A Herschel/PACS Far-infrared Line Emission Survey of Local Luminous Infrared Galaxies

Abstract

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