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Up: The ESO-Sculptor faint

9. First results

9.1. Star sample

Byproducts of deep imaging surveys at different galactic latitudes are stellar samples in different directions of our galaxy at very faint magnitudes (Shanks et al. 1980; Kron 1980; Infante 1986; Metcalfe et al. 1991). In Figs.  10 (click here)11 (click here), we show for our stellar sample the different colour histograms B-R, B-V, for three magnitude ranges in V. Note that the quasars are included in this sample. Also, at bright magnitudes (V < 17), a large fraction of stars are saturated, thus raising the uncertainty in the corresponding colours.
At faint magnitudes (tex2html_wrap_inline3019), two stellar populations can be seen. Brightward of V < 18, one broad blue peak is present near tex2html_wrap_inline3023. Faintward of this magnitude a second peak appears in the red part near tex2html_wrap_inline3025. These results agree well with the other existing data (see Table 9.1 (click here) above). The red peak is interpreted as being nearby M stars belonging to the disk population. The blue sequence is interpreted as being stars belonging to the galactic halo (Robin & Crézé 1986).
These observations are listed in Table 9.1 (click here) for tex2html_wrap_inline3027. The second column gives the colours used by the author, the third and fourth columns show the colours of the two peaks in this initial colour and the subsequent columns show the transformed position of the two peaks into the standard system Johnson-Cousins. The transformation equations are given in the corresponding publications, and for the Kron (1980) data we added the transformation equations given by Majewski (1992) as (J-F)=0.738(B-R) -0.02. Table 9.1 (click here) shows that our stellar color distributions are in good agreement with those resulting from other faint CCD surveys.

Table 4: Stellar colour peak for tex2html_wrap_inline3031

Figure 10: B-R colour histogram for the stars selected in three different ranges of V magnitude as specified within the graphs

Figure 11: same as Fig. 10 (click here) for B-V colour

9.2. Galaxy sample

9.2.1. Galaxy counts

a) Counts and slopes
We present the differential galaxy counts based on the total 0.4 square degree of the survey in the R, V and B bands to limiting magnitudes of 23.5, 24.0 and 24.5 respectively. These limiting magnitudes are defined as the last magnitude bin (tex2html_wrap_inline3069 mag) before the turn-off in the number counts. To these limits the galaxy catalogs contain about 13000, 12150 and 9500 objects in the R, V and B bands respectively. Note that there is no star-galaxy separation for tex2html_wrap_inline3077 and no correction for stellar contamination is done because the expected number of stars at this galactic latitude (tex2html_wrap_inline3079) is lower than 4% (corresponding to an offset of tex2html_wrap_inline3081 of the logarithmic number counts).
In Fig. 12 (click here), we show the differential galaxy number counts per square degree in 0.5 magnitude bins. The upper graphs show for each band the superimposed galaxy counts for all individual CCD fields. The dots displaced to the right of the columns of galaxy counts are the median counts offset by 0.2 mag for clarity. The error bars measure the 1tex2html_wrap_inline3083 rms field-to-field scatter. The field-to-field scatter is tex2html_wrap_inline3085 in the R band between 21 < R < 23.5, tex2html_wrap_inline3091 in the V band between 21 < V < 24. and tex2html_wrap_inline3097 in the B band between 22 < B < 24.5. These rms dispersions significantly exceed the expected Poisson variations in tex2html_wrap_inline3103 because of galaxy-galaxy clustering (Arnouts & de Lapparent 1997). In the bright part, the large values of the scatter are essentially due to the small number of bright galaxies per CCD frame. The lower graphs give the median number counts. In these graphs the error bars are given as:
The differential number counts (in degtex2html_wrap_inline3105 0.5 magtex2html_wrap_inline3107) are fitted by a power law in the same magnitude range as Metcalfe et al. (1991) for the R and B magnitudes. The exponent of the fit in the three bands are measured by a least squares fit in the tex2html_wrap_inline3113 plots. These fits are shown by solid lines in the lower graphs of Fig. 12 (click here) and are parameterized as follow:
for 20.5 tex2html_wrap_inline3115 24.5
for 20.0 tex2html_wrap_inline3117 24.0
for tex2html_wrap_inline3119.
In Table 9.2.1 (click here), we summarize the results of previous CCD surveys on galaxy counts in the visible bands. Our slopes are in good agreement with the other works.
In Fig. 12, we detect two magnitude bins between tex2html_wrap_inline3121 where the density systematically decreases and this effect is seen in the three bands (in the intervals tex2html_wrap_inline3123 and tex2html_wrap_inline3125). First of all, to see if this gap was caused by inhomogeneities in the projected distributions, we examined the angular distribution (RA, Dec) in different magnitude intervals, but no particular feature in the clustering of the projected distributions was visually detected. This investigation will be pursued quantitatively in a forthcoming study of the two-point angular correlation function for these data (Arnouts & de Lapparent 1997).

