Measuring the scatter in the mass richness relation of galaxy clusters for the dark energy survey using the correlation function

Student thesis: Doctoral thesis


The evolution of the abundance of cluster of galaxies is becoming a powerful tool to constrain cosmological parameters. This has motivated the design of a new wide-area cluster surveys such as Dark Energy Survey. This survey will have the potential to find hundred of thousands of clusters. The principal challenge to precision cosmology with this technique is the accurate calibration of the relation between the observables and halo masses. In this work we present a new method to measure the scatter in the mass observable relation of galaxy clusters for the future DES cluster catalog, based on the measurements of the bias of the correlation function. Our analysis is developed on N-body simulations. In particular, we use a light cone based on the Hubble Volume Simulations SO light cone that has 5000 deg2 (DES volume). In order to properly measure the large scale bias for clusters, first we need to understand the large scale properties of the dark matter halos. We study how the halos are biased respect to the underlying matter distribution using the halo model. It describes the clustering of dark matter halos and provides an analytical expression for the bias of halos as a function of halo mass. Since the basic element of the halo model is the mass function we also study the accuracy of the halo abundance models. We compare the measurements in simulations with the model predictions. To calculate the bias in simulations we measured the two point correlation function with Landy & Szalay estimator and study the statistical errors. Our results demonstrate that the uncertainty in the mass function produces a systematic error in our method because the halo bias depends on it. After we studied the bias in halos, we develop a bias model for a sample of clusters to compare with observations. We need to relate the mass to a easily observable quantity. In our case we model the bias for a richness threshold. Doing this requires a halo occupation distribution (HOD), where the number of galaxies is specified by the probability distribution. In particular we use a lognormal distribution with a mean given by an empirical mass richness relation and the standard deviation or scatter, lnM. We assign richness to the dark matter halos of the light cone by means of this distribution and study the precision to constrain the scatter. Our forecast of the new analysis technique shows how at the highest values of scatter we obtain the highest precision. We have a very competitive result to measure the expected scatter in the DES mass richness relation and it will be precise enough for the dark energy parameters won’t be significantly biased. In summary, the new method proposed could be used in the DES cluster catalog as a cross check method complementary to other such as self-calibration.
Date of Award26 Feb 2015
Original languageEnglish
SupervisorJuan Estrada (Director), Ignacio Sevilla-Noarbe (Director) & Enrique Fernandez Sanchez (Tutor)


  • Clusters of galaxies
  • Cosmology

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