J. A. Tauber

Planck pre-launch status: The HFI instrument, from specification to actual performance

Context. The High Frequency Instrument (HFI) is one of the two focal instruments of the Planck mission. It will observe the whole sky in six bands in the 100 GHz-1 THz range. Aims: The HFI instrument is designed to measure the cosmic microwave background (CMB) with a sensitivity limited only by fundamental sources: the photon noise of the CMB itself and the residuals left after the removal of foregrounds. The two high frequency bands will provide full maps of the submillimetre sky, featuring mainly extended and point source foregrounds. Systematic effects must be kept at negligible levels or accurately monitored so that the signal can be corrected. This paper describes the HFI design and its characteristics deduced from ground tests and calibration. Methods: The HFI instrumental concept and architecture are feasible only by pushing new techniques to their extreme capabilities, mainly: (i) bolometers working at 100 mK and absorbing the radiation in grids; (ii) a dilution cooler providing 100 mK in microgravity conditions; (iii) a new type of AC biased readout electronics and (iv) optical channels using devices inspired from radio and infrared techniques. Results: The Planck-HFI instrument performance exceeds requirements for sensitivity and control of systematic effects. During ground-based calibration and tests, it was measured at instrument and system levels to be close to or better than the goal specification.

Towards a model of full-sky Galactic synchrotron intensity and linear polarisation: A re-analysis of the Parkes data

We have analysed the angular power spectra of the Parkes radio continuum and polarisation survey of the Southern galactic plane at 2.4 GHz. We have found that in the multipole range l =4 0 250 the angular power spectrum of the polarised intensity is well described by a power-law spectrum with tted spectral index L = 2:37 0:21. In the same multipole range the angular power spectra of the E and B components of the polarised signal are signicantly flatter, with tted spectral indices respectively of E =1 :57 0:12 and B =1 :45 0:12. Temperature fluctuations in the E and B components are mostly determined by variations in polarisation angle. We have combined these results with other data from available radio surveys in order to produce a full-sky toy model of Galactic synchrotron intensity and linear polarisation at high frequencies ( > 10 GHz). This can be used to study the feasibility of measuring the Cosmic Microwave Background polarisation with forthcoming experiments and satellite missions.

Planck Pre-Launch Status: Expected LFI Polarisation Capability

We present a system-level description of the Low Frequency Instrument (LFI) considered as a differencing polarimeter, and evaluate its expected performance. The LFI is one of the two instruments on board the ESA Planck mission to study the cosmic microwave background. It consists of a set of 22 radiometers sensitive to linear polarisation, arranged in orthogonally-oriented pairs connected to 11 feed horns operating at 30, 44 and 70 GHz. In our analysis, the generic Jones and Mueller-matrix formulations for polarimetry are adapted to the special case of the LFI. Laboratory measurements of flight components are combined with optical simulations of the telescope to investigate the values and uncertainties in the system parameters affecting polarisation response. Methods of correcting residual systematic errors are also briefly discussed. The LFI has beam-integrated polarisation efficiency >99% for all detectors, with uncertainties below 0.1%. Indirect assessment of polarisation position angles suggests that uncertainties are generally less than 0°.5, and this will be checked in flight using observations of the Crab nebula. Leakage of total intensity into the polarisation signal is generally well below the thermal noise level except for bright Galactic emission, where the dominant effect is likely to be spectral-dependent terms due to bandpass mismatch between the two detectors behind each feed, contributing typically 1–3% leakage of foreground total intensity. Comparable leakage from compact features occurs due to beam mismatch, but this averages to < 5 × 10^(-4) for large-scale emission. An inevitable feature of the LFI design is that the two components of the linear polarisation are recovered from elliptical beams which differ substantially in orientation. This distorts the recovered polarisation and its angular power spectrum, and several methods are being developed to correct the effect, both in the power spectrum and in the sky maps. The LFI will return a high-quality measurement of the CMB polarisation, limited mainly by thermal noise. To meet our aspiration of measuring polarisation at the 1% level, further analysis of flight and ground data is required. We are still researching the most effective techniques for correcting subtle artefacts in polarisation; in particular the correction of bandpass mismatch effects is a formidable challenge, as it requires multi-band analysis to estimate the spectral indices that control the leakage.

Measurement of the Crab nebula polarization at 90 GHz as a calibrator for CMB experiments

CMB experiments aiming at a precise measurement of the CMB polarization, such as the Planck satellite, need a strong polarized absolute calibrator on the sky in order to accurately set the detectors polarization angle and the cross-polarization leakage. The Crab Nebula, as the most intense polarized source in the microwave sky at angular scales of few arcminutes, will be used for this purpose. Our goal was to measure the Crab nebula polarization characteristics at 90 GHz with an unprecedented precision. The observations were carried out with the IRAM 30m telescope employing the correlation polarimeter XPOL and using two orthogonally polarized receivers. We have processed the Stokes I, Q and U maps from our observations in order to compute the polarization angle and linear polarization fraction. The first is almost constant in the region of maximum emission in polarization with a mean value of alpha_Sky=152.1+/-0.2 deg and the second is found to reach a maximum of Pi=30% for the most polarized pixels. We find that a CMB experiment having a 5 arcmin circular beam will see a mean polarization angle of alpha_Sky=149.9+/-0.2 deg and a mean polarization fraction of Pi=8.8+/-0.2%.

