5.3 Galaxy groups and clusters at high redshift with Athena+
5.3.4 Source classification
To assess the detection of high-redshift galaxy groups with Athena+, extensive Monte Carlo simulations are performed (see Section5.3.2). Each simulation contains a population of AGNs and 7 identical galaxy groups of M500 = 5×1013M . The simulated galaxy groups are located at different off-axis angles: one is located on-axis, three at 12.70off-axis angle, and three at 18.60(see the bottom left panel in Fig.5.3). The galaxy groups properties are simulated at seven different redshifts z = 1, 1.5, 2, 2.5, 3, 3.5, 4, having 15 simulated pointings per redshift. A total of 105 Athena+ pointings of 100 ks are simulated. These simulations are analysed with the detection procedure described in Section5.3.3. All the detected sources are cross-identified with the input parameters using a correlation radius of 1000. In principle, any detected source which cannot be cross-identified with any input source is considered a spurious detec- tion. In this analysis, they are not quantified since the goal is only to measure the detection probability of high-redshift groups as extended sources. However, spurious detection must be taken into account for a deeper analysis (see Pacaud et al.2006).
For the detection and identification of high-redshift groups as extended sources, the output best-fitting parameter space is explored. This examination has the purpose of establishing a source classification criterion, and to estimate the contamination by misclassified/spurious sources. Following a similar approach as Pacaud et al. (2006), the extent−extension likelihood space is examined (see Fig. 5.4). The distinction between point and extended-like sources is clearly visible. By choosing the location of 4 < extent < 150 and extension likelihood > 20 (solid lines in Fig. 5.4) the majority of the galaxy groups are recovered and the level of contamination is kept as low as possible.
Figure5.5(blue-shaded histograms) shows the detection probability of galaxy groups of being detected as extended sources. In such histograms, detection probability equals 1 means that all simulated galaxy groups, at a given redshift and off-axis angle, have been detected as extended sources. The error in the detection probability is given by a binomial law since there are only two events to consider: detection or
5.3 Galaxy groups and clusters at high redshift with Athena+ 1 10 100 1000 Extension Likelihood 1 10 100 1000
Source Extent (arcsec)
Figure 5.4:Best-fitting values in the extent−extension likelihood plane. Point-like sources (AGN) are displayed as green triangles, while the simulated extended sources (galaxy groups) as blue diamonds. The solid lines at extent= 4, 150 and extension likelihood = 20 determine the selection criteria for extended-like sources.
non-detection of the input sources. The error is calculated through the Wilson interval approximation: σ(Ps)=
√
Nd(1 − Ps)+ 1/4
NsNi+ 1 , (5.1)
where Ni is the number of simulated images, Nsthe number of simulated sources, and Nd the number of detected sources. Psis the detection rate of one source (Ps = Nd/NiNs). One can observe that even at large off-axis angles, a 100 ks simulation allows detecting almost all simulated galaxy groups. Above z > 1, around ∼ 6.5 × 105 galaxy groups of M500 = 5 × 1013 M are expected to exist in the Universe (see Fig.5.2). Assuming that such groups are uniformly distributed in the sky, it is expected that Athena+ detects ∼ 6.5 × 105/41, 253 deg2/0.55 deg2∼9 galaxy groups per pointing. The contam- ination level, i.e. misclassified sources located at the chosen parameter space, is ∼ 11 false extended sources per pointing. This gives 55% contamination in the extended source detection. On the one hand, to maintain the contamination to a reasonable level, one can choose a more conservative criterion. For example, an extension likelihood= 120 (dashed line in Fig.5.4). This gives a false detection value of ∼1 source per simulation. However, this selection excludes a number of extended sources, and the de- tection probability decreases (see blue-filled pattern histograms in Fig.5.5). This decrement increases with off-axis angles, showing the effect of the PSF degradation and vignetting on extended sources: the maximum likelihood fitting gives a lower extent and extension likelihood values to sources located closer to the edges of the pointing because they get fainter and broader. On the other hand, by 2028 such high contamination levels can be handled by cross-identifying the data with other multi-wavelength measurements, e.g. from Euclid, and the Large Synoptic Survey Telescope (LSST).
Figure5.5also shows a fairly flat detection probability with redshift. This result is explained by different properties of the self-similar evolution model of galaxy groups and clusters (see Section2.2.4):
1. The X-ray cluster luminosity increases with redshift, by consequence more X-ray photons are emitted. In this work, the expected luminosity is calculated by using a non-self similar paramet-
On-axis 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Redshift 0.0 0.2 0.4 0.6 0.8 1.0 Detection Probability Off-axis - 12.7 arcmin 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Redshift 0.0 0.2 0.4 0.6 0.8 1.0 Detection Probability Off-axis - 18.6 arcmin 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Redshift 0.0 0.2 0.4 0.6 0.8 1.0 Detection Probability
Figure 5.5: The histograms show the source detection probability of M500 = 5 × 1013 M galaxy groups as
function of redshift in 100 ks simulations. Each panel shows a different off-axis angle where the galaxy groups are placed in the simulations (on-axis, 12.70, 18.60). The blue-shaded histograms display a less conservative
criteria (4 < extent < 150 and extension likelihood > 20 in Fig. 5.4) for selecting galaxy groups as extended sources than the blue-filled pattern ones (extension likelihood > 120).
erisation (Reichert et al.2011), which predicts a lower luminosity than the self-similar one but it also increases with redshift (see Fig.5.6).
2. The cluster temperature also increases with redshift, giving an enhancement of photons in the detection band.
3. The cluster intrinsic size (r500) decreases with redshift since the density of the Universe rises as a function of redshift (see Section2.2.4). Despite this lowering, the size of the simulated galaxy groups is always larger than the Athena+ PSF (> 1000). This also results in fewer number of background photons within the source area.
In summary, galaxy groups with masses M500 = 5 × 1013M can be detected with high probability as extended sources by Athena+, at any off-axis angle and out to high redshift.