4 Galaxy counts and colours

   
4.1 $\mathsfsl{I_{AB}}-$ and $\mathsfsl{B_{AB}}-$selected galaxy counts

In Figs. 9 and 10 we present I- and B-band galaxy counts as a function of AB magnitude derived from the CFDF survey (open thick circles) compared with the compilation of Metcalfe et al. (2000) (small points). For the I-band we have four independent fields, or approximately $\sim$10⁵ galaxies; for the B-band there are currently three fields with a total of $\sim$ $0.75\times 10^5$ sources. In both these filters we find an excellent agreement with the literature compilations extending over almost six orders of magnitude.

We find the fitted slope, $\alpha$, for 20 < I_AB < 24 to be $0.35\pm0.02$, which agrees quite well with the value quoted in Metcalfe et al. (2000) of 0.33 for the slightly deeper limits of $21 < I_{\rm kc} < 25$. Note that all our I-data, from $I_{AB}\sim18$ to $I_{AB}\sim24$(beyond which the effects of incompleteness begin to be important) is consistent with a constant slope in the number-magnitude relation.

We also compare our counts to those of Postman et al. (1998) (filled squares). This dataset is a $16\deg^2$ survey comprising 256separate exposures of 900 s each. This dataset is $\sim$20% below the CFDF faintwards of $I_{AB} \sim 22$ and remains so until the effects of incompleteness begin to dominate the CFDF counts. Furthermore, in our source counts we see no evidence of the inflection seen at $I_{AB} \sim 22$ in the Postman et al. counts.

For the B-counts we find a fitted slope in the range 20 < B_AB < 24 of $0.47 \pm 0.02$, consistent with the value of 0.50 quoted in Brunner et al. (1999).

Figure 9: IAB-band galaxy counts for the four fields in the CFDF survey (open circles). The dashed line shows a least-squares fit to the bright data ( 22<IAB<24). The error-bar on each point corresponds to the field-to-field variance. The filled squares show galaxy counts from Postman et al. (1998); the small points are from the compilation of Metcalfe et al. (2000).

Figure 10: As in Fig. 9 but for the BAB selected galaxy counts.

4.2 Galaxy colours

In Fig. 11 we show (V-I)_AB colours measured as a function of I_AB total magnitude for galaxies in the CFDF 11 hr field (for clarity only 1/8 of all galaxies have been shown). The dashed line represents the colour incompleteness computed using the limiting magnitudes given in Table 1. Our median (V-I)_AB colours in half magnitude slices (filled circles) shows good agreement with colours derived from the NTT deep field (Arnouts et al. 1999b), shown as open squares, and this agreement continues to the completeness limit of our data.

 

Figure 11: Median (V-I)AB galaxy colour, measured as a function of IAB total magnitude, for galaxies in the CFDF-11 hr field (for clarity only 1/8 of all galaxies are shown). The filled circles show the median galaxy colour in half magnitude intervals; the open squares show the median (V-I)AB colours measured in the NTT deep field (Arnouts et al. 1999b). The dashed line shows the nominal colour incompleteness, based on the detection limits given in Table 1.