Resolving the far-IR line deficit: Photoelectric heating and far-IR line cooling in NGC 1097 and NGC 4559

The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the interstellar medium over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C II] 158 μm, [O I] 63 μm, and [N II] 122 μm in NGC 1097 and NGC 4559, obtained with the Photodetector Array Camera & Spectrometer on board Herschel. We matched new observations to existing Spitzer Infrared Spectrograph data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C II] + [O I])/TIR, decreases as the far-infrared (far-IR) color, νf _ν(70 μm)νf _ν(100 μm), increases. In contrast, the ratio of far-IR cooling to total PAH emission, ([C II] + [O I])/PAH, is a near constant 6% over a wide range of far-IR color, 0.5 < νf _ν(70 μm)νf _ν(100 μm) ≲ 0.95. In the warmest regions, where νf _ν(70 μm)νf _ν(100 μm) ≳ 0.95, the ratio ([C II] + [O I])/PAH drops rapidly to 4%. We derived representative values of the local ultraviolet radiation density, G ₀, and the gas density, n _H, by comparing our observations to models of photodissociation regions. The ratio G ₀/n _H, derived from fine-structure lines, is found to correlate with the mean dust-weighted starlight intensity, 〈U〉, derived from models of the IR spectral energy distribution. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7/11.3 μm PAH ratio in regions that exhibit a deficit in ([C II] + [O I])/PAH, suggesting that small grains are ionized in these environments.