# Dynamical mass estimates of Sunyaev-Zel'dovich E_ect selected galaxy clusters in the millennium gas simulations.

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2016

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## Abstract

Abstract
To extract and make estimates of the cosmological parameters requires knowledge of the
cluster mass. Cluster mass is not directly observable but can be predicted by numerical
simulations of structure formation and can be inferred from observable proxies for mass.
One way to nd a cluster is by the Sunyaev-Zel'dovich (SZ) e ect, caused by the inverse
Compton scattering of photons from the cosmic microwave background (CMB) by hot gas
in clusters. The observable SZ e ect signal (Y , the integrated Comptonisation parameter)
does correlate well with cluster dynamical mass. The cluster mass can be estimated from
measuring the one dimensional (1D) line-of-sight (LOS) velocity dispersion ( v) of galaxies
in clusters, however, depending on the type of galaxies selected, such measurements
may be subject to biases. We investigate this issue using simulated cluster and galaxy
catalogues produced by the Millennium Gas Simulations Project. We aim to design an
optimal observing strategy which is important for future dynamical mass measurements of
Atacama Cosmology Telescope (ACT) clusters that aim to use the Southern African Large
Telescope (SALT) for much larger studies of dynamical mass measurements (M500).
We describe the methods used to make mock cluster catalogues by following the same
procedure used in multi-object spectroscopic observations with the Robert Stobie Spectroi
graph. In our case we applied a di erent number of slits masks for targeting the galaxy
clusters and investigate the impact it has on the recovered v and estimated M500. We
do this for both an idealized case (100% redshift z completeness), and for a realistic case,
where redshift completeness decreases for fainter objects. We calculate the velocity dispersion
( v) of each cluster at z = 0.3 using galaxies selected as members only, and then
use galaxy cluster scaling relations derived from N-body/hydrodynamic simulations to
estimate the cluster dynamical mass M500. The recovered velocity dispersion is almost
unbiased (1.52%) but with much bigger scatter (1218%). We found that the bias of the
estimated M500 for 100% z completeness is less than that for the realistic z incompleteness,
which is as expected. For realistic redshift completeness, the bias in recovered M500 ranges
from 1130%.
The ultimate goal for this project is to determine how many masks we need to use per
cluster, and how many clusters in total we need, to make a reasonable measurement of
the Y500D2A
v relation, since the observing time on a queue-scheduled telescope such
as SALT is quantized by how many masks are allocated to each cluster. Using a Markov
Chain Monte Carlo (MCMC) method to t the Y500D2A
v relation, we found that the
recovered slope of the relation has less bias when using a large sample of clusters with poor
quality v measurements, as compared to a smaller sample of clusters with high quality v
measurements.

## Description

Masters Degree. University of KwaZulu-Natal, Durban.