Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - EWC (Enabling Weak lensing Cosmology)

Teaser

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...

Summary

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. Various instrumental effects need to be corrected for, and the tremendous improvement in precision has to be matched with comparable advances in the modelling of astrophysical effects that affect the signal. 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. The Euclid Consortium has identified these as critical issues, which need to be addressed before launch, in order to maximise the science return of this exciting mission, and enable the dark energy science objectives of Europe

In order to achieve our objectives EWC will make use of currently available data that have unique aspects that will not be available from the Euclid data alone – so called calibration data. Our calibration data sets are the PauCAM Survey, the Hubble Space Telescope Archive, and the ESO Spectroscopy Large Programme.

Work performed

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 range coverage make it in a unique survey that can be used to calibrate the intrinsic alignment signal in weak lensing surveys like Euclid. We have worked to define the most efficient observational strategy to reach its goals. Once the images are taken, they need to be analysed properly. We are developing the algorithms to be able to measure the galaxy fluxes in all the narrow bands observed. This is complicated task given the large number of images and the fact that they have been taken in different observational conditions. We are also implementing the processing into a big data platform hosted at the Port d\'Informació Científica (PIC) as the data volume make it impossible to run in simple machines. We are also producing simulations of our observations to understand the measuring process and interpret the results.

WP2: Cosmology wants to understand the Universe as a whole. In order to do so, one needs to measure it. Galaxies are tracers that allow us to sample the universe. However, in order to fully exploit them we need to locate them precisely. Photometric redshifts provide an efficient tool to do so. Here, we are exploring how to best measure galaxy redshifts 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 redshifts accurately. We have been working in methods of analysing the PAU Survey data efficiently. In particular, the inclusion of emission lines in the galaxy spectra improves considerably the performance for emission line galaxies. Our results are consistent with the prediction we obtained in simulations.

WP3: In this period we defined the objectives for our EWC PDRA with the PAU Survey team, ready for the beginning of WP3 in the next period.

WP4: In this period we began our work on WP4 in M11, first getting familiar with relevant software (galsim). We also contributed to tests of the HST PSF model.

WP5: Not started yet

WP6: We have built a synthetic galaxy catalogue by combining a physically motivated model of galaxy formation with a high resolution N-body simulation. This catalogue includes photometric information about the model galaxies in the narrow band filters used by the PAUCam survey. These filters help to improve the estimation of the distances to galaxies, without going through the step of measuring a spectrum. Preparations are underway for the subsequent tasks in this WP which relate to adding gravitational lensing to the mock catalogues.

WP7: 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. We use a Self Organising Map for this purpose. We select galaxies that reside in regions of colour space without spectroscopy as targets for our survey. We designed spectroscopic observations in the optical and near-infrared to obtain the redshift of those colour space regions.

Final results

\"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 https://doi.org/10.1093/mnras/sty2491) that is freely available on an open access server (https://arxiv.org/abs/1807.03260).
• 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”, https://theconversation.com/the-universes-rate-of-expansion-is-in-dispute-and-we-may-need-new-physics-to-solve-it-100154 (Kitching, 2019) which received 81,266 views and was republished 15 times.

We have started our twitter account @weaklensing.

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Website & more info

More info: https://weaklensing-cosmology.org/.