Anatomy of a Photodissociation Region: High angular resolution images of molecular emission in the Orion Bar

We present observations of the molecular component of the Orion Bar, a prototypical Photodissociation Region (PDR) illuminated by the Trapezium cluster. The high angular resolution (6 sec-10 sec) that we have achieved by combining single-dish and interferometric observations has allowed us to examine in detail the spatial and kinematic morphology of this region and to estimate the physical characteristics of the molecular gas it contains. Our observations indicate that this PDR can be essentially described as a homogeneously distributed slab of moderately dense material (approximately 5 x 10(exp 4)/cu cm), in which are embedded a small number of dense (greater than 10(exp 6)/cu cm) clumps. The latter play little or no role in determining the thickness and kinetic temperature structure of this PDR. This observational picture is largely supported by PDR model calculations for this region, which we describe in detail in this work. We also find our model predictions of the intensities of a variety of atomic and molecular lines to be in good general agreement with a number of previous observations.

Scanning strategy for mapping the Cosmic Microwave Background anisotropies with Planck

We present simulations of different scanning strategies for the Planck satellite. We review the properties of slow- and fast-precession strategies in terms of uniformity of the integration time on the sky, the presence of low-redundancy areas, the presence of deep fields, the presence of sharp gradients in the integration time, and the redundancy of the scanning directions. We also compare the results obtained when co-adding all detectors of a given frequency channel. The slow-precession strategies allow a good uniformity of the coverage, while providing two deep fields. On the other hand, they do not allow a wide spread of the scan-crossing directions, which is a feature of the fast-precession strategies. However, the latter suffer from many sharp gradients and low-coverage areas on the sky. On the basis of these results, the strategy for Planck can be selected to be a slow (e.g. 4 month-period) sinusoidal or cycloidal scanning.

The Herschel and Planck Space Telescopes

The European Space Agency launches in 2009 two flagship missions in the domain of submillimeter space astronomy. The Herschel Space Observatory is a common user facility featuring a 3.5 m aperture Cassegrain telescope passively cooled to 80 K. The Planck survey mission includes a 1.5 m unobscured aperture off-axis aplanatic telescope passively cooled to 40 K. Herschel will make pointed target observations of astrophysical objects and phenomena in the frequency range 448 GHz to 5.3 THz (or 672 to 55 mum wavelength). Planck, on the other hand, will map the entire sky by strip scanning at a spin rate of one revolution per minute covering a frequency bandwidth of 30-857 GHz (or wavelength from 10 to 0.35 mm). Its spin axis is pointed antisunward and can be oriented within a 10deg cone around that direction. The telescope line of sight is fixed at an angle of 85deg to the spacecraft spin-axis. This paper describes briefly the specific telescopes developed for each mission; their design characteristics, the development process for each, their achieved performances (from on ground testing) and their expected performances in flight.

The angular power spectrum of radio emission at 2.3 GHz

We have analysed the Rhodes/HartRAO survey at 2326 MHz and derived the global angular power spectrum of Galactic continuum emission. In order to measure the angular power spectrum of the diuse com- ponent, point sources were removed from the map by median ltering. A least-square t to the angular power spectrum of the entire survey with a power law spectrum Cl/ l ,g ives =2 :43 0:01 in the l range 2 100. The angular power spectrum of radio emission appears to steepen at high Galactic latitudes and for observed regions withjbj > 20, the tted spectral index is =2 :92 0:07. We have extrapolated this result to 30 GHz (the lowest frequency channel of Planck) and estimate that no signicant contribution to the sky temperature fluctuation is likely to come from synchrotron at degree-angular scales.

Planck pre-launch status: Low Frequency Instrument calibration and expected scientific performance

We present the calibration and scientific performance parameters of the Planck Low Frequency Instrument (LFI) measured during the ground cryogenic test campaign. These parameters characterise the instrument response and constitute our optimal pre-launch knowledge of the LFI scientific performance. The LFI shows excellent 1/f stability and rejection of instrumental systematic effects; its measured noise performance shows that LFI is the most sensitive instrument of its kind. The calibration parameters will be updated during flight operations until the end of the mission.

A model for clumpy giant molecular clouds with external ultraviolet heating

A model of clumpy molecular clouds based on that of Kwan and Sanders (1986) has been developed, in which a clump filling factor which is very low at the cloud surface and increases to about 1 at the cloud center allows the UV radiation from an H II region located at the surface to permeate the volume of the cloud and to create a temperature gradient and varying abundance of CO within each clump. The model is in general agreement with the enhanced intensity of the (C-12)O J = 3-2 transition relative to the J = 1-0 line observed throughout the central 10-arcmin region of the Orion molecular cloud. 28 refs.