This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) to be launched onboard of ESA's Herschel Space Observatory, by 2008. The instrument is designed to be electronically tunable over a wide and continuous frequency range in the Far Infrared, with velocity resolutions better than 0.1 km s-1 and a high sensitivity. This will allow detailed investigations of a wide variety of astronomical sources, ranging from solar system objects, star formation regions to nuclei of galaxies. The instrument comprises 5 frequency bands covering 480–1150 GHz with SIS mixers and a sixth dual frequency band, for the 1410–1910 GHz range, with Hot Electron Bolometer Mixers (HEB). The Local Oscillator (LO) subsystem consists of a Ka-band synthesizer followed by 14 chains of frequency multipliers, 2 chains for each frequency band. A pair of Auto-Correlators and a pair of Acousto-Optic spectrometers process the two IF signals from the dual-polarization front-ends to provide instantaneous frequency coverage of 4 GHz, with a set of resolutions (140 kHz to 1 MHz), better than <0.1 km s-1. After a successful qualification program, the flight instrument entered the testing phase. We will also report on the first pre-flight test and calibration results together with the expected in-flight performance.

We present the observing modes that have been developed for maximizing the science to be obtained from the HIFI instrument on board Herschel during operations. The set of Astronomical Observation Templates and time estimator based on these observing modes are also illustrated.

SPIRE, the Spectral and Photometric Imaging Receiver, is Herschel's submillimetre camera and spectrometer. It comprises a three-band imaging photometer operating at 250, 350 and 500 μm, and an imaging Fourier Transform Spectrometer (FTS) covering 194–672 μm. The design of SPIRE is described, and the expected scientific performance is summarised, based on modelling and flight instrument test results.

The Photodetector Array Camera and Spectrometer on Herschel provides imaging line spectroscopy and imaging photometry in the 55–210 μm wavelength band. In photometry mode, two filled silicon bolometer arrays with 16×32 and 32×64 pixels, respectively, are used to simultaneously image two bands, 60–85 μm or 85–130 μm and 130–210 μm, over a field of view of ~1.75'×3.5', with Nyquist beam sampling in each band. In spectroscopy mode, two Ge:Ga photoconductor arrays (stressed and unstressed) with 16×25 pixels, each, are used to image a field of ~50''×50'', resolved into 5×5 pixels, with a spectral resolution of ~175 km s-1 and an instantaneous spectral coverage of ~1500 km s-1. In both modes the performance is expected to be not far from background-noise limited, with sensitivities (5σ in 1 h) of ~4 mJy or 3–20×10-18 W/m2, respectively. We summarise the design of the instrument and its subunits, describe the observing modes in combination with the telescope pointing modes, report results from instrument level performance tests of the Flight Model, and present our current prediction of the in-orbit performance of the instrument.

An important feature of ISO was its Central Programme. It covered a broad range of topics, ranging from planetary to stellar and interstellar investigations and the exploration of extragalactic sources and the Cosmos. Assurance that the data, gathered over a period of 28 months had been reliably obtained was provided by a deliberate policy to reserve one day in every seven for calibration and cross-calibration purposes. This made sure that data gathered at the beginning of the mission could be linked to data gathered at the end, and that information obtained with one instrument could be reliably backed up by observations obtained by one of the others. These precautions led to several major accomplishments recapitulated here. Spitzer built on some of ISO 's approaches and extended them further. The proposal tool Spot pioneered by Spitzer is facilitating Herschel proposal submission through its offspring HSpot. Spitzer's emphasis on Legacy Projects, large programs to create homogeneously gathered and archived data sets, has influenced the design of Herschel's Key Programmes. Spitzer's deliberate effort to establish collaborations with large ground based observatories and space-observatories covering complementing spectral ranges, led to further coordination in the pursuit of scientific goals. These efforts resulted in remarkable new findings.

The Herschel Satellite and the Acacama Large Millimeter Array (ALMA) are two very large sub-millimeter and Far Infra Red (FIR) astronomy projects that will come into operation in this decade. This report contains descriptions of the ALMA instrument only, since other papers in this volume contain discussions of the Herschel instrument. The emphasis here is on synergies and complementarities between ALMA and Herschel. A collection of projects designed to test the capabilites of ALMA is contained in the Design Reference Science Plan, DSRP, at http://www.str.leidenuniv.nl/alma.

The three instruments on the Herschel Space Observatory, HIFI, SPIRE, and PACS, cover together over a decade in frequency, from 450 GHz to 5000 GHz, with spectral resolutions δν/ν between 0.3 and 10-6. The equivalent temperature range, defined by T = hν/k is 21 K ≤ Teq ≤ 240 K, and thus for dust emission, Herschel will cover the peak of the Planck function in many sources associated with both quiescent and star-forming molecular clouds, both in the Milky Way and other galaxies. The temperature equivalence of the Herschel frequency range also means that essentially all portions of giant molecular clouds will be well probed by the many spectral lines in the Herschel wavelength range of 0.67 mm to 60 µm. My focus in this paper is on gas phase spectral lines of molecules, atoms, and ions, which will be well resolved with the heterodyne receivers on HIFI, but typically not resolved in Galactic sources with the other two instruments. Kinematic interpretation of spectral lines with HIFI is of great interest, along with determination of temperature, density, and column density. I briefly summarize in this review the most important species for Herschel observations, and discuss how one can derive important astronomical quantities from data obtained with the Herschel Space Observatory. Along with describing techniques for analyzing data from the Herschel instruments, I will cover briefly what supplementary ground–based observations are necessary to extract the most from the valuable observing time of the space–based facility.

