ESA Science & Technology - Publication Archive

We review the formalism and applications of the halo-based description of non-linear gravitational clustering. In this approach, all mass is associated with virialized dark matter halos; models of the number and spatial distribution of the halos, and the distribution of dark matter within each halo, are used to provide estimates of how the statistical properties of large scale density and velocity fields evolve as a result of non-linear gravitational clustering. We first describe the model, and demonstrate its accuracy by comparing its predictions with exact results from numerical simulations of non-linear gravitational clustering. We then present several astrophysical applications of the halo model: these include models of the spatial distribution of galaxies, the non-linear velocity, momentum and pressure fields, descriptions of weak gravitational lensing, and estimates of secondary contributions to temperature fluctuations in the cosmic microwave background.

The extragalactic background light at far-infrared wavelengths comes from optically faint, dusty, star-forming galaxies in the Universe with star formation rates of a few hundred solar masses per year. These faint, submillimetre galaxies are challenging to study individually because of the relatively poor spatial resolution of far-infrared telescopes. Instead, their average properties can be studied using statistics such as the angular power spectrum of the background intensity variations. A previous attempt at measuring this power spectrum resulted in the suggestion that the clustering amplitude is below the level computed with a simple ansatz based on a halo model. Here we report excess clustering over the linear prediction at arcminute angular scales in the power spectrum of brightness fluctuations at 250, 350 and 500 Œm. From this excess, we find that submillimetre galaxies are located in dark matter haloes with a minimum mass, Mmin, such that log₁₀[M_min/M_solar] = 11.5 (+0.7-0.2) at 350 µm, where M_solar is the solar mass. This minimum dark matter halo mass corresponds to the most efficient mass scale for star formation in the Universe, and is lower than that predicted by semi-analytical models for galaxy formation.

Gravitational lensing is a powerful astrophysical and cosmological probe and is particularly valuable at submillimeter wavelengths for the study of the statistical and individual properties of dusty star-forming galaxies. However, the identification of gravitational lenses is often time-intensive, involving the sifting of large volumes of imaging or spectroscopic data to find few candidates. We used early data from the Herschel Astrophysical Terahertz Large Area Survey to demonstrate that wide-area submillimeter surveys can simply and easily detect strong gravitational lensing events, with close to 100% efficiency.

The detection of circumstellar water vapour around the ageing carbon star IRC +10216 challenged the current understanding of chemistry in old stars, because water was predicted to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC +10216 using the Herschel satellite. This includes some high-excitation lines with energies corresponding to ~1000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances are much higher than hitherto predicted.

In this first paper on the results of our Herschel PACS survey of local ultra luminous infrared galaxies (ULIRGs), as part of our SHINING survey of local galaxies, we present far-infrared spectroscopy of Mrk 231, the most luminous of the local ULIRGs, and a type 1 broad absorption line AGN. For the first time in a ULIRG, all observed far-infrared fine-structure lines in the PACS range were detected and all were found to be deficient relative to the far infrared luminosity by 1-2 orders of magnitude compared with lower luminosity galaxies. The deficits are similar to those for the mid-infrared lines, with the most deficient lines showing high ionization potentials. Aged starbursts may account for part of the deficits, but partial covering of the highest excitation AGN powered regions may explain the remaining line deficits. A massive molecular outflow, discovered in OH and ¹⁸OH, showing outflow velocities out to at least 1400 km s^-1, is a unique signature of the clearing out of the molecular disk that formed by dissipative collapse during the merger. The outflow is characterized by extremely high ratios of ¹⁸O/¹⁶O suggestive of interstellar medium processing by advanced starbursts.

Volume 518 of Astronomy & Astrophysics is a special feature devoted to the first science results obtained with Herschel, an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. It includes 152 articles dealing with various subjects based on the first few months of science observing. A few papers describe the observatory and its instruments, and the rest are dedicated to observations of many astronomical targets from bodies in the Solar System to distant galaxies.

Aims. This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) that was launched onboard ESA's Herschel Space Observatory in May 2009.

Methods. The instrument is a set of 7 heterodyne receivers that are electronically tuneable, covering 480-1250 GHz with SIS mixers and the 1410-1910 GHz range with hot electron bolometer (HEB) mixers. The local oscillator (LO) subsystem comprises a Ka-band synthesizer followed by 14 chains of frequency multipliers and 2 chains for each frequency band. A pair of auto-correlators and a pair of acousto-optical spectrometers process the two IF signals from the dual-polarization, single-pixel front-ends to provide instantaneous frequency coverage of 2 × 4 GHz, with a set of resolutions (125 kHz to 1 MHz) that are better than 0.1 km s^-1.

Results. After a successful qualification and a pre-launch TB/TV test program, the flight instrument is now in-orbit and completed successfully the commissioning and performance verification phase. The in-orbit performance of the receivers matches the pre-launch sensitivities. We also report on the in-orbit performance of the receivers and some first results of HIFI's operations.

