For most of the nebulae in Table 1 (click here) older velocity measurements are
given in Schneider et al. (1983). However, few of these are of
comparable accuracy, where the uncertainty assigned by Schneider et
al. appears in some cases to be an underestimate. A paper discussing
the accuracies of the different data sets is in preparation (Durand et
al. 1997). The problem is serious for the Galactic Bulge region where
most of the velocities derived from two samples: Mayall (1964)
and Minkowski (1957). Both are published only as private
communications in the catalogue of Perek & Kohoutek (1967) and
may originate from a variety of separate measurements with different
accuracies. The unknown uncertainties in these data sets are a significant
problem for any attempt to study the dynamics of the inner Galaxy using PN
velocities. There are only two other large sources of velocity data
with comparable accuracy to the present survey: Kohoutek & Pauls
(1981), and Campbell & Moore (1918), where the latter
authors only study bright nebulae which consequently are mostly foreground
objects. Values given by Campbell & Moore agree to within 
:
the only deviating object is 358.3-21.6, where the velocity
difference is 
. This object has however a very complicated
velocity field and the result is sensitive to the placement of the
slit.
To illustrate that indeed the present nebulae are sampling the Galactic bulge, both distances and velocity distribution have to be shown to be consistent with expected values. The longitudinal distribution of the full sample of Galactic PN shows a sharp peak towards l=0 (Acker et al. 1992) indicating that there is a large Bulge population among the known PN. However, since the present objects were selected partly based on brightness, further confirmation is required.
The distances were estimated from the surface-brightness-distance
relation proposed by Van de Steene & Zijlstra (1994, 1995):
for the sample they are listed in Table 1 (click here). (For the two PN which
appear to be extra-galactic the distance is instead that of the host galaxy.
For neither PN can the distance scale be used for lack of a radio flux
density and diameter, and they are not included in Fig. 1 (click here).) A very
similar distance scale using optical flux determinations was
independently derived by Schneider & Buckley (1996); their
scale may better fit the more compact nebulae which may be affected by
optical depth effects at radio wavelengths. Use of the radio flux is
in general preferred in regions of high extinction. For the present
purpose both scales give the same result. The formula of Van de
Steene & Zijlstra is:
where D is the distance in pc, 
the 6-cm radio flux
in mJy and 
the angular radius in arcseconds. The use of
radio data overcomes the problem of extinction correction. From a
calibration to nearby nebulae and likely bulge nebulae, an average
uncertainty of 30% in the calculated distance was derived for this
scale, which compares favourably with other distance scales, but is
still a large uncertainty which could introduce significant bias in
any attempt to derive a rotation curve. The radio flux density and
angular radius as listed in Acker et al. (1992) were used,
except where superseded by more recent data. Single-dish radio flux
measurements were not used. The distances are in most cases between 4
and 
. This indicates the presence of a Bulge population.
However, the distribution is skewed towards distances less than the
Bulge, with a median distance of about 
. This skewness is
absent in the distance scale used (see Van de Steene & Zijlstra
1995) and indicates the presence of a bias. This does not
necessarily imply a large fraction of non-bulge objects, since the Bulge has
a large depth and the brightness selection could have favoured objects
within the near part of the Bulge.
Figure 1: Heliocentric velocity versus Galactic longitude of
Galactic planetary nebulae. The top panel shows all PN at distances 
from the Sun which are possible Bulge members. The bottom
panel shows PN nearer the Sun. The continuous lines indicate expected
radial velocities based on Galactic (disk) rotation at distances of
6. and 7.9 kpc (top panel), and 2 and 6 kpc (bottom panel). In the
bottom panel, the open symbol indicates the single object in the sample at a
distance of less than 2 kpc
The velocity distribution as function of longitude and derived
distance is shown in Fig. 1 (click here). In the top panel are shown all
objects with an estimated distance from the Sun. Indicated are
curves representing velocities expected from pure circular rotation
with 
km/s, for distances from the Sun of 6 and
. The distance to the Galactic Centre is taken as 8 kpc. The
velocity distribution is consistent with the objects being near the
Galactic Centre with significant dispersion of 
. The
distribution of the data points does not agree with disk rotation and
instead is dominated by the velocity dispersion. In contrast, PN with
estimated distance of less than 
, shown in the bottom panel
of Fig. 1 (click here), show a velocity-longitude distribution much more
consistent with the general Galactic rotation. The velocity
dispersion of all PN in the sample with distance of less than
is 
. Note that we cannot rule out that some of the nebulae in
Fig. 1 (click here)a are dynamically part of the inner disk rather than the
bulge. However, the derived velocity dispersion agrees well with other
determinations for the Bulge (e.g. Minniti 1996), which
indicates that sample confusion is relatively small.
The inner Galaxy contains, in addition to the Bulge and inner disk, a
stellar population belonging to the metal-poor halo (Minniti
1996). It is possible that some of the PN of Table 1 (click here) belong
to this population. The abundance distribution of Bulge nebulae shows a
tail of objects with 
(Ratag et al.
1992). These are expected to have a 
, based on models
of chemical evolution (e.g. Matteucci & Brocato 1990), and to
fall in the range of metallicities found for the halo component in the Bulge
region (Minniti 1996). However, it is known that the birth
rate for PN within the halo population is up to a factor of 10 lower than in
more metal-rich (younger?) populations (e.g. Zijlstra & Walsh
1996; Jacoby 1996).
Thus, even in the presence of a
significant halo population within the Bulge region, few PN would be
expected to belong to this.
We conclude that the nebulae in Table 1 (click here) with estimated distance
are mostly members of the Bulge. However, it is likely
that preferentially the near side of the Bulge was sampled.
To obtain a more complete sample for the Bulge, it will
be necessary to include fainter nebulae. This problem will be studied
further in the forthcoming paper by Durand et al. (1997).