Free Access
Issue
A&A
Volume 575, March 2015
Article Number A126
Number of page(s) 24
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201425262
Published online 10 March 2015

Online material

Appendix A: Observational material and astrometric and photometric calibrations

Table A.1

Observational material.

Appendix A.1: Astrometry

We transform the coordinates of our catalogues from x,y in pixels to Equatorial J2000 RA and Dec using stars in common between our catalogue and SDSS-DR9 (hereafter DR9, for brevity), using the latter catalogue as our astrometric standard. The transformation is performed with the dedicated code CataXcorr5 in two steps. First, a third-degree polynomial solution is fitted using DR9 sources classified both as stars or as galaxies as astrometric standard. The derived solution is based on a few hundred shared sources per field (from a minimum of 323 to a maximum of 612) and have a median rms of 0.11″ in both RA and Dec (from a minimum of 0.07″ to a maximum of 0.14″). Second, a first-degree polynomial solution for the derived coordinates is fitted using only DR9 sources classified as stars. The derived solutions are based, for each field, on several tens to a few hundred shared stars (from a minimum of 65 to a maximum of 330) and have a median rms of 0.08″ in both RA and Dec (from a minimum of 0.05″ to a maximum of 0.12″).

A second set of X,Y coordinates projected on the sky is also calculated for each field. The origin of these coordinates is located at the position of the centroid of the UCHVC associated with the considered field; X increases towards the west and Y increases towards the North.

Appendix A.2: Photometric calibration

The absolute photometric calibration is determined using stars in common with DR9 as secondary calibrators, as done in Bellazzini et al. (2011, 2014). Instrumental g,r magnitudes for each field have been transformed into the SDSS DR9 photometric system individually, using in-field calibrators. Here we adopted a two step procedure. We use Sextractor to reduce the 20 s images of four fields (A, J, K, R) that have many stars in common with DR9. Since our deep images saturate around r ~ 19.0, and DR9 g,r photometry has photometric errors 0.03 mag only for r< 20.0, the catalogues from these short exposures have a much larger (useful) magnitude range that overlaps more with DR9 than those from the long exposures. This leads to better comparisons between instrumental (gi,ri) magnitudes and standard magnitudes (gDR9,rDR9) since they rely on the large numbers of DR9 stars observed at high signal-to-noise, i.e. with more accurate photometry (see Bellazzini et al. 2011, for a discussion). We use the hundreds of stars in common for each field within the relevant colour range to fit the coefficient of a first-degree colour term to transform instrumental magnitudes into natural magnitudes (gnat,rnat): (A.1)The equations are found to provide an adequate description of the data over the colour range of interest for our scientific purposes. We adopt the mean of the four values as our final coefficients to be used for all our photometry: ag = 0.084 with standard deviation σag = 0.002 and ar = 0.040 with standard deviation σar = 0.012.

These equations are used to transform all of our DAOPHOTII photometry into the natural system. We then use the stars in common between our deep photometry catalogues and DR9 to determine a single constant for each field and passband. To estimate these photometric zero points, ZPr, ZPg, we carefully inspect, for each field, the magDR9 − magnat vs. magDR9 diagrams to select the set of unsaturated common stars showing low scatter in magDR9 − magnat that were finally used to estimate the zero points, with typical rms 0.02 mag. All the natural magnitudes were transformed into the standard systems with the equations: (A.2)

Appendix B: Colour–magnitude diagrams: the complete data set

We present here the CMDs for the fields that are not shown in the main text. The arrangement and the symbols are the same as in Fig. 6.

thumbnail Fig. B.1

Same as Fig. 6 but for Fields G to P.

Open with DEXTER

thumbnail Fig. B.2

Same as Fig. 6 but for Fields Q to Z.

Open with DEXTER

Appendix C: Density maps: the complete data set

We present here the density maps for the fields not shown in the main text. The arrangement and the symbols are the same as in Fig. 9. We remind the reader that the r27 maps are obtained using the entire, colour-selected sample, while r25 stars are obtained from the sub-sample of stars with r< 25.0.

thumbnail Fig. C.1

r27 maps and r25 maps for fields D, E, F, and G. The arrangement and the meaning of the symbols are the same as in Fig. 9.

Open with DEXTER

thumbnail Fig. C.2

r27 maps and r25 maps for fields H, I, K, and L. The arrangement and the meaning of the symbols are the same as in Fig. 9.

Open with DEXTER

thumbnail Fig. C.3

r27 maps and r25 maps fields M, N, O, and P. The arrangement and the meaning of the symbols are the same as in Fig. 9.

Open with DEXTER

thumbnail Fig. C.4

r27 maps and r25 maps for fields Q, R, S, and T. The arrangement and the meaning of the symbols are the same as in Fig. 9.

Open with DEXTER

thumbnail Fig. C.5

r27 maps and r25 maps the fields U, V, W, X. The arrangement and the meaning of the symbols are the same as in Fig. 9.

Open with DEXTER

thumbnail Fig. C.6

r27 maps (upper panels) and r25 maps (lower panels) the fields Y and Z. The meaning of the symbols is the same as in Fig. 9.

Open with DEXTER


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