Free Access
Issue
A&A
Volume 542, June 2012
Article Number A3
Number of page(s) 23
Section Galactic structure, stellar clusters and populations
DOI https://doi.org/10.1051/0004-6361/201118531
Published online 24 May 2012

Online material

Appendix A: False positive clusters

A.1. False positive clusters caused by artefacts

Clustering all sources without filtering the data fails. Clustering only sources with K magnitude brighter than 17m, k_1ppErrBits  < 524   288 and mergedClass  =  + 1 improves the results remarkably, but visual inspection of the images of the candidate areas revealed a large fraction of the cluster candidates to be blatant false positives:

  • i)

    Bright stars tend to create artefacts in the catalogue dataappearing as mergedClass  =  + 1 classifications. This happens specially in the direction of the 8 spikes of the diffraction pattern from the two spiders supporting the secondary and the guider auxiliary lens (see Sect. 7.6 in Dye et al. 2006). We fetched 2MASS stars brighter than 10m in K, and examined their surroundings in the UKIDSS GPS images and catalogues creating thus rules according to which non-stellar sources are discarded both very near the bright star, and also farther away in the direction of the 8 diffraction spikes. The brighter the star, the greater the distance to which it produces false classifications. Here we note that bits 2 and 20 for the quality error bit flag are not yet implemented. The issue for the former is “Close to a bright source” and for the latter “Possible diffraction spike artefact/contamination” (WSA 2012). An example is shown in Fig. A.1.

  • ii)

    Bright stars at or just outside an array edge tend to create mergedClass  =  + 1 classifications. To find such potential locations we compare the coordinates of 2MASS stars brighter than 8m in K against parameters minRa, minDec, maxRa and maxDec in the UKIDSS table CurrentAstrometry, and check that parameter multiframeID equals in the tables CurrentAstrometry and gpsDetection. Each cluster candidate is compared to these locations in order to automatically remove false positives. This method might remove also true positives as is the case with e.g. [BDB2003] G094.60-01.80. An example is shown in Fig. A.2.

  • iii)

    Beams, “bow-ties”, cross-talk images and persistence images create clusters of non-stellar sources. The “bow-tie” is a low-level feature in the PSF produced by haloes of bright stars (see Sect. 7.6 in Dye et al. 2006). As for the cross-talk images the WSA states that the GPS cannot ever be cross-talk flagged with the current algorithm parameters as its fields are just simply too crowded (WSA 2012). The first three types of these false positives are not numerous. It would be useful to remove the persistence image clusters but at present we cannot separate them from true positive clusters using the catalogue data. Examples are shown in Fig. A.3.

A.2. False positive clusters caused by surface brightness

In Figs. A.4 and A.5 two examples of a false positive candidate caused by surface brightness are shown. In Fig. A.4 the false positive cluster at (l = 12.841°,b = 0.544°) is caused by the interplay of extinction and the reflection of the interstellar radiation field from the dust cloud. In Fig. A.5 the object at (l = 16.799°,b = 0.126°) at the centre of the image is a millimetre radio-source and classified as a possible planetary nebula. This seems to be an outflow coming from a hole in a dark cloud. Excess surface brightness due to the bright central source makes the stars appear as non-stellar and in addition seems to produce non-existent sources.

thumbnail Fig. A.1

Example of a false positive cluster caused by the bright star 2MASS J18360783-0644313. The red points and crosses are UKIDSS GPS sources brighter than 17m and classified as non-stellar. The red crosses mark the filtered out points.

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thumbnail Fig. A.2

False positive cluster caused by the star 2MASS J19013383+1116532 at the array edge. The red points are UKIDSS non-stellar sources brighter than 17m in K.

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thumbnail Fig. A.3

Examples of false positive clusters due to: a) a beam, b) cross-talk images, c) “bow-ties” (the bright star causing the artefact is 5′ towards the upper right corner) and d) a persistence image.

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thumbnail Fig. A.4

False positive cluster at (l = 12.841°,b = 0.544°) caused by surface brightness. The panels on the left show the catalogue data and the panels on the right the corresponding on K band images. The red points are the UKIDSS sources classified as non-stellar and brighter than 17m in the K band. The blue circles mark the cluster members given by the algorithm. The blue points are sources listed in 2MASS but not in UKIDSS GPS. Image orientation is North up and East left.

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thumbnail Fig. A.5

False positive cluster at (l = 16.799°,b = 0.126°) caused by surface brightness. The catalogue data is shown on the left and the corresponding K band image on the right. The symbols are as in Fig. A.4.

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Appendix B: Examples of cluster candidates

Example cluster candidates are shown in Figs. B.1B.6. The different panels in the figures are as follows. Upper left: The catalogue data in the direction of the cluster candidate. The red points are UKIDSS non-stellar sources brighter than 17 mag, black points other sources brighter than 17m, and yellow points sources fainter than 17m in the K filter. Brown points are stars listed in 2MASS but not in UKIDSS GPS. Sources encircled by a yellow line do not have coverage in all three bands. The bright stars encircled by a green line in the catalogue data plot and the grey scale image lower left cause non-stellar classifications and produce false positive clusters: the algorithm removes the sources indicated with a green cross. Two bright stars encircled by a green line in the catalogue data plot and the WSA grey scale plot in the lower left. The WSA false colour blow-up image (J image coded in blue, H image in green and K image in red) of the cluster candidate is shown to the right of the grey scale image. All the UKIDSS GPS sources within the catalogue data plot are plotted in the (HK,JH) colour–colour plot on the right. Blue dots mark sources brighter than 17m and green dots sources fainter than 17m in K. The red filled circles mark UKIDSS sources (both stellar and non-stellar) in the cluster direction brighter than 17m in K. The approximate unreddened main sequence is plotted with a continuous line. Approximate main sequence reddening lines are shown with dashed lines. The numbers on the reddening lines show the optical extinction in case the star originates from the early or late main sequence. The arrow indicates an optical extinction of 5 mag.