This contribution aims to give a brief overview of the nature of the dust in the interstellar medium, its rôle there, where it comes from, what happens to it while it resides there and what are some key outstanding dust issues that yet need further attention.

The analysis of spectra obtained with ground based and space based facilities relies on the availability of high quality atomic and molecular data. In this paper, we shortly describe the most relevant data bases for the actual and forthcoming facilities at millimeter, submillimeter and far infrared wavelengths. We discuss how the data bases are organized and implemented, and we present recent advances in the general framework of the Virtual Observatory, aimed at making the access to the molecular physics data both easier and better documented to the final user. We stress that this essential scientific activity should receive proper acknowledgment from astronomers, through referencing of the use of both the original data and the data bases.

The full opening of the submillimeter range with the operation of Herschel will provide a new window for the study of Solar System objects. The main area of anticipated progress will be the study of water and related chemistry in planetary and cometary atmospheres. The photochemistry of the martian atmosphere, the origin of water in the outer planets, and the mechanisms of water sublimation and radiative transfer in comets will be investigated in details. Several other compositional aspects, notably the D/H ratio in Mars, the Giant Planets, and comets, will also be studied by Herschel. Photometric measurements of small bodies will be performed, to characterize the size and albedo distribution in the Kuiper Belt.

Most of the mass of the interstellar medium is in neutral phases that are heated by far-ultraviolet radiation from star forming regions. The gas, in turn, emits line radiation in the infrared and submillimeter that potentially has great diagnostic value. In this paper, we will review the physical processes (including heating, cooling, gas phase and grain surface chemistry) that give rise to this line emission and present diagnostic diagrams. We will also discuss several examples where models of neutral gas emission have been used to interpret line emission, including observations of H2O and O2 in molecular clouds and an example which points out the caution that must be used when applying the diagnostics to kiloparsec-sized regions in other galaxies.

Herschel will explore the last window on the Universe which has remained until now almost inaccessible to us, namely the Far Infrared to submillimeter wavelength range. This window is perfectly suited to study most of the phases of star formation, which occur at relatively low temperatures. The first step starts in very cold cores, with dust temperatures as low as 6 or 7 K, and proceeds up to the phase of hot corinos, where temperatures reach 100 K. Even larger temperatures, up to a few 1000 K, are found in the outflow shocks caused by the interaction of the outflows emanating from the newly born protostar with the surroundings. All these phases will be studied with the three instruments aboard Herschel: PACS and SPIRE will mostly probe the continuum dust emission, whereas HIFI will be devoted to the study of the molecular emission. This lecture especially focuses on the importance of molecules to study star formation.

We review the theories and the observations relevant for the formation of low-mass stars, particularly emphasizing the aspects that Herschel will contribute to develop.

We review the theories and observations of high-mass star formation emphasizing the differences with those of low-mass star formation. We hereafter describe the progress expected to be achieved with Herschel, thanks notably to Key Programmes dedicated to the earliest phases of high-mass star formation.

Debris disks are faint disks surrounding main-sequence stars. Their occurrence at ages beyond the typical dissipation time scales of YSO disks suggests that they are secondary in nature, being refurbished by the collisions of bodies of not-so-well known sizes, hence the importance of the subject for our understanding of the evolution of planetary systems. IRAS has discovered debris disks, ISO and now especially Spitzer have substantially refined our understanding of them. With Herschel we will have a unique opportunity to study debris disks in the wavelength range where their SEDs peak, and with the spatial resolution needed to unravel their structure and composition.

Mass loss is an important ingredient in the evolution of stars, in particular during late stages of stellar evolution, when mass loss often is the dominant factor. The far-IR window enables to probe the outflows of evolved stars at different depths, providing direct access to the driving mechanisms. The spatial and spectral resolution of Herschel offer important clues to study the geometry and the composition of the circumstellar gas and dust, and to describe and understand the spatial and temporal variability of the mass loss from initially spherical stars.

Infrared spectroscopy in the mid- and far-infrared provides powerful diagnostics for studying the emission regions in active galaxies. The large variety of ionic fine structure lines can probe gas conditions in a variety of physical conditions, from highly ionized gas excited by photons originated by black hole accretion to gas photoionized by young stellar systems. The critical density and the ionization potential of these transitions allow to fully cover the density-ionization parameter space. Some examples of line ratios diagrams using both mid-infrared and far-infrared ionic fine structure lines are presented. The upcoming space observatory Herschel will be able to observe the far-infrared spectra of large samples of local active galaxies. Based on the observed near-to-far infrared emission line spectrum of the template galaxy NGC 1068, are presented the predictions for the line fluxes expected for galaxies at high redshift. To observe spectroscopically large samples of distant galaxies, we will have to wait for the future space missions, like SPICA and, ultimately, FIRI.

Modelling the UV/optical - infrared/submm SEDs of spiral galaxies observed with Herschel will be an essential tool to quantitatively interpret these observations in terms of the present and past star-formation activity of these systems. In this lecture we describe the SED modelling technique we have developed, its applications and tests of its predictions. We show that both the panchromatic SED modelling of individual galaxies and the B-band attenuation-inclination relation of large statistical samples suggest that spiral galaxies in the nearby Universe behave as optically thick systems in their global properties and large-scale distribution of light (central face-on B-band opacity of τBf~4). However disk galaxies are very inhomogeneous systems, having both optically thick components (e.g. spiral arms), and optically thin components (e.g. the interarm regions), the latter making galaxies transparent to background galaxies.