SPIRE, the Spectral and Photometric Imaging REceiver, is the Herschel Space Observatory's submillimetre camera and spectrometer. It contains a three-band imaging photometer operating at 250, 350 and 500 mm, and an imaging Fourier-transform spectrometer (FTS) covering 194-671 mm (447-1550 GHz).

In this paper we describe the initial approach taken to the absolute calibration of the SPIRE instrument using a combination of the emission from the Herschel telescope itself and the modelled continuum emission from solar system objects and other astronomical targets. We present the photometric, spectroscopic and spatial accuracy that is obtainable in data processed through the "standard" pipelines.

The overall photometric accuracy at this stage of the mission is estimated as 15% for the photometer and between 15 and 50% for the spectrometer. However, there remain issues with the photometric accuracy of the spectra of low flux sources in the longest wavelength part of the SPIRE spectrometer band.

The spectrometer wavelength accuracy is determined to be better than 1/10th of the line FWHM. The astrometric accuracy in SPIRE maps is found to be 2 arcsec when the latest calibration data are used.

The photometric calibration of the SPIRE instrument is currently determined by a combination of uncertainties in the model spectra of the astronomical standards and the data processing methods employed for map and spectrum calibration.

Improvements in processing techniques and a better understanding of the instrument performance will lead to the final calibration accuracy of SPIRE being determined only by uncertainties in the models of astronomical standards.

The Spectral and Photometric Imaging REceiver (SPIRE), is the Herschel Space Observatory's submillimetre camera and spectrometer. It contains a three-band imaging photometer operating at 250, 350 and 500 mm, and an imaging Fourier-transform spectrometer (FTS) which covers simultaneously its whole operating range of 194-671 mm (447-1550 GHz).

The SPIRE detectors are arrays of feedhorn-coupled bolometers cooled to 0.3 K. The photometer has a field of view of 4'× 8', observed simultaneously in the three spectral bands. Its main operating mode is scan-mapping, whereby the field of view is scanned across the sky to achieve full spatial sampling and to cover large areas if desired. The spectrometer has an approximately circular field of view with a diameter of 2.6'. The spectral resolution can be adjusted between 1.2 and 25 GHz by changing the stroke length of the FTS scan mirror. Its main operating mode involves a fixed telescope pointing with multiple scans of the FTS mirror to acquire spectral data. For extended source measurements, multiple position offsets are implemented by means of an internal beam steering mirror to achieve the desired spatial sampling and by rastering of the telescope pointing to map areas larger than the field of view.

The SPIRE instrument consists of a cold focal plane unit located inside the Herschel cryostat and warm electronics units, located on the spacecraft Service Module, for instrument control and data handling. Science data are transmitted to Earth with no on-board data compression, and processed by automatic pipelines to produce calibrated science products. The in-flight performance of the instrument matches or exceeds predictions based on pre-launch testing and modelling: the photometer sensitivity is comparable to or slightly better than estimated pre-launch, and the spectrometer sensitivity is also better by a factor of 1.5 - 2.

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 mm wavelength regime.

In photometry mode, it simultaneously images two bands, 60-85 mm or 85-125 mm and 125-210 mm, 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 kms^-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.

Herschel was launched on 14 May 2009, and is now an operational ESA space observatory offering unprecedented observational capabilities in the far-infrared and submillimetre spectral range 55-671 mm. Herschel carries a 3.5 m diameter passively cooled Cassegrain telescope, which is the largest of its kind and utilises a novel silicon carbide technology.

The science payload comprises three instruments: two direct detection cameras/medium resolution spectrometers, PACS and SPIRE, and a very high-resolution heterodyne spectrometer, HIFI, whose focal plane units are housed inside a superfluid helium cryostat. Herschel is an observatory facility operated in partnership among ESA, the instrument consortia, and NASA.

The mission lifetime is determined by the cryostat hold time. Nominally approximately 20 000 h will be available for astronomy, 32% is guaranteed time and the remainder is open to the worldwide general astronomical community through a standard competitive proposal procedure.

We summarize the first results from the Gould Belt Survey, obtained toward the Aquila rift and Polaris Flare regions during the science demonstration phase of Herschel. Our 70-500 micron images taken in parallel mode with the SPIRE and PACS cameras reveal a wealth of filamentary structure, as well as numerous dense cores embedded in the filaments. Between ~350 and 500 prestellar cores and ~45-60 Class 0 protostars can be identified in the Aquila field, while ~300 unbound starless cores and no protostars are observed in the Polaris field. The prestellar core mass function (CMF) derived for the Aquila region bears a strong resemblance to the stellar initial mass function (IMF), already confirming the close connection between the CMF and the IMF with much better statistics than earlier studies. Comparing and contrasting our Herschel results in Aquila and Polaris, we propose an observationally-driven scenario for core formation according to which complex networks of long, thin filaments form first within molecular clouds, and then the densest filaments fragment into a number of prestellar cores via gravitational instability.