The automated search uses by default the AperMag3 magnitudes (2.0′′ aperture diameter). For the colour–colour plots we experimented also with the AperMag1 (1.0′′ aperture diameter) and AperMag4 (2.8′′ aperture diameter) extended source magnitudes. For cluster candidates 20, 114 and 116 the colour−colour

plots use the AperMag1 magnitudes because they seem to give better precision. For the remaining cluster candidates (9, 43 and 110) the colour–colour plots use the AperMag3 magnitudes. For ppErrbits we apply the same  <    524   288 limit as in the automated search knowing that by using this limit we don’t take advantage of all the photometric warning flags. However for these six cluster candidates only for a negligible portion of the data ppErrbits  > 255.

We use in the figures a reddening slope of 1.6. We recognise that reddening bands in colour–colour diagrams are delimited by curves rather than vectors (e.g. Golay 1974; Stead & Hoare 2009). The value of 1.6 is the mean of all the reddening tracks in Stead & Hoare (2009). Irrespective of the uncertainty of the reddening vector the colour–colour plots allow to estimate the reddening in the direction of the cluster candidates. The notable difference between cluster candidates is the larger number of field stars, especially giants, in the direction of the inner Galaxy (cluster candidates 9, 20 and 43) with respect to the number of stars in the outer Galaxy (cluster candidates 110, 114 and 116). In the inner Galaxy the field stars, i.e. the stars not classified as non-stellar, lie within the approximate reddening path outlined by the reddening lines. In the outer Galaxy the statistics is poor because of the small number of field stars and the high extinction. The spread of the sources in the direction of the cluster candidates and classified as non-stellar is much higher than for the field stars. The photometry of these sources suffers from the faintness of the stars and the high background surface brightness. However, these were the objects used to locate the cluster candidates. In general it can be noted that the extinction towards these sources is on the average higher than for the general field stars population. The extinctions range from 10 mag up to 20 mag and more. Dedicated observations are needed for detailed analysis of the cluster candidates.

thumbnail Fig. B.1

2.5′ by 2.5′ box around cluster candidate 43. The red points in the panel upper left are UKIDSS non-stellar sources brighter than 17m in K, black points other sources brighter than 17m in K and yellow points sources fainter than 17m in K. Brown points are sources listed in 2MASS but not in UKIDSS GPS. Sources encircled by a yellow line do not have coverage in all three bands. The stars encircled by a green line in the catalogue data plot and the WSA grey scale image lower left cause non-stellar classifications and produce false positive clusters: the algorithm removes the sources marked with a green cross. The WSA false colour blow-up image (J image coded in blue, H image in green and K image in red) of the cluster candidate is shown to the right of the grey scale image. Image orientation is North up and East left. All the UKIDSS GPS sources within the catalogue data plot are plotted in the (HK,JH) colour–colour plot on the right. Blue dots are sources brighter than 17m and green dots fainter than 17m in K. The red filled circles mark UKIDSS sources (both stellar and non-stellar) in the cluster direction brighter than 17m in K.

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thumbnail Fig. B.2

As Fig. B.1 for cluster candidate 20. The box size is 1.9′ by 1.9′.

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thumbnail Fig. B.3

As Fig. B.1 for cluster candidate 110. The box size is 1.2′ by 1.2′.

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thumbnail Fig. B.4

As Fig. B.1 for cluster candidate 116. The box size is 1.4′ by 1.4′.

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thumbnail Fig. B.5

As Fig. B.1 for cluster candidate 9. The box size is 0.9′ by 0.9′.

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thumbnail Fig. B.6

As Fig. B.1 for cluster candidate 114. The box size is 1.8′ by 1.8′.

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Appendix C: Zone of avoidance galaxies

A zone of avoidance galaxy has been reported in the direction of three of the new cluster candidates: 109, 110, 112 and 137. False colour images of these candidates produced from WSA fits files are shown in Figs. C.1C.4. J image is coded in blue, H in green and K in red. North up and East left. A cluster of individual stars can be seen in the direction of these three cluster candidates. This would not be the case if the sources were extragalactic.

thumbnail Fig. C.1

False colour image of cluster candidate 109. The J, H and K bands are coded in blue, green and red, respectively. North up and East left. The box size is 4′ by 4′.

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thumbnail Fig. C.2

As Fig. C.1 for cluster candidate 110. The box size is 4′ by 4′.

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thumbnail Fig. C.3

As Fig. C.1 for cluster candidate 112. The box size is 4′ by 4′.

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thumbnail Fig. C.4

As Fig. C.1 for cluster candidate 137. The box size is 4′ by 4′.

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