Designing future dark energy space missions

II. Photometric redshift of space weak lensing optimized surveys

¹ Laboratoire d’Astrophysique de Marseille, CNRS-Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
e-mail: s.jouvel@ucl.ac.uk
² University College London, Gower Street, London WC1E 6BT, UK
³ University of Pennsylvania, 4N1 David Rittenhouse Lab 209 S 33rd St, Philadelphia, PA 19104, USA
⁴ Institute of Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁵ University of California, Space Sciences Laboratory, Berkeley, CA 94720, USA
⁶ Centre de Physique de Particule de Marseille, 163 Av. de Luminy, Case 902, 13288 Marseille Cedex 09, France
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁸ CFHT, 65-1238 Mamalahoa Hwy, Kamuela, HI 96743, USA

Received: 4 April 2010
Accepted: 19 April 2011

Abstract

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 ℛ ~ 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 I_AB = 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.