Master-2 internship 2019: Measuring cosmic magnification in the Euclid deep fields

By | October 14, 2018

The objective of this internship is to make a measurement of the cosmic magnification effect in prototype Euclid deep field data.


ESA’s Euclid satellite aims to uncover the nature of dark matter and dark energy by precisely measuring shapes and positions of a billion galaxies taken with the wide-field VIS camera. Dark matter induces tiny distortions — cosmic shear — in these observed galaxy shapes. By measuring these distortions and combining them with distance information cosmological parameters can be inferred.

“Cosmic magnification” is a related effect and refers to the modification of the source counts of background objects by light rays passing through foreground large-scale structures. Magnification induces a positional correlation between objects which are spatially separated. The amplitude of the magnification signal can provide an independent measurement of relationship between galaxies and dark matter (essential to test models of galaxy formation) and also provide complementary constraints on cosmological parameters compared to the traditional cosmic shear signal.

Unlike the cosmic shear signal, which results in shapes of galaxies being correlated at small scales, measuring cosmic magnification does not require shape measurement. It is therefore is unaffected by systematics related to our knowledge of the instrumental point spread function. Accurate shape measurement is one of the most challenging aspects of the cosmic shear measurement and therefore it is important to assess what role can cosmic magnification play is deriving parameters of the cosmological model.

However, magnification measurements require very precise photometry in order to accurately separate background and foreground galaxies using photometric redshifts. In addition to the relatively wide Euclid survey, a second deeper survey will cover forty square degrees with approximately forty times longer integration times. Half of this area will be be covered by very deep optical and infrared space- and ground- based data. These Euclid deep fields are being processed at CALET.


The objective of this stage is as follows. 1) Assess how well the cosmic magnification signal can be recovered in current state-of-the art catalogues like COSMOS and SXDS 2) From this data, based on simulated and actual complementary observations the Euclid deep fields, estimate how well Euclid will recover the cosmic magnification signal.

Location, supervision, renumeration:

This project will be carried out at the IAP (Institut d’Astrophysique de Paris, 98 bis Boulevard Arago, Paris 75014) under the direction of Henry J. McCracken and Raphael Gavazzi. The stage will last six months.


A knowledge of astronomical image analysis would be an advantage. The stage will involve (a) developing cosmic magnification estimators (b) applying them to simulated and real data.

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