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.

The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESA's far infrared and submillimetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16 × 25 pixels, each, and two filled silicon bolometer arrays with 16 × 32 and 32 × 64 pixels, respectively, to perform integral-field spectroscopy and imaging photometry in the 60−210 μm wavelength regime. In photometry mode, it simultaneously images two bands, 60−85 μm or 85−125 μm and 125−210 μm, over a field of view of ∼1.75 × 3.5 , with close to Nyquist beam sampling in each band. In spectroscopy mode, it images a field of 47 × 47 , resolved into 5 × 5 pixels, with an instantaneous spectral coverage of ∼ 1500 km s −1 and a spectral resolution of ∼175 km s −1 . We summarise the design of the instrument, describe observing modes, calibration, and data analysis methods, and present our current assessment of the in-orbit performance of the instrument based on the performance verification tests. PACS is fully operational, and the achieved performance is close to or better than the pre-launch predictions. Key words. space vehicles: instruments -instrumentation: photometers -instrumentation: spectrographsHerschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

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