2017ApJ...846...32D

We present an analysis of [OI]₆₃, [O III]₈₈, [N II]₁₂₂, and [CII]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, [OI]₆₃, and [CII]₁₅₈ as a function of FIR color and infrared luminosity surface density, Σ_IR. The median electron density of the ionized gas in LIRGs, based on the [N II]₁₂₂/[N II]₂₀₅ ratio, is n_e = 41 cm^–3. We find that the dispersion in the [CII]₁₅₈ deficit of LIRGs is attributed to a varying fractional contribution of photodissociation regions (PDRs) to the observed [CII]₁₅₈ emission, f([C II]₁₅₈^PDR) = [C II]₁₅₈^PDR/[CII]₁₅₈, which increases from ∼60% to ∼95% in the warmest LIRGs. The [OI]₆₃/[C II]₁₅₈^PDR ratio is tightly correlated with the PDR gas kinetic temperature in sources where [OI]₆₃ is not optically thick or self-absorbed. For each galaxy, we derive the average PDR hydrogen density, n_H, and intensity of the interstellar radiation field, G, in units of G₀ and find G/n_H ratios of ∼0.1-50 G₀ cm³, with ULIRGs populating the upper end of the distribution. There is a relation between G/n_H and Σ_IR, showing a critical break at Σ_IR^* ≃ 5 x 10¹⁰ L_☉ kpc^–2. Below Σ_IR^* , G/n_H remains constant, ≃0.32 G₀ cm³, and variations in Σ_IR are driven by the number density of star-forming regions within a galaxy, with no change in their PDR properties. Above Σ_IR^* , G/n_H increases rapidly with Σ_IR, 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.