Screening steps

The complete XDCP screening and classification procedure is illustrated in the flow chart of Fig. 6.13. The selection process can be structured into the two independent parts: (i) the X-ray quality assessment of the source and (ii) the optical counterpart identification. The third part in Fig. 6.13 displays some rounded preliminary success rates of follow-up observations, covered in Chap. 7, in order to complete the start-to-end source accounting. Numbers circled in blue refer to approximate absolute extended source counts at the given screening stage, the green circled percentages are relative to the starting sample at screen level 0.

The sample completenessis a prime factor for the XDCP science applications. Conse- quently care has to be taken not to lose any real clusters during the screening procedure. Ambiguous sources at a given screening stage are therefore passed to the next level for a re-evaluation before they are rejected. In the end, the survey completeness and the follow- up efficiency have to be balanced,i.e. the false positive fraction should be acceptable for a given completeness level. The source flagging and classification scheme should thus be flex- ible enough to allow re-adjustments of the selection cuts as a consequence of observational follow-up feedback.

X-ray selection

The starting point for the selection process is theraw extended source list orscreen level 0, which is compiled in a automated and objective way. The first aim of this starting source sample is the complete coverage of the single band scheme detections on the 2.5σ extent significance level, which contribute about 75% (see Fig. 6.13). The second aim is to test for possible selection effects introduced by a given detection scheme. We thus include the extended sources detected with the spectral matched filter schemeon the 2.0σ significance level which are not on the list and comprise about 20% of the initial sample. Below an extent significance of 2.0σ most additional detected sources are very uncertain. For the standard scheme results at the 1.5σ extent level, only those new sources are added to the list, which are also confirmed as extended by the wavelet method. With these last 5% of sources detected at lower significance, theraw extended source listwith a total of just over 2 000 extended sources is completed. From this point on, all subsequent classification steps (screen levels 1–3) rely on human intervention.

As a first screening level, the raw X-ray source catalog is cleaned of obvious spurious detections based on the summary plots as shown in the upper right panel of Fig. 6.10. At this time, the majority of the source-in-source detections, FoV boundary sources, wing detections of bright point sources, and pronounced out-of-time residual detections are removed. With about 15% of the X-ray sources flagged as ‘artifact detections’, the cleaned screen level 1sources are passed on two the next level.

The detailed X-ray source screening of the second level makes use of the zoomed X-ray data overlays as shown in panels 1 & 2 of Fig. 6.12 and (up to) three sets of returned source parameters of the maximum likelihood fitting procedure. At this stage, sources with more

subtle reasons for a spurious extent or a spurious detection are identified. The most com- mon causes are misclassified point sources, point source blends, high noise environments, poorly determined backgrounds, and detections at high off-axis angle due to PSF residuals. Besides the visual confirmation of the existence and extent using the overlay panels 1 & 2, the source quality is evaluated based on (i) the wavelet detection result, (ii) the number of extent detections and their significance within the three schemes, (iii) the stability of the extent determination, and (iv) the hardness ratio of the source. Spurious extents of point sources can often be identified based on cross-checks with items (i), (ii), and (iv); spurious detections on the other hand typically do not exhibit stable extent solutions of item (iii) or are not re-detected at all for tests (i) and (ii).

An additional ∼20% of the initial source sample are identified as spurious sources in this way and flagged accordingly. Extended sources that passed the X-ray quality screening but are close to the cut threshold are marked as sources with lower X-ray significance and retained in the object list. The screen level 2 source list has been cleaned of about 35% of the initial sample. This large fraction of spurious sources is to be predominantly attributed to systematic effects inherent to the available SAS source detection and calibration tools and not to the expectation rate of false positives of the statistical likelihood. The main reason for the built-in redundancies in the XDCP source detection procedure is the im- proved identification and removal of this spurious source fraction while retaining the full sensitivity. The screen level 2 extended source list completes the X-ray object selection and characterization and should now closely reflect the statistical expectations of a sample with a 2–2.5σ significance threshold.

Optical classification

The screened extended source list is the input for the optical classification, which consti- tutes the second part of the candidate selection process. The main tools for the optical counterpart search are the X-ray-DSS overlays for each source as shown in panels 3 & 4 of Fig. 6.12. As a first step of the optical classification, the non-cluster sources (e.g. single nearby galaxies) are visually identified using the DSS imaging data and removed from the sample. The remaining 60% percent of the input sample comprise thescreen level 3cluster candidate source list of roughly 1 200 objects, still including multiple detections of the same source in different fields.

In the last step, these sources are visually classified either as DSS-identified cluster candidates or as XDCP distant cluster candidates based on their optical counterpart or the lack thereof. The classification scheme will be discussed in the following Sect. 6.4.2. About three quarters of the screen level 3 cluster candidates (45% of the initial sample) can be identified with an optical cluster counterpart signature on the X-ray-DSS overlays. These approximately 750 distinct objects are flagged as low–intermediate redshift cluster candidates and are saved for secondary science projects beyond the XDCP scope.

The ∼250 extended X-ray sources without an DSS-identified optical counterpart enter the distant cluster candidate master list, which constitutes the basis of the subsequent follow-up imaging program. In order to provide a full accounting of the extended X-ray source population at the XDCP sensitivity level, approximate cluster yields are stated in

Fig. 6.13 as derived from the follow-up imaging programs of Chap. 7. The numbers related to the DSS-identified candidates are based on a preliminary evaluation of the wide-field follow-up program in the South Pole Telescope (SPT) survey region (see Sect. 9.2 & 12.1). The first important finding from these data is the confirmation that the sources flagged as spurious are indeed notassociated with optical clusters out the data limit of z∼0.9.

The final cluster accounting establishes the following approximate cluster yields for the distant cluster candidates: About one out of six candidates is a true z >_∼1 system, half of the sources are associated with z <1 clusters, and roughly a third are false positives (FP),i.e.spurious sources. For the DSS-identified candidate sources, about 5/6 arez <_∼0.8 clusters or groups with a false positive rate of 1/6.

The XDCP candidate selection task (including spurious sources) can hence be globally summarized as follows (see Fig. 6.13). (i) About 1/40 of all extended X-ray sources are associated with the targeted population of z >_∼1 galaxy clusters. (ii)∼1/10 of all extended sources are clusters beyond the DSS identification limit. (iii) ∼1/2 of all extended sources are galaxy clusters. (iv) ∼3/4 of all cluster sources can be identified with an optical DSS counterpart.

Concerning the required cleaning of spurious sources, the following picture is obtained. (v) For each distant cluster candidate more than two spurious sources were removed from the initial sample in early cleaning stages. (vi)∼3/4 of all spurious sources can be identified as such within the screening procedure without follow-up observations. (vii) ∼1/10 of all spurious sources pass the screening procedure and need to be identified as false positive sources in the follow-up data of the distant cluster candidate sample. (viii) After removing all spurious sources from the object catalog, >_∼90% of the non-spurious extended sources in the high galactic latitude survey fields are associated with galaxy clusters. Additional source diagnostic plots will be presented in Sect. 6.6.