2 Field selection

A critical aspect for a deep field study is the selection of a suitable sky area. Since we intended to obtain a representative deep cosmological probe of the Universe, one condition was that the galaxy number counts were not disturbed by a galaxy cluster in the field. To go as deep as possible also requires low galactic extinction ( E(B-V) < 0.02 mag). For the same reason, the field had to be devoid of strong radio or X-ray sources (potentially indicating the presence of galaxy clusters at medium redshifts). On the other hand, we decided to include a high-redshift (z > 3) radio-quiet QSO to study the IGM along the line-of-sight to the QSO and the QSO environment. To facilitate the observations in other wavebands, low HI column density (< $2 \times 10 ^{20}~ {\rm cm}^{-2}$) and low FIR cirrus emission was required. Moreover, stars brighter than 18th mag had to be absent to allow reasonably long exposures, to avoid saturation of the CCD and to minimize readout time losses. Because of the latter conditions, the HDF-S region was not suitable for our study (see Fig. 1). Additionally, stars brighter than 5th mag within $5\hbox{$^\circ$ }$ of the field had to be absent to avoid possible reflexes and stray-light from the telescope structure. Finally, the field had to have a good observability and, therefore, had to pass close to the zenith at the VLT site.

 

 

Table 1: Characteristics of the FORS Deep Field.

Field center	$1^{\rm h}6^{\rm m}3\hbox{$.\!\!^{\rm s}$ }6\ \ -25\hbox{$^\circ$ }45\hbox{$^\prime$ }46\hbox{$^{\prime\prime}$ }$ (2000)
mean E(B-V)	0.018
H I column density	$1.92 \times 10^{20}~ {\rm cm}^{-2}$
Radio sources (NVSS)	none with flux > 2.5 mJy
${\rm IRAS Cirrus}~ (100~ \mu{\rm m})$	0.035 Jy
Bright stars (<5 mag)	none within $5\hbox{$^\circ$ }$

 

 

Table 2: Observing log of the FDF observations.

Tel./Inst.	Dates	Filters	Comments
FORS1/UT1	Aug. 13-17 1999	g, R	mostly non-phot.
FORS1/UT1	Oct. 6-13 1999	U, B, g, R, I	during 3 nights
FORS1/UT1	Nov. 3-6 1999	U, B, R, I	3 $\times$ 0.5 nights
FORS1/UT1	Dec. 2-6 1999	U, B, R, I	4 $\times$ 0.3 nights
FORS1/UT1	July/Aug. 2000	B, I	3.5 hours each
SofI/NTT	Oct. 25-28 1999	J, Ks

Due to these constraints, the south galactic pole region was searched for a suitable field. We started by selecting all the QSOs from the catalog of Véron-Cetty & Véron (7th edition, 1997) with z > 3 within $10^{\circ}$ of the south galactic pole. This resulted in 32 possible field candidates. Next we did an extensive search in the literature from radio up to the X-ray regime (FIRST, IRAS maps, RASS etc.), checked visually the digitized sky survey and used the photometry provided by the COSMOS scans to select 4 promising field candidates containing a z > 3 QSO. For these 4 field candidates short test observations were carried out during the commissioning phase of FORS1, which showed that 3 of them were not useful (they either contained conspicuous galaxy clusters or, in one case, did not provide suitable guide stars for the active optics of the VLT). Finally, a field with the center coordinates $\alpha_{2000} = 1^{\rm h} 6^{\rm m} 3\hbox{$.\!\!^{\rm s}$ }6, \delta_{2000} = -25\hbox{$^\circ$ }45\hbox{$^\prime$ }46\hbox{$^{\prime\prime}$ }$ containing the QSO Q 0103-260 (z = 3.36, Warren et al. 1991) was chosen as the FDF. The characteristics of this field are summarized in Table 1. The Digital Sky Survey (DSS) prints in Fig. 1 provides a comparison of the FDF and the HDF-S, showing the great advantage of the FDF in relation to the HDF-S concerning the presence of bright stars.