With the biggest mirror yet flown in space, Europe's new Herschel Space Observatory will peer through a new wavelength window at the cool regions of the universe. It will be launched together with Planck, a mission to map the cosmic microwave background radiation in unprecedented detail. In addition to studying the formation of galaxies and stars, astronomers hope to use Herschel to study comets, asteroids, and planetary atmospheres in our solar system and how debris disks around stars form into planets.

In 2008, an Ariane-5 will lift off from French Guiana carrying ESA's two pioneering Herschel and Planck deep space observatories to explore previously unknown regions of the Universe. Their target is the 'bright' part of the far-infrared spectrum that has tantalised scientists for decades. Until now, the technology has not existed to make precise observations of a distant domain that touches the very beginning of time.

Abstract
Since ten years ASTRIUM has developed sintered Silicon Carbide (SiC) technology for space applications. Its unique thermo-mechanical properties, associated with its polishing capability, make SiC an ideal material for building ultra-stable lightweight space based telescopes or mirrors. SiC is a cost effective alternative to Beryllium and the ultra-lighweighted ULE. In complement to the material manufacturing process, ASTRIUM has developed several assembly techniques (bolting, brazing, bonding) for manufacturing large and complex SiC assemblies. This technology is now perfectly mature and mastered. SiC is baselined for most of the telescopes that are developed by ASTRIUM. SiC has been identified as the most suitable material for manufacturing very large crygenic telescopes. In this paper we present the development of a 3.5m-diameter telescope for the Herschel Mission. Herschel's main goal is to study how the first stars and galaxies were formed and evolved. The Herschel Space telescope, using silicon carbide technology will be the largest space imagery telescope ever launched. The Herschel telescope will weight 300 kg rather than the 1.5 tons required with standard technology. The Herschel telescope is to be delivered in 2005 for a launch planned for 2007.

FIRST is one of the satellites of the next ESA scientific mission FIRST/Planck, which will be launched in 2007 to the 2nd Lagrangian libration point L₂. It will be a multi-user observatory, watching the universe in the infrared and sub-millimetre wavelength range from 60 to 670 µm. The payload Module (PLM) of FIRST will accommodate 3 instruments built by large scientific consortia: SPIRE (Spectral and Photometric Imaging REceiver), PACS (Photo-conductor Array Camera & Spectrometer), and HIFI (Heterodyne Instrument for FIrst). All of them have detectors or mixers operating in the range 0.3 K-2 K. It must also support a 3.5 m diameter telescope. The design of the payload module is inspired from the one of ISO (Infrared Space Observatory). It is based on a large superfluid helium (HeII) Dewar (2560 litres at < 1.7 K), with cooling of the instruments either directly on the tank, or on the helium vent line (4 K and 15 K). The last cooling stage to 0.3 K is performed inside the instruments using a recyclable ³He-sorption cooler. The paper will describe the latest design status of the cryostat, and its interfaces to the instruments and the telescope.

SPIE symposium 'UV, Optical, and IR Space Telescopes and Instruments', held 29-31 March 2000 in Munich.

In this paper we describe the design of PACS which has been developed by consortium of European research institutions with the goal to build and operate the instrument and the associated Instrument Control Centre. This design is the outcome of an iterative optimization process toward best observing efficiency regarding the key science of Herschel and toward simplicity of operation, and in the context of complementary missions like SOFIA or SIRTF. SPIE symposium 'UV, Optical, and IR Space Telescopes and Instruments', held 29-31 March 2000 in Munich. More info on http://astro.estec.esa.nl/FIRST/overview.html

UV, Optical, and IR Space Telescopes and Instruments FIRST, the 'Far InfraRed and Submillimeter Telescope', is the fourth cornerstone mission in the European Space Agency science program. It will perform photometry and spectroscopy in the far infrared and submillimeter part of the spectrum, covering approximately the 60 - 670 micrometers range.

The FIRST/Planck ESA program combines two ESA missions of the HORIZON 2000 program, the cornerstone mission of the Far InfraRed and Submillimeter Telescope and the third medium sized mission, Planck. An overview is given in this paper of the current system design, the performance parameters and an outlook on the spacecraft development.

The Heterodyne Instrument for FIRST (HIFI) will cover at least the frequency range 492 GHz to 1113 GHz and will provide sensitive observations with resolving powers ranging from less than 5x10⁵ to 1.2x10⁷. The instrument is optimised for the measurement of weak, broad spectral lines of distant galaxies and for performing fast line surveys of galactic objects. This paper describes the performance, observing modes, and calibration modes of the planned instrument. Companion papers in these Proceedings describe the instrument optics, mixers and spectrometer systems.

In the Proceedings of 'The Far InfraRed and Submillimetre Universe', an ESA symposium, held 15-17 April 1997 in Grenoble, France.