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 cm 3 , with ULIRGs populating the upper end of the distribution. There is a relation between G / and , showing a critical break at ≃ 5 × 10 10 L ⊙ kpc −2 . Below , G / remains constant, ≃0.32 cm 3 , 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.