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Subsections

3 Target selection

 
  
\begin{figure}
\includegraphics [width=7.6cm]{ds7976f2.eps}
 

\includegraphics [width=7.6cm]{ds7976f03.eps}\end{figure} Figure 2: Color-color diagram for EIS point-sources. Left-panel: those detected in all three passbands. Right-panel: those detected in V and I but not B, for which the lower limit in (B-V) is indicated
Figure 2 left panel shows the color-color diagrams for the 3233 point sources detected simultaneously in the B, V and I band with $(S/N)_I\gtrsim 5$;Fig. 2 right panel shows the 345 objects only detected in V and I and not in B-band with $(S/N)_I\gtrsim 5$.The plots include all objects brighter than I=23. For non-detections in B (hereafter B-dropouts) an estimate of the B limiting magnitude has been measured on the best seeing frame. The limiting magnitude is defined to be a 1$\sigma$ detection within the area corresponding to the seeing-disk as measured in the I-band. From this an estimate of the lower limit on the (B-V) color is calculated. In addition to the B-dropouts, there are objects only detected in the I-band, for which the lower limit in (V-I) is similarly computed. While a large number of these objects is expected if one considers the sample as a whole (because of the relative bright limiting magnitudes of the V images), objects brighter than $I\sim
21$ are the most interesting and are the ones considered in more detail below.
  
\begin{figure}
\includegraphics [width=8.8cm]{ds7976f04.eps}\end{figure} Figure 3: Theoretical color-color plot for different type of objects. The solid line shows the location of main sequence, sub- and red-giant branch stars of an old halo, low metallicity, stellar population model taken from Bertelli et al. (1994). The dotted line shows the location of main sequence, sub- and red-giant branch stars of a young disk, solar metallicity, stellar population model taken from Bertelli et al. (1994). The short-dashed line shows the location of a WD pure Hydrogen cooling sequence taken from Bergeron et al (1995). The long-dashed line shows the location of 5 Gyr old BD stars with solar metallicity, taken from Baraffe et al. (1998). The color track for QSOs at different redshifts (3.05<z<5.00) are shown by triangles while the dots indicate the typical scatter around the median for the different parameters of the spectral properties and absorbers of high-redshift quasars (see text). Also shown (stars) are the EIS colors of the known quasars present in the EIS catalog which have redshifts in the range 0.4<z<2.96

For comparison with the previous figures, Fig. 3 shows the locus of main sequence, giants, white dwarf and brown dwarf stars. The stellar locus for main sequence, subgiant- and red-giant branch stars typical of the old low-metallicity halo and the young solar-type metallicity disk was taken from the models of Bertelli et al. (1994) extending down to 0.6 $M_{\odot}$. The color-color cooling sequence for pure-Hydrogen WD was taken from Bergeron et al. (1995). Finally, the locus for very low mass stars and/or brown dwarfs down to 0.08 $M_{\odot}$ is taken from the models of Baraffe et al. (1998). These curves are presented in the Jonhson-Cousins system, close to the EIS magnitude system except for the B-band (Paper III). However, the differences are relatively small and have no significant impact on the adopted selection criteria described below.

Also shown in Fig. 3 is the track of quasars in the color-color diagram as a function of redshift and the typical color scatter along the sequence due to the different assumptions for their typical spectra and intervening absorption. QSO colors were simulated using synthetic QSO spectra, which cover a range of intrinsic spectral properties, and the response functions of the EIS filters (Paper I). The method is the same as that used by Warren et al. (1994) and Hall et al. (1996), and is a modified version of the method of Warren et al. (1991). QSO spectra were synthesised assuming that the QSO continuum has the form of a single power law with spectral index $\alpha$(${S}(\nu)\propto\nu^{\alpha}$) and assuming fixed emission line strengths relative to Ly$\alpha +NV$. Three different values of the spectral index $\alpha=(-0.25, -0.75, -1.25)$ were used, and three different values for the emission line strength, defined by the Ly$\alpha +NV$ rest-frame equivalent width, EW(Ly$\alpha+NV)=(42$, 84 and 168 Å). For each set of assumptions, spectra were generated at intervals of 0.1 in z over the range (3.0 < z < 5.0). Absorption by intervening HI was taken into account by simulating absorption spectra, following the method of Warren et al. (1994) and based on the work of Møller & Jacobsen (1990). For each set of intrinsic properties, ten QSO spectra were generated at each z step, each using a different realization of the absorption spectrum appropriate for that redshift. Thus at each redshift a total of 90 spectra were generated. Because patch B is close to the South Galactic Pole galactic extinction was neglected in the present calculation. Figure 3 shows the median and the scatter corresponding to the various simulations as a function of redshift.

