September 2017 • 2017ApJ...846...32D
Abstract • We present an analysis of [O I]63, [O III]88, [N II]122, and [C II]158 far-infrared (FIR) fine-structure line observations obtained with Herschel/PACS, for ∼240 local luminous infrared galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey. We find pronounced declines ("deficits") of line-to-FIR continuum emission for [N II]122, [O I]63, and [C II]158 as a function of FIR color and infrared luminosity surface density, ΣIR. The median electron density of the ionized gas in LIRGs, based on the [N II]122/[N II]205 ratio, is ne = 41 cm-3. We find that the dispersion in the [C II]158 deficit of LIRGs is attributed to a varying fractional contribution of photodissociation regions (PDRs) to the observed [C II]158 emission, f([C II]158PDR) = [C II]158PDR/[C II]158, which increases from ∼60% to ∼95% in the warmest LIRGs. The [O I]63/[C II]158PDR ratio is tightly correlated with the PDR gas kinetic temperature in sources where [O I]63 is not optically thick or self-absorbed. For each galaxy, we derive the average PDR hydrogen density, nH, and intensity of the interstellar radiation field, G, in units of G0 and find G/nH ratios of ∼0.1-50 G0 cm3, with ULIRGs populating the upper end of the distribution. There is a relation between G/nH and Σ IR, showing a critical break at ΣIR* ≃ 5 × 1010 L⊙ kpc-2. Below ΣIR*, G/nH remains constant, ≃0.32 G0 cm3, and variations in ΣIR are driven by the number density of star-forming regions within a galaxy, with no change in their PDR properties. Above Σ IR*, G/nH increases rapidly with ΣIR, signaling a departure from the typical PDR conditions found in normal star-forming galaxies toward more intense/harder radiation fields and compact geometries typical of starbursting sources.