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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 . 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
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 (
) and assuming fixed emission line
strengths relative to Ly
. Three different values of the
spectral index
were used, and three
different values for the emission line strength, defined by the
Ly
rest-frame equivalent width, EW(Ly
, 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 ; in
Cols. (6) and (7) the (B-V) color and
its error estimate
; in
Cols. (8) and (9) the
(V-I) color and its error estimate
; 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.
One of the interesting regions of the color-color diagram is the
region redder than (region I). Objects in this region
extend well beyond the track defined by main-sequence stars with
masses greater than
. Therefore, this region should be
populated primarily by very low mass stars (
)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 (
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
, all brighter than I=20 (listed in Table 3);
and 14 objects with
, 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,
, 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
,
,
and
,
,
. 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 .These blue objects could be either relatively hot (young) disk white
dwarfs or blue horizontal branch (HB), low-metallicity halo
stars. However, for
HB stars would be located at
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.
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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) |
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
and B-dropouts
candidates, respectively.
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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 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
(
) 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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