Jean L. Puget

The optically dark side of galaxy formation

Deep optical surveys, probe the rest-frame ultraviolet luminosities of high-redshift galaxies. They can therefore be used to infer star formation rates, under assumptions about young stellar populations. Current data suggest that the global star-formation rate of the Universe peaked at a redshift of z = 1, then subsequently declined,, leading to claims that the bulk of star formation in the Universe has been seen. However, the large uncertainties inherent in correcting for ultraviolet absorption by dust associated with young stars suggest that these formation rates might be substantially underestimated in high-redshift galaxies. Here we circumvent this problem by considering the dust thermal emission at infrared (and submillimetre) wavelengths. We propose an improved determination of the long-sought cosmic infrared background (built up from the accumulated infrared light of faint galaxies along the line of sight), from which we are able to estimate the required population of high-redshift, dust-enshrouded starburst galaxies. We argue that most of the star formation at high redshift may be hidden by dust, and we define the necessary characteristics of a feasible far-infrared survey that could detect this population.

The Planck high-frequency instrument: a third-generation CMB probe and the first submillimeter surveyor

The High Frequency Instrument of the Planck satellite is dedicated to the measurement of the anisotropy of the Cosmic Microwave Background (CMB). Its main goal is to map the CMB with a sensitivity of ΔT/T=2.10-6 and an angular resolution of 5 arcmin in order to constrain cosmological parameters. Planck is a project of the European Space Agency based on a wide international collaboration, including United States and Canadian laboratories. The architecture of the satellite is driven by the thermal requirements resulting from the search for low photon noise. Especially, the passively cooled telescope should be at less than 50K, while a cascade of cryo-coolers will ensure the cooling of the HFI bolometers down to 0.1K. This last temperature will be produced by a gravity insensitive 3He/4He dilution cooler. This will be achieved at the L2 Lagrangian point of the Sun-Earth system. The whole sky will be observed two times in the 14 months mission with a scanning strategy based on a 1RPM rotation of the satellite. In addition to the cosmological parameters that can be derived from the CMB maps, Planck will deliver nine high sensitivity submillimeter maps of the whole sky that will constitute unique data available to the whole astronomical community.

Optical Design of the EPIC-IM Crossed Dragone Telescope

The Experimental Probe of Inflationary Cosmology - Intermediate Mission (EPIC-IM) is a concept for the NASA Einstein Inflation Probe satellite. EPIC-IM is designed to characterize the polarization properties of the Cosmic Microwave Background to search for the B-mode polarization signal characteristic of gravitational waves generated during the epoch of Inflation in the early universe. EPIC-IM employs a large focal plane with 11,000 detectors operating in 9 wavelength bands to provide 30 times higher sensitivity than the currently operating Planck satellite. The optical design is based on a wide-field 1.4 m crossed-Dragone telescope, an aperture that allows not only comprehensive measurements of Inflationary B-mode polarization, but also measurements of the E-mode and lensing polarization signals to cosmological limits, as well as all-sky maps of Galactic polarization with unmatched sensitivity and angular resolution. The optics are critical to measuring these extremely faint polarization signals, and any design must meet demanding requirements on systematic error control. We describe the EPIC-IM crossed Dragone optical design, its polarization properties, and far-sidelobe response.

Planck-HFI thermal architecture: from requirements to solutions

The Planck-High Frequency Instrument (HFI) will use 48 bolometers cooled to 100mK by a dilution cooler to map the Cosmic Microwave Background (CMB) with a sensitivity of ΔT/T~2.10-6 and an angular resolution of 5 minutes of arc. This instrument will therefore be about 1000 times more sensitive than the COBE-DMR experiment. This contribution will focus mainly on the thermal architecture of this instrument and its consequences on the fundamental and instrumental fluctuations of the photon flux produced on the detectors by the instrument itself. In a first step, we will demonstrate that the thermal and optical design of the HFI allow to reach the ultimate sensitivity set by photon noise of the CMB at millimeter wavelength. Nevertheless, to reach such high sensitivity, the thermal behavior of each cryogenic stages should also be controlled in order to damp thermal fluctuations that can be taken as astrophysical signal. The requirement in thermal fluctuation on each stage has been defined in the frequency domain to degrade the overall sensitivity by less than 5%. This leads to unprecedented stability specifications that should be achieved down to 16mHz. We will present the design of the HFI thermal architecture, based on active and passive damping, and show how its performances were improved thanks to thermal simulations.