The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/Blue ratios (indicating star-formation activity and optical depth).In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L [CII] /L FIR with increasing F ν (60 µm)/F ν (100 µm) and L FIR /L B , spanning a range of a factor of more than 50. Two galaxies, at the warm and active extreme of the range have L [CII] /L FIR < 2 × 10 −4 (3σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains.The ratio of the two principal photodissociation region (PDR) cooling lines L [OI] /L [CII] shows a tight correlation with F ν (60 µm)/F ν (100 µm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II](122 µm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. (1999) with observed ratios of (a) L [OI] /L [CII] and (L [CII] + L [OI] )/L FIR and (b) L [OI] /L FIR and F ν (60 µm)/F ν (100 µm) yields far-UV flux G 0 and gas density n. The G 0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5 respectively in more than half the sources.The derived G 0 and n correlate with each other, and G 0 increases with n as G 0 ∝ n α , where α ≈ 1.4 . We interpret this correlation as arising from Strömgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270 − 900 K) and pressure (6 × 10 4 − 1.5 × 10 7 K cm −3 ) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G 0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G 0 and n have larger and/or denser star-forming clouds.

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We present Spitzer IRS spectra (R ∼ 600, 10 -38 µm) of 38 positions in the Galactic Center (GC), all at the same Galactic longitude and spanning ±0.3 • in latitude. Our positions include the Arches Cluster, the Arched Filaments, regions near the Quintuplet Cluster, the "Bubble" lying along the same line-ofsight as the molecular cloud G0.11−0.11, and the diffuse interstellar gas along the line-of-sight at higher Galactic latitudes. From measurements of the, and H 2 S(0), S(1), and S(2) lines we determine the gas excitation and ionic abundance ratios. The Ne/H and S/H abundance ratios are ∼ 1.6 times that of the Orion Nebula. The main source of excitation is photoionization, with the Arches Cluster ionizing the Arched Filaments and the Quintuplet Cluster ionizing the gas nearby and at lower Galactic latitudes including the far side of the Bubble. In addition, strong shocks ionize gas to O +3 and destroy dust grains, releasing iron into the gas phase (Fe/H∼ 1.3 × 10 −6 in the Arched Filaments and Fe/H∼ 8.8 × 10 −6 in the Bubble). The shock effects are particularly noticeable in the center of the Bubble, but O +3 is present in all positions. We suggest that the shocks are due to the winds from the Quintuplet Cluster Wolf-Rayet stars. On the other hand, the H 2 line ratios can be explained with multi-component models of warm molecular gas in photodissociation regions without the need for H 2 production in shocks.

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