1. THE GROWTH & OBSERVABILITY OF GALAXY CLUSTERS
Galaxy cluster evolution supports a hierarchical scenario of large scale structure development: progenitors coalesce and merge to form the largest gravitationally-bound objects in the
Universe. We investigate this phenomenon and how it impacts cluster observability, with particular attention paid to identifying the protoclusters of those in the XXL survey using
photometric and spectroscopic techniques. We are in the process of combining these results with data from the Millennium Simulation, which models the formation and growth of dark
matter haloes, and determining whether observations and simulated predictions agree.
1 INTRODUCTION
Most clusters did not begin to form in the inaccessibly remote past, but rather at
redshifts which can be observed by using a variety of proxies and detection
techniques. Developing systems such as merging protoclusters are of most relevance
to the investigation due to their dynamical diversity and interesting observable
properties. Preliminary research includes visualising how detected clusters of varying
mass and redshifts will evolve theoretically. This was achieved by:
(i) Standardising cluster masses and luminosities in the [0.1−2.4] keV flux band to
an overdensity of 500 by using X-ray simulations and scaling relations to ensure
the overall population is consistent and comparable, as in Piffaretti et al. (2011).
(ii) Numerically integrating the cluster mass accretion rates deduced by Fakhouri &
Ma (2010) to compute the coloured isocontours shown in Fig. 1.
3 DISCUSSION
It is often difficult to distinguish between the X-ray signals of abundant, active
galactic nuclei and the hot intracluster gas of galaxy clusters. A selection algorithm
(pipeline) is used to identify clusters based on their X-ray source extent (angular
core radius) and extent likelihood, however its accuracy is inevitably limited. An X-
ray source in the vicinity of a potential cluster progenitor (pp-1) shown in Fig. 5, is
just below the pipeline cluster confirmation threshold. Despite this, it still remains an
extremely good protocluster candidate due to spectroscopic and dual-cut
photometric overdensities in precisely the same region.
2 ANALYSIS
2.1 Photometry
Clusters contain populations of passively evolving, red galaxies which follow an
intrinsic colour-magnitude relationship known as a ”red sequence”. This allows
member galaxies to be selected via colour cuts: excluding those galaxies which are
unlikely to have formed at the same epoch as the cluster. Multiple colour cuts in
different bands allow us to further isolate galaxies with similar spectral energy
distributions to the relevant cluster.
This material is the basis of the project and has led to the investigation of larger
scale structure and the search for merging systems in the vicinity of XXL clusters via
photometric (colour) and spectroscopic (redshift) analysis of the CFHT and VIPERS
survey data respectively; this region is shown in Fig. 2.
2.2 Spectroscopy
Galactic redshifts can also be used in the cluster member selection process by
creating a subset of galaxies which are at a similar redshift to the central cluster and
therefore likely to be components of the same structure.
The separation between n0286 and its potential progentitors exceeds its virial
radius, suggesting they are independent structures. In the future more rigorous
comparison with observational results will be conducted by calculating progentitor
masses from their luminosities (X-ray fluxes), combined with a study of merger
frequencies as seen in the Millennium Simulation.
Fig 1
Jacob Ider Chitham,Tom Wigg
Astrophysics Group, H.H.Wills Physics Laboratory, University of Bristol,Tyndall Avenue, Bristol BS8 1TL
Figure 1: Mass-redshift scatter plot for four different cluster surveys overlaid on top of an
isocontour plot which represents the evolution of a cluster’s mass as a function of redshift.
Figure 3: CFHT galaxies around cluster n0286 with the relevant colour cut about the cluster’s
red sequence.
Figure 2: Number density plot for the CFHT photometric objects in the XXL-VIPERS overlap region. All XXL survey clusters with redshifts greater than 0.5 are circled; these are likely to be the most
evolutionarily active of the survey. The 0.25 deg2 area around the cluster of interest n0286 which is described throughout this poster, is also shown.
Figure 4: Large scale structure is easily observable surrounding clusters in the form of overdense
regions, filaments and voids in the RA−z plane, after preliminary spatial and redshift restrictions of
0.25 deg2 and zcluster ± 0.1 about each cluster.
4 ACKNOWLEDGEMENTS
We thank Prof M. Bremer, Dr B. Maughan, Miss K. Husband and Dr
Mark Taylor for guidance throughout the duration of the
investigation. For more information about galaxy cluster research
within the University of Bristol School of Physics please visit:
www.bristol.ac.uk/physics/research/astrophysics/research/clusters-galaxies/
Figure 5: Left: Combined photometric and spectroscopic data displayed as a two-dimensional
number density histogram and contours within a 0.25 deg2 square annulus centred on the cluster
n0286. Overdensities at a physical distance of ∼3.31 Mpc are labelled as potential progenitors; pp-1
and pp-2. Right: Angular core radius against extension likelihood for all X-ray sources in the XXL-
North catalogue. Regions corresponding to confirmed (C1 and C2) clusters are highlighted along
with n0286 and the pp-1 and pp-2 source candidates.
REFERENCES
Fakhouri O., Ma C.-P., 2010, MNRAS, 401, 2245
Piffaretti R.,Arnaud M., Pratt G.W., Pointecouteau E., Melin J.-B., 2011,A&A, 534