next previous
Up: Radial velocities of

3. Discussion

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 tex2html_wrap_inline1331: the only deviating object is 358.3-21.6, where the velocity difference is tex2html_wrap_inline1335. 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, tex2html_wrap_inline1341 the 6-cm radio flux in mJy and tex2html_wrap_inline1343 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 tex2html_wrap_inline1345. 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 tex2html_wrap_inline1347. 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 tex2html_wrap_inline1349 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 tex2html_wrap_inline1349 from the Sun. Indicated are curves representing velocities expected from pure circular rotation with tex2html_wrap_inline1353km/s, for distances from the Sun of 6 and tex2html_wrap_inline1355. 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 tex2html_wrap_inline1357. 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 tex2html_wrap_inline1359, 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 tex2html_wrap_inline1359 is tex2html_wrap_inline1363. 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 tex2html_wrap_inline1365 (Ratag et al. 1992). These are expected to have a tex2html_wrap_inline1367, 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 tex2html_wrap_inline1349 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).

next previous
Up: Radial velocities of

Copyright by the European Southern Observatory (ESO)