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Enabling Weak lensing Cosmology

Periodic Reporting for period 2 - EWC (Enabling Weak lensing Cosmology)

Reporting period: 2019-04-01 to 2020-03-31

Enabling Weak lensing Cosmology (EWC) is a team of cosmologists working across Europe to prepare for the next generation of astronomical data that will come from the European Space Agency Euclid mission. Euclid will map 3/4 of the extra-galactic sky back in time over 3/4 the age of the Universe. Its science objective is to determine the nature of dark energy – that is causing the expansion of the Universe to accelerate – by mapping how dark matter grows over time. One of the primary ways that Euclid will do this is by measuring an effect called weak lensing: the effect where dark matter structure distort the observed images of galaxies. Euclid will measure this signal by imaging 1.5 billion galaxies with a resolution similar to that of the Hubble Space Telescope.
Although Euclid is designed to minimize observational systematics, the observations are still limited by two factors. The objective of our team is to make significant progress on both fronts. To do so, we will (i) quantify the morphology of galaxies using archival HST observations; (ii) carry out a unique narrow-band photometric redshift survey to obtain state-of-the-art constraints on the intrinsic alignments of galaxies that arise due to tidal interactions, and would otherwise contaminate the cosmological signal; (iii) integrate these results into the end-to-end simulation pipeline; (iv) perform a spectroscopic redshift survey to calibrate the photometric redshift technique.
WP1: The PAU Survey has been designed to measure accurately the redshift of galaxies using photometric techniques with narrow band filters. Its wide area coverage, the depth reached and the wavelength rage coverage make it in a unique survey that can be used to calibrate the intrinsic alignment signal in weak lensing surveys like Euclid. We take observations at the William Herschel Telescope of the Observatorio del Roque de los Muchachos and automatically transfer them to our institutions and archive them. We are developing the algorithms to be able to measure the galaxy fluxes in all the narrow bands observed. We are also implementing the processing into a big data platform hosted at the Port d'Informació Científica (PIC) as the data volume makes it impossible to run in simple machines.
WP2: As part of our work we need to locate galaxies very precisely. Photometric redshifts provide an efficient tool to do so. Here, we are exploring how to best measure galaxy distances using the narrow band data collected from PAUCam. With PAUCam, we sample the spectral energy distribution at 40 different wavelengths. Such fine resolution allows us to determine the galaxy distances accurately. We have been working in methods of analysing the PAU Survey data efficiently including the use of machine learning techniques.
WP 3: Galaxies are clouds of stars in space. They can take many forms, some are elliptical and some are spiral galaxies. We have studied how the clouds of stars orientate themselves in space, and in particular if they tend to orientate themselves to lumps of dark matter. We have found that they do align with dark matter lumps and this means that if this was not accounted for we would not be able to measure how gravity between us and the galaxies distorts the observed shapes of galaxies using the Euclid telescope, a measurement we want to use to determine what the Universe is made of. Fortunately, now we have a good model for this “intrinsic alignment” of galaxies so we can now account for this and make accurate and precise measurement of the gravity-induced distortion using the Euclid telescope.
WP 4: This WP is tasked has testing and recalibrated Hubble Space Telescope optical models, in particular the blurring caused introduced in the observations, know as the Point Spread Function (PSF). A particular focus has been on the development of general PSF validation statistics. The main results has been a revised weak lensing calibration for HST weak lensing measurements, including corrections for noise bias, selection effects, and the impact of neighbours, with a focus on high-redshift source populations and the application for HST cluster weak lensing measurements
WP 5: This WP is tasked with determining the impact of observing multi-colour galaxies with a grey-scale telescope (as is the case in Euclid). Significant theoretical progress has been made in understanding how these effects can be calibrated, and work on simulations to test these theoretical approaches will begin the next phase of our project.
WP 6: Surveys of galaxies are useful for understanding both the formation and growth of galaxies like our Milky Way, and also the expansion of the Universe as a whole. Astronomers can observe real galaxies, but interpreting those observations requires something to compare them to. We run supercomputer simulations of alternative universes with slightly different laws of physics. In this project, we are converting those raw simulations into mock observations - encompassing limitations of telescopes in as realistic a way as possible, so we will be able to decide which set of physical laws is closest to reality.
WP 7: The weak lensing cosmological analysis requires a precise knowledge of where the galaxy sources and the galaxies that act as lenses are located in order to optimize the information to be obtained. Photometric redshifts techniques will be used in Euclid to provide this knowledge. However, these techniques require spectroscopic data for their training, testing and validation. Here, we are conducting a survey to obtain the spectroscopic data required for Euclid. We first study the galaxies colour space to see which colours do not have spectroscopic data. With the data obtained, we have been filling in the regions of colour space where there was no spectroscopy before.
"We have published several papers in academic journals:
• The mock catalogue has been released to the PAUCam survey team. This work has been presented at PAUCam survey team meetings and at EWC meetings. The PAUCam survey team has performed validation work on this catalogue that has uncovered a redshift systematic. The work has been reported in a published refereed paper (Stothert et al. 2018 that is freely available on an open access server (
• Eriksen et al, 2019, ""The PAU Survey: early demonstration of photometric redshift performance in the COSMOS field"", MNRAS, 484, 4200. This paper presents the first results of the photometric performance of the PAU Survey. It is the result of work done in the context of WP1.
• The PAU Survey has been reporting its initial data products and data pipeline descriptions in these papers: Tonello, N., Tallada, P., Serrano, S., et al. 2018, arXiv e-prints , arXiv:1811.02368.; Cabayol L., et al., 2019, MNRAS, 483, 529; Tortorelli L., et al., 2018, Journal of Cosmology and Astro-Particle Physics, 2018, 35.
We have published an article in The Conversation on cosmology: “The universe’s rate of expansion is in dispute – and we may need new physics to solve it”, Kitching, 2019 which received 81,266 views and was republished 15 times.
We have started our twitter account @weaklensing."