board Herschel. The [CII] lumino
sities of the LIRGs in GOALS range from ~10^7 to 2x10^9 Lsun. We find that LIRGs show a tight correlation of [CII]/FIR with far-IR flux density ratios, with a strong negative trend spanning from ~10^-2 to 10^-4, as the average temperature of dust increases. We find correlations between the [CII]/FIR ratio and the strength of the 9.7um silicate absorption feature as well as with the luminosity surface density of the mid-IR emitting region (Sigma_MIR), suggesting that warmer, more compact starbursts have substantially smaller [CII]/FIR ratios. Pure star-forming (SF) LIRGs have a mean [CII]/FIR ~ 4x10^-3, while galaxies with low 6.2um PAH equivalent widths (EWs), indicative of the presence of active galactic nuclei (AGN), span the full range in [CII]/FIR. However, we show that even when only pure SF galaxies are considered, the [CII]/FIR ratio drops by an order of magnitude, from 10^-2 to 10^-3, with Sigma_MIR and Sigma_IR, implying that the [CII] luminosity is not a good indicator of the star formation rate (SFR) for most LIRGs, for it does not scale linearly with the warm dust emission. Moreover, even in LIRGs in which 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 most AGNs do not contribute significantly to the far-IR emission. We provide an empirical relation between the [CII]/FIR and the specific SFR (SSFR) for SF LIRGs. Finally, we present predictions for the starburst size based on the observed [CII] and far-IR luminosities which should be useful for comparing with results from future surveys of high-redshift galaxies with ALMA and CCAT.
We have studied the molecular hydrogen energetics of the edge-on spiral galaxy NGC,891, using a 34-position map in the lowest three pure rotational H$_2$ lines observed with the Spitzer Infrared Spectrograph. The S(0), S(1), and S(2) lines are bright
with an extinction corrected total luminosity of $sim2.8 times 10^{7}$ L$_{odot}$, or 0.09% of the total-infrared luminosity of NGC,891. The H$_2$ line ratios are nearly constant along the plane of the galaxy -- we do not observe the previously reported strong drop-off in the S(1)/S(0) line intensity ratio in the outer regions of the galaxy, so we find no evidence for the very massive cold CO-free molecular clouds invoked to explain the past observations. The H$_2$ level excitation temperatures increase monotonically indicating more than one component to the emitting gas. More than 99% of the mass is in the lowest excitation (T$_{ex}$ $sim$125 K) ``warm component. In the inner galaxy, the warm H$_2$ emitting gas is $sim$15% of the CO(1-0)-traced cool molecular gas, while in the outer regions the fraction is twice as high. This large mass of warm gas is heated by a combination of the far-UV photons from stars in photo-dissociation regions (PDRs) and the dissipation of turbulent kinetic energy. Including the observed far-infrared [OI] and [CII] fine-structure line emission and far-infrared continuum emission in a self-consistent manner to constrain the PDR models, we find essentially all of the S(0) and most (70%) of the S(1) line arises from low excitation PDRs, while most (80%) of the S(2) and the remainder of the S(1) line emission arises from low velocity microturbulent dissipation.
