Results

X-ray versus MIR flux measures

Figure6.6compares the cluster 3.6µm aperture flux to the X-ray aperture flux measured for all XMM- LSS and SpARCS z > 0.8 clusters. A total of 18 XMM-LSS and 95 SpARCS clusters were ultimately measured. The remaining clusters (2 XMM-LSS and 3 SpARCS) were located sufficiently close to the survey data footprint that their aperture measurements were compromised.

Two initial impressions are apparent from this comparison. Firstly, there is a broad correlation defined by X-ray faint, MIR faint ranging to X-ray bright, MIR bright clusters. This work does not attempt to quantify this trend in the current study as the adopted measurement approach is deliberately simple and

2_{The virial temperature of a dark matter halo is a measure of the binding energy of the halo material, i.e. the halo can only}

Figure 6.6: A comparison of X-ray and MIR 10 aperture brightness values for XMM-LSS (blue squares) and SpARCS (black squares) clusters. XMM-LSS and SpARCS clusters which are located within 30_{on the sky (and}

thus potentially represent the same source) are indicated with an additional cyan or magenta circle, respectively. Error bars indicate the 68% interval of the posterior background subtracted flux distribution for each source. is designed to provide a robust comparison between clusters of widely different properties. In contrast to this broadly defined correlation between X-ray gas emission and stellar emission within both cluster samples, there is a population of MIR selected clusters which can be described as X-ray faint (effectively consistent with zero emission) and MIR bright.

Based upon the distribution of the SpARCS clusters on the X-ray/MIR plane displayed in Fig.6.6, three sub-samples can be defined - the aim being to determine if the visually apparent sub-samples display recognisable physical differences with the XMM-LSS sample. An X-ray bright MIR selected clusters sample is defined by those galaxy clusters displaying fX ≥ 0.6 × 10−14ergs s−1 cm−2. A further split of the X-ray faint ( fX < 0.6 × 10−14 _{ergs s}−1_cm−2_{) MIR selected clusters is done. These X-ray faint} clusters are divided into those that are MIR bright ( f3.6µm ≥ 650µJy) and those that are MIR faint ( f3.6µm< 650µJy). The final numbers of clusters present in each sample are as follows: XMM-LSS, 18, SpARCS X-ray-bright, 15, SpARCS X-ray-faint+MIR-bright, 17, SpARCS X-ray-faint+MIR-faint, 63. Despite these differences, each sub-sample presents statistically identical distributions of observables such as redshift and off-axis angle in XMM-Newton pointings.

MIR properties

The angular surface brightness distribution was evaluated, and a visual assessment of the clusters in each subsample was performed (see Willis et al., in preparation, for further details). The main findings are summarized here:

• The X-ray selected galaxy clusters display a more compact distribution of stellar light. This

indicates that X-ray selection is biased towards compact high surface brightness systems. This is confirmed by the visual check, where these objects appear as centrally concentrated systems of galaxies.

6.4 Wavelength bias in distant galaxy cluster samples: A case study using the XMM-LSS and SpARCS cluster samples

• X-ray-faint+MIR-faint objects and X-ray-bright+MIR-bright systems have similar surface bright-

ness distributions. The visual inspection reveals that the X-ray-bright+MIR-bright galaxy clusters also appear as centrally concentrated systems of galaxies, while the X-ray-faint+MIR-faint clusters objects show a range of appearances: from concentrated systems to sparce objects.

• X-ray-faint+MIR-bright galaxy clusters have a decrement in the central surface brightness, this

results indicates that such systems are physically different from the other subsamples. This is confirmed by the visual assessment, which reveals that X-ray-faint+MIR-bright systems have a lack of concentrated systems of galaxies.

Stacked X-ray images

The creation of stacked X-ray images for each cluster sub-sample permits the average X-ray emission properties of each to be discussed. Furthermore, the low noise properties of stacked images allow a sensitive test of the average emission from the SpARCS X-ray faint sub-samples to be investigated. Stacked images of the cluster sub-samples were created using the data from the XMM-LSS survey. The stacking method is described in Section6.3. The cut-outs have a radius of 20_{and were extracted in the} [0.5 − 2] keV energy band. Figure6.7shows the final stacked images for each sub-sample. As a simple test of our stacking procedure, a set of randomly selected positions from the XMM-LSS region was also analysed in an identical way.

Highly significant X-ray emission is seen in both the XMM-LSS and X-ray-bright+MIR-bright sub- samples when compared to the image created by combining 100 random locations. Weaker emission is also detected in the stacked image corresponding to the X-ray-faint+MIR-faint sub-sample. There is some evidence from the stacked images that X-ray emission in the SpARCS sub-samples is associ- ated more closely with the cluster BCG position compared to the barycentre position. This is indicated visually in the stacks for the X-ray bright and X-ray-faint+MIR-faint sub-samples where more compact, centrally-peaked X-ray emission is generated when stacking on the BCG position. In contrast to this, the stacked image for the X-ray-faint+MIR-bright SpARCS sub-sample displays no centrally concen- trated X-ray emission when stacking on either the BCG or barycentre position. Some peripheral X-ray emission is noted in the stacked image yet is only marginally significant when compared to the stacked random location image.

The visual trends noted in the stacked images are reinforced by inspection of the angular X-ray surface brightness distribution in each cluster sub-sample shown in Fig.6.8. The angular surface brightness was computed using the relation

f_X,cum = PN

i fX,i

r_i2 , (6.15)

where fX,iis the X-ray flux summed as a function of increasing radius ri. The X-ray centroid is used as the reference position for XMM-LSS clusters and the barycentre of the galaxy light is used for SpARCS. The results confirm that the X-ray flux in the SpARCS sub-samples is centred on the BCG instead of the barycentre position and that the X-ray faint, MIR bright sub-sample is devoid of central X-ray emission.

Figure 6.7:Stacked X-ray images for each cluster sub-sample. Each image is smoothed with a Gaussian kernel of sigma equal to two pixels. The scale bar indicates photon count rate per second. Top row: middle: XMM- LSS clusters; right: Stack of 100 random positions. Middle row: SpARCS clusters stacked on the catalogue barycentre position: left: X-ray-bright; middle: X-ray-faint+MIR-faint; right: X-ray-faint+MIR-bright. Bottom panel: SpARCS clusters stacked on the catalogue BCG position. The order follows the middle row. The blue circle in each panel represents the 10_{radius aperture used to measure individual cluster X-ray fluxes.}