In addition, in Fig. 3 all the 19 known quasars present in the field are shown in their measured EIS magnitudes. These quasars have redshifts, taken from the literature, in the range 0.4<z<2.96.

Comparison of the color-color diagram for the data and model predictions shows at least four regions of potential interest. These regions are schematically shown in Fig. 3 and their limits are given in Table 1. Objects in region I are candidate very low mass stars (VLM) or brown dwarf stars (BD), those in region II are candidate white dwarfs (WD). Candidate quasars (QSO) at different redshifts should lie in regions III and IV. Below preliminary lists for these objects are presented in tables which give: in Col. (1) the object name; in Cols. (2) and (3) the J2000 coordinates; in Cols. (4) and (5) the I magnitude and its error estimate $\varepsilon_{I}$; in Cols. (6) and (7) the (B-V) color and its error estimate $\varepsilon_{(B-V)}$; in Cols. (8) and (9) the (V-I) color and its error estimate $\varepsilon_{(V-I)}$; and in Col. (10) notes or comments on the individual objects, whenever necessary. In the cases where the (B-V) and/or (V-I) colors are lower limits, the measure is preceded by a > sign and the error in the color is the error in the magnitude in the passband in which the object is detected. For objects not detected in two passbands the error in the color is set to zero in the tables.


  
Table 1: Definition of regions of interest in Fig. 3 for candidate objects

\begin{tabular}
{lll}
\hline \hline\noalign{\smallskip}
Region & Cand. Objects &...
 ...e and $-0.25<(B-V)<0.25$\space \\ \noalign{\smallskip}\hline \hline\end{tabular}

3.1 Rare stellar-type candidates

One of the interesting regions of the color-color diagram is the region redder than $(V-I) \geq 3.5$ (region I). Objects in this region extend well beyond the track defined by main-sequence stars with masses greater than $0.6\,M_{\odot}$. Therefore, this region should be populated primarily by very low mass stars ($0.6 \gt M/M_{\odot} \gt 0.1$)in the disk and/or brown dwarfs. Another possibility is that they are asymptotic giant and red giant branch stars. However, this is unlikely because there should be few of them in this color and magnitude range since they would have to be high metallicity objects at very large distances from the Sun ($\sim\! 100$ kpc). Even though unlikely, considering the size of the area covered by the EIS multicolor data, this region of the color space could also be populated by very high-redshift QSOs with very large (B-V), which could appear as B non-detections. In this region there are 18 detections (listed in Table 2; 22 B-dropouts with $(V-I)
\ge 3.5$, all brighter than I=20 (listed in Table 3); and 14 objects with $I~\rlap{$<$}{\lower 1.0ex\hbox{$\sim$}}\,21$, which are only detected in the I-band (listed in Table 4). In the tables with "rare'' stellar objects (2, 3 and 4), the following naming convention has been adopted: VLM, for very low mass candidates, VLMB, for very low mass B-dropouts, and VLMI, for the objects only detected in the I-band.

Since extreme colors could be caused by some unexpected artifact all these cases have been visually inspected, and all seem to be legitimate candidates. Note, however, that in the course of the inspection the two brightest objects in this sample exhibited a strange morphology in the coadded image appearing to be a "double'' star, with the two objects having almost exactly the same magnitude, $I=17.46 \pm0.01$, and a few arcsecs of separation. This prompted the examination of the two single frames, which showed a single slightly elongated object that occupies different positions in the two single exposure images. The object was observed at $\alpha= 00^{\rm h} 49^{\rm m}
37\hbox{$.\!\!^{\rm s}$}71$, $\delta= -29^\circ 50' 58\hbox{$.\!\!^{\prime\prime}$}7$, $\rm JD=50696.3174202$ and $\alpha= 00^{\rm h} 49^{\rm m} 37\hbox{$.\!\!^{\rm s}$}76$, $\delta= -29^\circ 50' 56\hbox{$.\!\!^{\prime\prime}$}7$,${\rm JD}=50696.32054438$. This fact strongly suggests that this object is probably a relatively fast moving asteroid. However, no known asteroids were found to be at the observed position during the nights the observations were conducted. This example of a serendipitous source demonstrates the need to implement tools in the EIS pipeline to search for transient phenomena present in the survey such as high proper-motion objects, variables, supernovae.

