The VPM search is based on indices for star-like image structure, positional stationarity, overall variability, and long-term variability measured on 57 B plates taken with the Tautenburg Schmidt plates between 1964 and 1994, i.e. with a time-baseline of three decades. The B magnitudes given in the present paper are mean magnitudes from this database. The strategy for the QSO candidate selection of the VPM search in the M 3 field and the definitions of the indices are outlined at length in Paper I. Here we review only the basic ideas and describe the modifications in the selection procedures.
The proper motion index,
,
is expressed simply by the measured proper motion in units
of the proper motion error. The overall variability index,
,
is assessed by the deviation of the individual magnitudes about the mean
magnitude, and is normalised by the average magnitude scatter
for star-like objects in the same magnitude range. Finally, an
index for long-term variability,
,
is defined
by means of structure function analysis and is computed for all
star-like objects with
(see below) and B<20.
The selection thresholds for the indices were derived
from the statistics of the previously known QSOs in the field.
There are 90 such QSOs identified with star-like objects measured on
at least 7 of our B plates.
We found that a good compromise between the success rate
(i.e., the fraction of candidates that turn out to be QSOs)
and the completeness (i.e., the fraction of all QSOs found by
the survey) is achieved for the following set of constraints:
,
and
.
(In Paper I, the long-term variability
index was denoted RS100.) The pre-estimated values
for the success rate and the completeness are 90% and 40%,
respectively, for a limiting magnitude
.
(Note that the limiting magnitudes of the individual B plates
vary from 19.5 to 21.3.)
For stationary objects, the probability
to measure a
non-zero proper motion follows a Weibull-distribution and
depends only on the proper motion index (Brunzendorf & Meusinger
2001). An object with
has a probability
of
for non-zero proper motion.
As illustrated by Fig. 1, the proper motion
selection is in particular efficient for brighter magnitudes
where the proper motion errors are smaller. For B < 18.5,
the typical proper motion error is about 1 mas yr-1, and
83% of the star-like objects have
.
For 19<B<20, on the other hand, the typical proper motion
error is about 3 mas yr-1, and only about 21% of the star-like
objects have
.
In a flux-limited sample, most of the objects have magnitudes
close by the limit. Hence, the proper motion selection appears
not very efficient for the whole survey with a limit at
.
However, at brighter magnitudes, the number-magnitude relation
for QSOs is much steeper than for the foreground stars.
This means that the contamination of the variability-selected
QSO candidate sample by foreground stars is stronger at
brighter magnitudes where the proper motion selection works
more efficiently. At fainter magnitudes,
the zero proper motion constraint is important in particular
for the efficient rejection of nearby variable late-type
main sequence stars (see Paper I).
The selection starts with 32 700 objects detected on a deep master
plate. About 24 600 objects were identified on at least two further plates,
among them are about 21 500 objects with star-like images.
A basic object sample for the variability selection is defined
by the 12 800 star-like objects measured on at least 7 B plates.
After excluding
the objects in the crowded cluster region (distance to the centre
of M 3 less than 24'), this sample is reduced to 8582 objects
in total and to 4614 objects in the magnitude range
,
respectively. About 65% of the objects from this reduced sample
are rejected due to the zero proper motion constraint
.
Finally, the variability constraints strongly reduce the candidate
sample to a manageable size.
priority class | high | medium | low |
number of candidates | 80 | 95 | 607 |
already catalogued | 26 | 17 | 15 |
newly observed | 54 | 68 | 27 |
QSOs/Sey1s | 75 | 36 | 20 |
NELGs | 2 | - | 1 |
stars | 3 | 49 | 21 |
For practical reasons, the candidate sample is devided into
three subsamples of different priority. A similar approach was
used for the VPM search in the M 92 field
(cf. Brunzendorf & Meusinger 2001). However, the
variability indices defined there are slightly different from
those used in the present study, and the priority classification
in the two VPM fields are not completely identical.
Here, the priority depends mainly on the variability indices.
In addition, the B magnitudes and the crowding of the
field (expressed by the distance
from the centre
of M 3) are taken into account. For all priority classes,
star-like objects are considered with
,
and
arcmin. The high-priority subsample
consists of the strongly variable objects
with
.
The medium-priority subsample contains the objects with
smaller variability indices
and
.
In addition, we included the few objects
with somewhat higher variabiliy indices (
and
)
in the stronger crowded region
arcmin.
Finally, the low-priority subsample comprises
the objects having only one
of the two variability indices above the threshold
(i.e.,
or
).
In addition, we consider also objects with
and
arcmin as low-priority
candidates if at least one of the two variability indices
exceeds the threshold.
The variability selection is illustrated by Fig. 2.
As discussed in Paper I, the U band variability index may serve as an additional selection criterion. In practice, however, the fainter objects are measured on only a small number of U plates. Therefore, the U variability index was invoked only in one case: the QSO No. 51 from Table 3 has insignificant B variability indices but shows significant variability in the U band.
The numbers of selected candidates are 80, 95, and 607, respectively, for the subsamples of high, medium, or low priority (Table 1). It is expected that the fraction of QSOs/Sey1s strongly decreases with decreasing priority. In particular, the low-priority subsample is expected to be strongly contaminated by galactic stars with relatively enhanced photometric errors.
A cross-check of the candidate list against the
NED (2002, February)
yields the identification (identification radius 10 arcsec)
of 57 QSOs/Sey1s and one narrow emission line galaxie (NELG)
with catalogued redshifts.
Over the whole magnitude range, 104 objects with catalogued
redshifts z>0 were identified (100 QSOs/Sey1s, 4 NELGs).
The overwhelming fraction of the QSOs/Sey1s are from the CFHT
blue grens survey (e.g., Crampton et al. 1990)
which covers approximately half of our survey field.
Copyright ESO 2002