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.

Abstract. Infrared to millimetre spectral energy distributions (SEDs) have been obtained for 41 bright ultraluminous infrared galaxies (ULIRGs). The observations were carried out with ISOPHOT between 10 and 200 µm and supplemented for 16 sources with JCMT/SCUBA at 450 and 850 µm and with SEST at 1.3 mm. In addition, seven sources were observed at 1.2 and 2.2 µm with the 2.2 m telescope on Calar Alto. These new SEDs represent the most complete set of infrared photometric templates obtained so far on ULIRGs in the local universe. The SEDs peak at 60-100 µm and show often a quite shallow Rayleigh-Jeans tail. Fits with one single modified blackbody yield a high FIR opacity and small dust emissivity exponent β < 2. However, this concept leads to conflicts with several other observational constraints, like the low PAH extinction or the extended filamentary optical morphology. A more consistent picture is obtained using several dust components with β = 2, low to moderate FIR opacity and cool (50 K > T > 30 K) to cold (30 K > T > 10 K) temperatures. This provides evidence for two dust stages, the cool starburst dominated one and the cold cirrus-like one. The third stage with several hundred Kelvin warm dust is identified in the AGN dominated ULIRGs, showing up as a NIR-MIR powerlaw flux increase. While AGNs and SBs appear indistinguishable at FIR and submm wavelengths, they differ in the NIR-MIR. This suggests that the cool FIR emitting dust is not related to the AGN, and that the AGN only powers the warm and hot dust. In comparison with optical and MIR spectroscopy, a criterion based on the SED shapes and the NIR colours is established to reveal AGNs among ULIRGs. Also the possibility of recognising evolutionary trends among the ULIRGs via the relative amounts of cold, cool and warm dust components is investigated.

Abstract.Mining the ISO data archive we provide the complete ISO view of PG quasars containing 64 infrared spectral energy distributions between 5 and 200 µm. About half of the sample was supplemented by MAMBO and SCUBA (sub-)millimetre data. Since the PG quasars were selected optically, the high infrared detection rate of more than 80% suggests that every quasar possesses luminous to hyperluminous dust emission with dust masses comparable to Seyferts and ultraluminous IR galaxies (ULIRGs). The gas-to-dust mass ratio (of those sources where CO measurements are available in the literature) is consistent with the galactic value providing further evidence for the thermal nature of the IR emission of radio quiet quasars. The SEDs represent templates of unprecedented detail and sensitivity. The power-law like near-to mid-IR SEDs (F ν ∝ ν α ) are smooth up to far-infrared wavelengths, favouring dust heating by the central AGN, and we conclude that, in particular for our hyperluminous quasars at z = 1, starbursts play only a minor role for powering the dust emission, even in the FIR. The IR spectral slopes α 1−10 µm range from -0.9 to -2.2 with a mean of −1.3 ± 0.3. They neither correlate with the optical spectral slope α 0.3−1 µm , nor with the IR luminosity, nor with the FIR/MIR luminosity ratio, nor with inclination-dependent extinction effects in the picture of a dusty torus. We suggest that the diversity of the SEDs reflects largely the evolution of the dust distribution, and we propose a classification of the SED shapes as well as an evolutionary scheme in which this variety can be understood. During the evolution the surrounding dust redistributes, settling more and more into a torus/disk like configuration, while the SEDs show an initial FIR bump, then an increasing MIR emission and a steeper near-to mid-infrared slope, both of which finally also decrease. Strikingly, based on the sensitive ISO data now we do not only see the coarse IR differences between ULIRGs and quasars, but also the details and a possible evolution of the dust distribution and emission even among the optically selected PG sample. Regarding cosmic evolution, our hyperluminous quasars in the "local" universe at z = 1 do not show the hyperluminous (L FIR > ∼ 10 13 L ) starburst activity inferred for z = 4 quasars detected in several (sub-)millimetre surveys. In view of several caveats this difference should be established further, but it already suggests that in the early dense universe stronger merger events led to more powerful starbursts accompanying the quasar phenomenon, while at later cosmic epochs any coeval starbursts obviously do not reach that high power and are outshone by the AGN.

Abstract. The FIRBACK (Far Infrared BACKground) survey is one of the deepest imaging surveys carried out at 170 µm with ISOPHOT onboard ISO, and is aimed at the study of the structure of the Cosmic Far Infrared Background. This paper provides the analysis of resolved sources. After a validated process of data reduction and calibration, we perform intensive simulations to optimize the source extraction, measure the confusion noise (σc = 45 mJy), and give the photometric and astrometric accuracies. 196 galaxies with flux S > 3σc are detected in the area of 3.89 square degrees. Counts of sources with flux S > 4σc present a steep slope of 3.3 ± 0.6 on a differential "log N-log S" plot between 180 and 500 mJy. As a consequence, the confusion level is high and will impact dramatically on future IR deep surveys. This strong evolution, compared with a slope of 2.5 from Euclidian geometry, is in line with models implying a strongly evolving Luminous Infrared Galaxy population. The resolved sources account for less than 10% of the Cosmic Infrared Background at 170 µm, which is expected to be resolved into sources in the 1 to 10 mJy range.Key words. cosmology: miscellaneous -galaxies: infrared -galaxies: evolution -galaxies: statistics Send offprint requests to: H. Dole, e-mail: hdole@as.arizona.edu Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.Tables 7 to 13 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via

Abstract. The Sc galaxy M 33 has been mapped with ISOPHOT in the far-infrared, at 60, 100, and 170 µm. The spatial resolution of these FIR maps allows the separation of spiral arms and interarm regions and the isolation of a large number of star-forming regions. The spectral energy distribution in the FIR indicates a superposition of two components, a warm one originating from dust at ∼45 K, and a cold one, at ∼16 K. The warm component is concentrated towards the spiral arms and the star-forming regions, and is likely heated by the UV radiation from OB stars. The cold component is more smoothly distributed over the disk, and heated by the diffuse interstellar radiation. For the about 60 star-forming regions detected the Hα/FIR flux ratio increases significantly with the distance from the galaxy center, probably due to decreasing extinction. An anti-correlation of F Ha /F 60 with F 170 suggests the intrinsic extinction to be related to the cold dust surface brightness according to A V /S 170 ∼ 0.03 mag MJy −1 sr. For the total galaxy the star formation rate (SFR) derived from the FIR is in agreement with that derived from the de-extincted Hα emission. For individual star-forming regions, a consistency between SFRs derived from the optical and from the FIR requires only a fraction of the UV radiation to be absorbed locally. The individual star-forming regions also show a local radio-FIR correlation. This local correlation is, however, due to quite different components than to those that lead to the well-known global radio-FIR correlation for entire galaxies.