Another potentially interesting population is that defined by objects in region II of Fig. 3. These objects are clearly visible in Fig. 1 at magnitudes $V \gtrsim 19.5$.These blue objects could be either relatively hot (young) disk white dwarfs or blue horizontal branch (HB), low-metallicity halo stars. However, for $V \gtrsim 20$ HB stars would be located at $\gtrsim 100$ kpc, where the density should be extremely small for standard galactic structure models. There are 32 objects in region II which are listed in Table 5. The adopted cut-off in (V-I) (see Table 1) was chosen based on cooling sequence of disk white dwarfs (Bergeron et al. 1995) shown in Fig. 3. We emphasize that the criterion adopted is somewhat arbitrary and it is used simply to illustrate the possible identification of these objects. As can be seen from Fig. 3, this sample can be contaminated by low redshift quasars. In fact Table 5 contains 2 already known quasar which are identified (name and redshift from the Simbad database). The U-band data will be useful to sort out these cases.

Finally, Fig. 4 shows the spatial distribution of these various candidates. Note that the northeast edge of the patch has been removed because of the incompleteness of the B-band catalogs. Similarly, a region along the southern edge was removed because of the incompleteness in the I-band catalog. A small trimming of the whole region has also been done yielding a total area of 1.27 square degrees.

  
\begin{figure}
\includegraphics [width=8.8cm]{ds7976f05.eps}\end{figure} Figure 4: Projected distribution of star-like objects which shows: all stellar objects detected in the selected area of patch B (dots); low-mass candidates found in region I of the color-color diagram of Fig. 3 (filled circles); and WD candidates in region II (filled triangles)

3.2 Quasar candidates

From simulations of QSO tracks (Fig. 3) high redshift QSOs (3<z<5) can be found in region III of the color-color diagram, while the available sample of known low redshift QSOs populate region IV (see Fig. 3, Osmer et al. 1998). The rough criteria used to define region III (Table 1) were chosen based on the simulated QSO track. The blue part was chosen to be parallel to the stellar locus but shifted to minimize the contamination by stars. Several improvements in the selection can be made to take into account the errors in colors, as a function of the magnitude, and to optimize the yield based on the expected density of objects of different types. Since the parent sample is public, interested groups are likely to make significant refinements to the selection criteria adopted here.

In region III there are 70 objects detected in all three passbands. These are listed in Table 6. In addition, there are 126 objects that are detected in V and I but not detected in B (hence have lower limits in (B-V)) that could also lie in region IV. These objects are listed in Table 7. Note that, since the depth of the B images varies across the patch, the limits on (B-V) are more meaningful in some areas than others. The depth of the B frames corresponding to each object can be calculated from the V magnitude and the (B-V) limits given in Table 6. In the tables the following naming convention has been adopted: QSO and QSOB stand for objects in region III detected in all three bands $(\gtrsim 3.0)$ and B-dropouts candidates, respectively.

  
\begin{figure}
\begin{center}

\includegraphics [width=8.8cm]{ds7976f06.eps}\end{center}\end{figure} Figure 5: Projected distribution of quasar candidates at low (filled circles), intermediate and high redshift (filled triangles). The adopted selection criteria are discussed in the text

Adopting the criteria given in Table 1 for region IV, where QSOs with $z~\rlap{$<$}{\lower 1.0ex\hbox{$\sim$}}\,3$ are likely to be found, one finds 48 stellar objects which are listed in Table 8. This table includes 6 known QSOs, as indicated (name and redshift are from the Simbad database). In the table QLZ stands for low redshift ($z~\rlap{$<$}{\lower 1.0ex\hbox{$\sim$}}\,3.0$) quasars. Note, however, that with the follow-up observations in U-band to be carried out later this year, it will be possible to select low-z QSOs more efficiently.

Figure 5 show the projected sky distribution of the QSO candidates. This figure should be compared with those for the seeing and the limiting magnitudes presented in Paper III to investigate possible correlations between the QSO candidates and the quality of the data, especially the B-dropouts or those detected only in the I-band. At first glance there is no obvious correlation as the QSO candidates seem to be uniformly distributed over the surveyed area.


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