Euclid preparation. XLVII. Improving cosmological constraints using a new multitracer method with the spectroscopic and photometric samples
Abstract
Future data provided by the Euclid mission will allow us to better understand the cosmic history of the Universe. A metric of its performance is the figureofmerit (FoM) of dark energy, usually estimated with Fisher forecasts. The expected FoM has previously been estimated taking into account the two main probes of Euclid, namely the threedimensional clustering of the spectroscopic galaxy sample, and the socalled 3×2pt signal from the photometric sample (i.e., the weak lensing signal, the galaxy clustering, and their crosscorrelation). So far, these two probes have been treated as independent. In this paper, we introduce a new observable given by the ratio of the (angular) twopoint correlation function of galaxies from the two surveys. For identical (normalised) selection functions, this observable is unaffected by sampling noise, and its variance is solely controlled by Poisson noise. We present forecasts for Euclid where this multitracer method is applied and is particularly relevant because the two surveys will cover the same area of the sky. This method allows for the exploitation of the combination of the spectroscopic and photometric samples. When the correlation between this new observable and the other probes is not taken into account, a significant gain is obtained in the FoM, as well as in the constraints on other cosmological parameters. The benefit is more pronounced for a commonly investigated modified gravity model, namely the γ parametrisation of the growth factor. However, the correlation between the different probes is found to be significant and hence the actual gain is uncertain. We present various strategies for circumventing this issue and still extract useful information from the new observable.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 October 2024
 DOI:
 10.1051/00046361/202450368
 arXiv:
 arXiv:2404.12157
 Bibcode:
 2024A&A...690A..30E
 Keywords:

 cosmological parameters;
 dark energy;
 largescale structure of Universe;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 16 pages, 7 figures, submitted to A&