Euclid preparation. XII. Optimizing the photometric sample of the Euclid survey for galaxy clustering and galaxygalaxy lensing analyses
Abstract
Photometric redshifts (photozs) are one of the main ingredients in the analysis of cosmological probes. Their accuracy particularly affects the results of the analyses of galaxy clustering with photometrically selected galaxies (GC_{ph}) and weak lensing. In the next decade, space missions such as Euclid will collect precise and accurate photometric measurements for millions of galaxies. These data should be complemented with upcoming groundbased observations to derive precise and accurate photozs. In this article we explore how the tomographic redshift binning and depth of groundbased observations will affect the cosmological constraints expected from the Euclid mission. We focus on GC_{ph} and extend the study to include galaxygalaxy lensing (GGL). We add a layer of complexity to the analysis by simulating several realistic photoz distributions based on the Euclid Consortium Flagship simulation and using a machine learning photoz algorithm. We then use the Fisher matrix formalism together with these galaxy samples to study the cosmological constraining power as a function of redshift binning, survey depth, and photoz accuracy. We find that bins with an equal width in redshift provide a higher figure of merit (FoM) than equipopulated bins and that increasing the number of redshift bins from ten to 13 improves the FoM by 35% and 15% for GC_{ph} and its combination with GGL, respectively. For GC_{ph}, an increase in the survey depth provides a higher FoM. However, when we include faint galaxies beyond the limit of the spectroscopic training data, the resulting FoM decreases because of the spurious photozs. When combining GC_{ph} and GGL, the number density of the sample, which is set by the survey depth, is the main factor driving the variations in the FoM. Adding galaxies at faint magnitudes and high redshift increases the FoM, even when they are beyond the spectroscopic limit, since the number density increase compensates for the photoz degradation in this case. We conclude that there is more information that can be extracted beyond the nominal ten tomographic redshift bins of Euclid and that we should be cautious when adding faint galaxies into our sample since they can degrade the cosmological constraints.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 November 2021
 DOI:
 10.1051/00046361/202141061
 arXiv:
 arXiv:2104.05698
 Bibcode:
 2021A&A...655A..44E
 Keywords:

 galaxies: distances and redshifts;
 techniques: photometric;
 cosmological parameters;
 surveys;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 21 pages, 13 figures. Abstract abridged