Figure 12: Galaxy number-magnitude counts by square degree per 0.5 mag interval in the B (left), V (center) and R (right) bands. The upper graph shows the counts for each observed CCD field and the median values with error bars given as the tex2html_wrap_inline3133 field-to-field fluctuation. The lower graph shows the median number count value in each bin. The error bars are given by Eq. (13). The solid line is the estimated slope by least squares fit. a) Galaxy counts in B band; b) Galaxy counts in V band; c) Galaxy counts in R band.

Table 5: The galaxy number count exponents for several CCD photometric surveys

b) Counts comparison
In Figs. 13 (click here)-15 (click here) we compare the differential number counts from our data with those from the other CCD surveys. For the data given in other systems than the standard Johnson-Cousins system, we apply the different transformations provided by the authors. For the data from Metcalfe we apply a transformation only for the B band as tex2html_wrap_inline3177.
For the data from Tyson, the transformation equations are not given. By default we use the transformations into the photographic system (tex2html_wrap_inline3179) given in Metcalfe et al. (1991) combined with the transformations from photographic bands to the standard system given by Shanks et al. (1984). We obtain the approximate transformations defined as tex2html_wrap_inline3181 and tex2html_wrap_inline3183.
For the data from Driver no transformations are done because the color terms are small. Except for Metcalfe et al. (1991), these other works are significantly deeper than our data, and the number counts at very faint magnitudes (tex2html_wrap_inline3185 and tex2html_wrap_inline3187) are corrected for confusion.
Figures  13 (click here)14 (click here)15 (click here) show that our counts in B and R are in good agreement with the results from Metcalfe et al. (1991) in both the slope and the amplitude of the logarithmic number counts in the red band but a small shift in the blue band exists as we will see below in the discussion of the colour distributions (Sect. 9.2.2). The R number counts of our survey are significantly higher (tex2html_wrap_inline3195) than those from Tyson in the common range of magnitudes. This difference has been interpreted by Tyson (1988) as being due to the a priori choice of fields devoid of bright galaxies, and Metcalfe et al. (1991) suggest that this difference can originate from the use of isophotal magnitudes by Tyson (1988) in contrast to the "total'' magnitudes used by the others authors. The data of Driver et al. (1994) also show a small deficit in galaxy number counts compared to ours at the tex2html_wrap_inline3197 level. Driver specifies that the Hitchhiker data suffers from a calibration uncertainty, so a small shift in zero-point could explain the deficit in the three visible bands but this effect does not alter the slopes of the counts. In the data from Smail et al. (1995), the plotted points correspond to the average of two single fields. Their V counts are in very good agreement with ours but their R counts show a significant number excess by a factor of about 1.2 compared with all the others authors below R<24.
Finally we compare our deep counts with the recent bright galaxy counts in B and R bands performed by Bertin & Dennefeld (1997). These counts are in good agreement with ours and we use them to normalize the non-evolving model kindly provided by M. Fioc. Bertin & Dennefeld (1997) suggest: tex2html_wrap_inline3209 in the B band (estimated at tex2html_wrap_inline3213), tex2html_wrap_inline3215 in R band and we adopt an intermediate value of tex2html_wrap_inline3219 in the V band (tex2html_wrap_inline3223 is defined by tex2html_wrap_inline3225). The other parameters of the luminosity function come from Guiderdoni & Rocca-Volmerange (1990): tex2html_wrap_inline3227 and tex2html_wrap_inline3229.

