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 WiggAstrophysics 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