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Astron. Astrophys. 341, 641-652 (1999)

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6. Discussion and conclusions

We have presented the first results of a deep multi-colour (B, V, r, I) field observed at ESO with the NTT telescope. A full description of the data reduction and analysis has been given. The final images and catalogue are available on the WEB site of ESO.

A comparative study with a WFPC2-HST image of the same field allows us to estimate the image quality of the NTTDF. Despite the broadening of the stellar profiles due to the effect of the seeing, the comparison of the the star/galaxy classification of the NTTDF objects and the one of the WFPC2 shows good agreement. Reliable star/galaxy separation is obtained down to I[FORMULA]24 where the star contamination of the galaxy sample is [FORMULA]1.5%. The comparison of the stellar content of the NTTDF with the one of the HDF indicates that the star density in the NTTDF is [FORMULA] times higher, and of the 26 stars in the magnitude range [FORMULA], 6 are low mass stars or possible brown dwarfs with a notable object having (V-I)[FORMULA].

We have shown that the galaxy number counts at faint magnitudes ([FORMULA]) converge to a slope [FORMULA] in all bands. The flattening in the blue band removes the divergence in the extragalactic background light (EBL) and shows that the dominant galaxies of the EBL have B[FORMULA]25 (Madau 1997). As shown by Guhathakurta et al. (1990) the contribution of high redshift galaxies ([FORMULA]) is not more than 10% of the total number counts at [FORMULA]25. In our sample the contribution to the number counts of star forming galaxies at [FORMULA]4 is [FORMULA] 5% for 24[FORMULA] 25 and [FORMULA] 3% for 25[FORMULA]26. If the flattening observed in B is due to the reddening (combination IGM + Lyman break) of galaxies at z[FORMULA], it cannot explain the similar flattening in the V band. Thus, the high redshift star forming galaxies are not a dominant population to explain the excess of faint sources.

At faint magnitudes, the number counts can be dominated by the faint-end luminosity function (LF) at increasing redshift (with a decreasing contribution of the bright galaxies - [FORMULA] - beyond z[FORMULA] 0.6-1), thus the observed slope implies that the slope of the faint-end LF is as steep as [FORMULA] (by assuming a Schechter function for the LF gives [FORMULA]). Such a steep slope is supported by HST observations (Driver et al., 1995) where a similar value is obtained for the late-type galaxies which are the dominant population responsible for the number excess. This evidence is supported by the rapid evolution of the faint-end luminosity function for bluer samples observed in the deep spectroscopic surveys (Lilly et al.,1996, Ellis et al., 1996) up to z[FORMULA].

The median color evolution shows a rapidly blueing with increasing magnitude (up to [FORMULA]). From [FORMULA] to [FORMULA]25.5, our data suggest that the blueing trend is reduced, in agreement with Smail et al. (1995), and stops beyond [FORMULA]. Analyzing colors as a function of the I magnitude, a similar effect is observed beyond I[FORMULA] 25. The stabilization of the blueing trends observed in the data is consistent with the convergence of the slope in the galaxy number-counts (assuming that we observe the same population in different bands).

Using the approach described by Giallongo et al. (1998), we have selected a sample of high-z galaxies in the range [FORMULA]. The derived surface density in our field is 0.8[FORMULA] arcmin-2 at [FORMULA] and 2.0 [FORMULA]0.6 arcmin-2 at [FORMULA]. This sample requires spectroscopic follow-up on 8-m class telescopes (for example the VLT with the FORS instrument). We have derived a lower limit for the star formation rate per unit comoving volume at [FORMULA] for galaxies with [FORMULA] of [FORMULA] for [FORMULA] ([FORMULA]). Our estimation should be regarded as a lower limit due to the selection technique which is biased against Lyman [FORMULA] emission line galaxies (Hu et al. 1998) and dusty galaxies undetectable with optical surveys (Cimatti et al., 1998).

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998