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Galaxy Cluster identification

The extracted catalogue for the extended sources was used to identify a number of candidate clusters by a model independent algorithm, which is described in more detail in [Puddu et al. 2000] and [Puddu et al. 2001] (submitted). The procedure is briefly summarised here: the spatial galaxy distribution in the cleaned catalogue is binned into equal area square bins to produce a density map, such that the mean number of galaxies per pixel is $\sim 1$. SEx is run on the resulting density map image to identify and extract all the over-densities, with number density $2 \sigma$ above the mean background and a minimum detection area of 16 pixels on the map. For the detection of the over-densities a convolution filter was used; it is a Gaussian filter with width of 300-500 kpc, which is the typical dimension of a cluster core at z=0.4.

The resulting candidates identified as over-densities in the smoothed density map of the galaxy counts are shown in Figure 14. The circles indicate the positions of the candidates with a S/N > 10. As an example, the R-band image of the candidate n.1, marked with the double circle in Figure 14, is shown at full spatial resolution in Figure 15.

Figure: Detected over-densities in the smoothed map of the OACDF2 with a S/N >10 (circles). The double circle indicates the centre of the over-density corresponding to the cluster candidate described in the text. The image covers the entire OACDF2 field.
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Figure: R-band detail of the region marked with the double circle in Figure 16. The small circles indicate the galaxies detected in our matched catalogue. The image covers a field of ?? x ?? square arc-min. The North is up and East to the left. verificare area sull'immagine
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It is well known that early-type galaxies are preferentially located in the cluster core and that they present a small scatter in the colour-magnitude diagram ([Dressler 1980]; [Dressler & Gunn 1992]; [Stanford et al. 1998]). These properties turn the colour-magnitude diagram into a powerful tool to distinguish true clusters from over-densities of galaxies caused by random object alignment along the line of sight.

In order to identify the members of the candidate cluster n.1 shown in Figure 15, the colour-magnitude diagram is used (cf. Figure 16): we first obtain the colour-magnitude diagram for the cluster region (upper panel in Figure 16) using an aperture proportional to the isophotal area (cf. Figure 14), and a comparison field (middle panel in Figure 16), in an annular region centred on the cluster, but with a much larger radius. In order to enhance the early-type sequence of the candidate cluster we then perform a statistical subtraction of the background contribution in the colour-magnitude diagram. The resulting colour-magnitude diagram is shown in the bottom panel of Figure 16.

Figure: Colour magnitude diagram, V-R vs R, in the region coincident with the cluster. The sequence of the early-type galaxies is evident. rivedere questa immagine con il nuovo catalogo estratto dopo le calibrazioni
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The galaxy cluster n.1 turns out to have a photometric redshift of $\simeq 0.2$ (estimated from the V-R colour of the early-type sequence; [Fukugita et al. (1995)]) and an estimated richness (at rest) of $\simeq 50$.

A population synthesis code, like the one by Bruzual and Charlot (2000), can be used to determine the evolution of magnitudes and colours of stellar populations using different parameters like initial mass function (IMF) and star formation rate (SFR). Adopting a cosmological model (H0=50 km/s/Mpc & q0=0.05) and accounting for the K and evolutionary corrections, we determined colours and magnitudes as function of redshift. Our ``standard model'' was calibrated using the colours of ellipticals in the nearby Universe, and the colour behaviour as function of redshifts (Kristian et al. 1978). This model reproduces quite well the behaviour of the B-V and V-R for Es and S0s in clusters as function of redshift. The estimated redshift for the candidate cluster n.1 is roughly 1.2


next up previous
Next: Conclusions Up: Selection of galaxy cluster Previous: Masking and star/galaxy separation
Juan Alcala
2002-02-05