Figure 13: Comparison with others published galaxy number counts transformed into the B Johnson filter. The error bars for our data show the tex2html_wrap_inline3233 estimated in Eq. (13). The dashed line shows the differential number counts expected for a non evolving model using a tex2html_wrap_inline3235 normalized to the bright galaxy number counts from Bertin & Dennefeld (1997) as described in the text. a) Comparison with others deep CCD galaxy number counts. b) Comparison with bright photographic galaxy number counts from Bertin & Dennefeld (1997)

Figure 14: Same as Fig. 13 (click here) in the V Johnson filter

Figure 15: Same as Fig. 13 (click here) in the R Cousins filter. a) Comparison with others deep CCD galaxy number counts. b) Comparison with bright photographic galaxy number counts from Bertin & Dennefeld (1997)

9.2.2. Galaxy colours

In this section, we present the first results of the colour distributions for our galaxy catalogue in the B, V and R bands. As shown in Fig. 7 (click here), there is a large fraction of objects identified in all 3 filters for B< 24 (tex2html_wrap_inline3265 of objects). In the following, we restrict the sample to these 7500 common objects.
In Figs. 16-18 we plot the mean observed colours B-R, B-V, V-R as a function of magnitude for the 7500-object sample. The solid lines draw the 1tex2html_wrap_inline3273 envelope of the measured colours, tex2html_wrap_inline3275 being the rms dispersion of the colour histogram within the corresponding magnitude bin. This envelope is larger in B-R than in B-V and V-R because the expected colour in B-R varies in a large range for the different galaxy types. The B-R and B-V colours clearly show a tendency to become bluer at fainter magnitude as first observed by Kron (1980) and subsequently confirmed by several groups (Tyson 1988; Metcalfe et al. 1991, 1995).
At tex2html_wrap_inline3289, the typical mean colour is tex2html_wrap_inline3291 and a blueing shift to tex2html_wrap_inline3293 is seen between 22 < V < 24. The same tendency is visible in the B-V colour distribution. It decreases from tex2html_wrap_inline3299 at V < 22 to tex2html_wrap_inline3303 at V = 24. Note that in Fig. 16 (click here) , at B > 24 mag, the completeness level drops to tex2html_wrap_inline3309 (see Fig. 7 (click here)) and the mean colours (B-R) and (B-V) become redder due to the incompleteness in the R and V bands where only the brighter objects are identified and contribute to shift the colour toward redder colours.
In contrast, the (V-R) colours in Figs. 16-18 show no evidence of colour evolution with magnitude up to tex2html_wrap_inline3321. The same stability is obtained by Driver et al. (1994).
In addition, we compare our observed mean colours B-R with those from Metcalfe et al. (1991, 1995) and our mean V-R colours with those of Smail et al. (1995). Metcalfe et al's B-R colours are systematically 0.1 mag redder than ours. Half of this shift is expected due to reddening in the fields of Metcalfe et al. (Metcalfe 1995). Comparison with the V-R colours of Smail also show a small offset, but the restricted overlap in the magnitude ranges covered does not allow us to draw any firm conclusions about the agreement between the data sets.

Figure 16: Galaxy mean colours as a function of 0.5 bin of B magnitude. The solid lines represent the 1tex2html_wrap_inline3333 envelope of the measured colours. The upper left graph shows the mean B-R from our data (filled circles) and from the data of Metcalfe et al. (1991, 1995) (crosses). The error bars give the quadratic errors in the magnitudes obtained from the simulations

Figure 17: same as Fig. 16 (click here) for V magnitude

Figure 18: same as Fig. 16 (click here) for R magnitude. The upper left graph shows the mean B-R from our data (filled circles) and from the data of Metcalfe et al. (1991, 1995) (crosses). The lower graph compares the mean V-R from our data (filled circles) with those from Smail et al. (1995) (crosses)

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