Cosmology

Abstract:

Exploiting the full statistical power of future cosmic shear surveys will necessitate improvements to the accuracy with which the gravitational lensing signal is measured. We present a framework for calibrating shear with image simulations that demonstrates the importance of including realistic correlations between galaxy morphology, size and more importantly, photometric redshifts. This realism is essential so that selection and shape measurement biases can be calibrated accurately for a tomographic cosmic shear analysis. We emulate Kilo-Degree Survey (KiDS) observations of the COSMOS field using morphological information from {\it Hubble} Space Telescope imaging, faithfully reproducing the measured galaxy properties from KiDS observations of the same field. We calibrate our shear measurements from lensfit, and find through a range of sensitivity tests that lensfit is robust and unbiased within the allowed 2 per cent tolerance of our study. Our results show that the calibration has to be performed by selecting the tomographic samples in the simulations, consistent with the actual cosmic shear analysis, because the joint distributions of galaxy properties are found to vary with redshift. Ignoring this redshift variation could result in misestimating the shear bias by an amount that exceeds the allowed tolerance. To improve the calibration for future cosmic shear analyses, it will be essential to also correctly account for the measurement of photometric redshifts, which requires simulating multi-band observations.

Abstract:

As the forthcoming generation of Cosmic Microwave Background observatories move towards the use of large format detector arrays operating at ~100 mK, the need for test cryostats capable of operating in this temperature regime is becoming more pronounced. This has strongly driven the development of several related systems, including the continuous miniature dilution refrigerator (MDR) reported here. The MDR is comprised of a thermally separated mixing chamber, step heat exchangers, twin stills and twin condensation pumps. The pumps are alternately cooled to ~300 mK by a pair of single-shot 1He sorption coolers (cycled in anti-phase) to circulate 3He in the system. The system is therefore closed-cycle, with the circulation of 3He, both in the MDR and sorption coolers, contained to the cold stage. As a result, the reliability of the system is improved through a mechanically simple design and the absence of external connections, gas handling systems, and cold o-rings.