Euclid: Fast two-point correlation function covariance through linear construction

Euclid Collaboration, E. Keihanen, V. Lindholm, P. Monaco, L. Blot, C. Carbone, K. Kiiveri, A. G. Sánchez, A. Viitanen, J. Valiviita, A. Amara, N. Auricchio, M. Baldi, D. Bonino, E. Branchini, M. Brescia, J. Brinchmann, S. Camera, V. Capobianco, J. CarreteroM. Castellano, S. Cavuoti, A. Cimatti, R. Cledassou, G. Congedo, L. Conversi, Y. Copin, L. Corcione, M. Cropper, A. Da Silva, H. Degaudenzi, M. Douspis, F. Dubath, C. A. J. Duncan, X. Dupac, S. Dusini, A. Ealet, S. Farrens, S. Ferriol, M. Frailis, E. Franceschi, M. Fumana, B. Gillis, H. Hoekstra, M. Kunz, P. Schneider, C. Sirignano, A. N. Taylor, E. A. Valentijn, Y. Wang, J. Weller

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Abstract

We present a method for fast evaluation of the covariance matrix for a two-point galaxy correlation function (2PCF) measured with the Landy- Szalay estimator. The standard way of evaluating the covariance matrix consists in running the estimator on a large number of mock catalogs, and evaluating their sample covariance. With large random catalog sizes (random-to-data objects'ratio M≫ 1) the computational cost of the standard method is dominated by that of counting the data-random and random-random pairs, while the uncertainty of the estimate is dominated by that of data-data pairs. We present a method called Linear Construction (LC), where the covariance is estimated for small random catalogs with a size of M = 1 and M = 2, and the covariance for arbitrary M is constructed as a linear combination of the two. We show that the LC covariance estimate is unbiased. We validated the method with PINOCCHIO simulations in the range r = 20-200 h-1 Mpc. With M = 50 and with 2 h-1 Mpc bins, the theoretical speedup of the method is a factor of 14. We discuss the impact on the precision matrix and parameter estimation, and present a formula for the covariance of covariance.

Original languageEnglish
Article numberA129
Number of pages17
JournalAstronomy and Astrophysics
Volume666
Issue numberOctober 2022
DOIs
Publication statusPublished - 14-Oct-2022

Keywords

  • Cosmology: observations
  • Large-scale structure of Universe
  • Methods: data analysis
  • Methods: statistical

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