Preparing The Euclid Dark Energy Survey with Clusters

The primary objective of cosmological research in the coming decade is to understand the accelerated expansion of the Universe, attributed to either a dark energy component or a modification to gravity on cosmic scales.  ESA’s Euclid mission is dedicated to probing the physical origin of the acceleration through measures of large-scale structure.  Galaxy clustering, gravitational lensing and galaxy cluster abundance are the three central observational probes. 

     The APC laboratory is heavily invested in the Euclid mission.  This includes responsibility for providing simulations of the ground-based complement of imaging data essential for photometric redshift determinations, and co-lead of the Galaxy Cluster Science Working Group (J.G. Bartlett).  The thesis topic involves both aspects, with significant contribution to our ground-segment development responsibilities and to preparation of the cluster cosmology pipeline.  The context for both is set by the ground-segment Science Challenge (SC) and Science Performance Verification (SPV) cycles 2 and 3; cycle 2 begins in Feb. 2017 and cycle 3, a more extensive study, in Sept. 2018.  This schedule is well-suited to a thesis starting in Sept. 17.

     The first part of the thesis will focus on developing and delivering simulations of the ground-based imaging  data needed for the SC and SVP.  The student will work with our group on different aspects, including the incorporation of important physical and instrumental effects into the simulations and their validation.  This involves understanding the characteristics of the ground-based surveys (e.g., DES, KiDS, LSST), modelling their systematics, atmospheric effects, observing strategies and instrument performance (e.g., PSF spatial and temporal variations, filter variations across the focal plane and with observing conditions, etc.).  It also means he/she will follow the outputs downstream through the Euclid pipeline to help identify the impact of the ground-based data quality on higher level data products, such as photometric redshift determinations.

     In the second part of the thesis, the student will contribute to our evaluation of Euclid’s ability to measure cluster masses through lensing.  This is a critical part of the cluster cosmology pipeline, and an area where Euclid will enable significant scientific advance.  Developing, implementing and testing cluster mass measurement methods requires cosmological simulations linking clusters in mock catalogs to the lensing signal of their dark matter halo. The simulated cosmic lensing maps produced by the Euclid Consortium for the SPV do not resolve the lensing signal on individual cluster scales, because the mass distribution from the N-body simulation will smoothed before simulating the lensing.  The student will develop a method of adding the signal from clusters to the simulations by injecting smaller scale information from the N-body halo catalog into the smoothed lensing simulation.  The Galaxy Cluster Science Working Group will then be able to use the modified simulations to develop cluster mass measurement techniques. 

     The two parts of the thesis will give the student experience with both the technical and scientific preparation of the Euclid mission.  They come together as she/he participates in the development of mass measurement methods where his/her lensing simulations will be essential, as will photometric redshifts whose performance depends critically on evaluation of the ground-based data quality through the image simulations she/he will provide to the Consortium.


James G. Bartlett






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