Designing future dark energy space missions - II. Photometric redshift of space weak lensing optimized surveys

摘要

Context. With the discovery of the accelerated expansion of the universe, different observational probes have been proposed to investigate the presence of dark energy, including possible modifications to the gravitation laws by accurately measuring the expansion of the Universe and the growth of structures. We need to optimize the return from future dark energy surveys to obtain the best results from these probes. Aims. A high precision weak-lensing analysis requires not an only accurate measurement of galaxy shapes but also a precise and unbiased measurement of galaxy redshifts. The survey strategy has to be defined following both the photometric redshift and shape measurement accuracy. Methods. We define the key properties of the weak-lensing instrument and compute the effective?PSF and the overall throughput and sensitivities. We then investigate the impact of the pixel scale on the sampling of the effective?PSF, and place upper limits on the pixel scale. We then define the survey strategy computing the survey area including in particular both the Galactic absorption and Zodiacal light variation accross the?sky. Using the Le Phare photometric redshift code and realistic galaxy mock catalog, we investigate the properties of different filter-sets and the importance of the u-band photometry quality to optimize the photometric redshift and the dark energy figure of merit?(FoM). Results. Using the predicted photometric redshift quality, simple shape measurement requirements, and a proper sky model, we explore what could be an optimal weak-lensing dark energy mission based on FoM?calculation. We find that we can derive the most accurate the photometric redshifts for the bulk of the faint galaxy population when filters have a resolution ??~?3.2. We show that an optimal mission would survey the sky through eight filters using two cameras (visible and near infrared). Assuming a five-year mission duration, a?mirror size of 1.5?m and a?0.5?deg2?FOV with a visible pixel scale of?0.15′′, we found that a homogeneous survey reaching a survey population of IAB?= 25.6?(10σ) with a sky coverage of ?~11?000?deg2 maximizes the weak lensing?FoM. The effective number density of galaxies used for?WL is then ?~45?gal/arcmin2, which is at least a factor of two higher than ground-based surveys. Conclusions. This study demonstrates that a full account of the observational strategy is required to properly optimize the instrument parameters and maximize the FoM of the future weak-lensing space dark energy mission.