4 Analysis and results

4.1 Positions

In Table 1 we list the position of the peak flux density per beam of the PN at 6 cm together with the optical positions as determined from the H$\alpha$or [S III]$\lambda$9532 images (Jacoby & Van de Steene 2001, in preparation). The peak of the radio emission is adopted as the PN position. If the PN is extended in the radio, the radio position may be off center. The radio position will be better determined for higher peak flux values and smaller PNe. It is also for this reason that we chose to use the value at 6 cm and not at 3 cm, where the resolution is twice as high and the signal to noise per beam lower for extended sources.

We note that the radio positions have a tendency to be offset towards the west of the optical position. In declination there is no clear tendency noticeable in the offsets.

The optical and radio positions agree very well. PNe for which the radio position differs more than 2 $^{\prime\prime}$ in RA or DEC from the optical position are extended and the peak in the radio is usually not centered.

Of the 64 PNe observed, 7 were not detected: JaSt 7, 21, 45, 80, 88, 92, and 96. Most likely they are very extended and have a surface brightness that is too low to be detected in the radio. They are also very faint in the H$\alpha$ images and their H$\alpha$ flux values are very uncertain (Jacoby & Van de Steene 2001, in preparation). All but JaSt 96 were also faint and extended in the [S III]$\lambda$9532 images.

4.2 Radio flux densities

Table 2 gives the radio flux densities at 6 and 3 cm.

If a Gaussian model provided a satisfactory fit to the surface brightness, the total Gaussian flux density was adopted. This was mostly the case for small, unresolved PNe. If the PN was extended, the intensities within the 2 or 3 $\sigma$ level contour were summed. This value was compared with the statistics over a larger region across the nebula to obtain an error estimate (Fomalont 1989).

For JaSt 69 only a small 3 $\sigma$ blob at the right position indicated the presence of a PN. No flux or size values could be determined. In some cases several blobs indicated the presence of a PN at 3 cm and hence their flux is a lower limit. Some objects detected at 6 cm, but having low peak flux density per beam, could not be seen at 3 cm.

PNe are normally optically thin at 6 cm in which case its 3 cm flux density is about 95% of the 6 cm flux density. In our case this means that the flux densities are similar within the error-bars. However, when a PN is already well resolved at 6 cm, more flux may have been missed at 3 cm where the beam is half the size, especially if the nebula is extended and of low surface brightness. It is clear that due to these factors, the flux density at 3 cm is generally less well-determined than at 6 cm, except for the bright and compact PNe. When a PN is optically thick at 6 cm, its 3 cm flux density is expected to be three times the flux density at 6 cm. There are some PNe like JaSt 65 and JaSt 79 for which the 3 cm flux density is clearly higher than the 6 cm flux density and which are point sources. In this case the PNe may not yet be completely optically thin at 6 cm and should be quite young.

In Fig. 1 we plotted the histogram of the radio flux of the known and new galactic bulge PNe. We selected a sample of known galactic bulge PNe for which radio flux densities and angular diameters are available, as in Van de Steene & Zijlstra (1995). The new bulge PNe are within 2 degrees of the galactic center. None has a radio flux larger than 100 mJy and their angular sizes are smaller than 20 $^{\prime\prime}$. Hence they fulfill the same selection criteria as these previously known bulge PNe.

There is a larger number of PNe with low flux densities among the new bulge PNe than among the known ones. 67% of the new PNe have a radio flux less than 15 mJy, while this is only 45% for the known ones. Of the 7 known ones within 2 degrees of the galactic center only 2 have a radio flux below 15 mJy. The median flux for the new PNe is 11.3 mJy, while the median for the known bulge PNe is 17.0 mJy. Our rms noise level in the maps is similar to the 1 $\sigma$ noise of 0.1 mJy in the 6 cm maps of Zijlstra et al. (1989). Apparently these faint and small PNe have just been missed in optical surveys done to date.

Figure 1: Histogram of the 6 cm flux values of previously known galactic bulge PNe with radio data and the new galactic bulge PNe (striped).

4.3 Angular size

Table 2 also gives the angular size of the detected PNe. We chose to determine the angular size at 6 cm because at this wavelength the resolution was lower and thus gives the best signal to noise ratio for the extended nebulae. The diameter at 3 cm is given if it is better determined than at 6 cm, such as for very small and bright PNe.

Diameters may differ considerably depending upon how they are calculated. The diameter was derived by one of several ways depending on the structure of the brightness distribution. If a two-dimensional Gaussian fit provided a satisfactory model to the observed structureless surface distribution, its deconvolved FWHM major and minor axis are given. The equivalent diameter is the square root of their product. To obtain the full diameter, this value must be multiplied with a conversion factor which is a function of the beam FWHM and depends upon the intrinsic surface distribution of the source. We assumed a spherical constant emissivity shell of 0.5 and used formula 5 and Table 1 from van Hoof (2000) to estimate the true radii. For small objects, if the Gaussian deconvolution was well determined and similar at 3 and 6 cm, the FWHM at 3 cm is given. If the deconvolution produced a point source at 6 cm, the source size at 3 cm is given. If the source was still a point source at 3 cm, the beam-size is an upper limit. If the source was extended, a Gaussian model was usually not a good representation of the radio source. The diameter of the PN was measured on the contour at 50% of the peak and deconvolved with the beam size. To determine the full diameter we determined the ratio of the flux density within the 3 $\sigma$ contour with the flux density within the 50% contour. Hence, we assumed that the flux decreased linearly with radius outside the 50% contour. We checked that this procedure seemed to give very good agreement with the size measured based on the 3 $\sigma$ contour. We didn't use the contour level at 10% of the peak (Zijlstra et al. 1989; Aaquist & Kwok 1990), because this was often below noise level.

It was noticed in the review paper by Pottasch (1992) that there is a selection against discovering both large and small PNe in the galactic bulge. 36% of the new PNe have a diameter smaller than 5 $^{\prime\prime}$, while this is 71% for the known ones. The median is 6 $.\!\!^{\prime\prime}$6 for the new PNe and only 3 $.\!\!^{\prime\prime}$2 for the known bulge PNe with radio data. We seem to identify relatively more larger PNe than in previous surveys. In regions with large extinction the [SIII]$\lambda$9532 line appears efficient in picking out the larger, low surface brightness PNe, and not only the small and dusty ones. Obviously these are the PNe which may have been missed in optical surveys.

 

 

Table 3: The statistical distance is shown in Col. 2, the scale height, radius, and ionized mass calculated using this distance in Cols 3, 4, and 5 respectively, and $E(B-V) = {\rm c}_{\rm H_{\beta }}$/1.46 in Col. 6. If the object was not detected in H$\alpha ,$ E(B-V) is marked with X. If no H$\alpha$image was obtained E(B-V) is marked with Z. In this case the E(B-V) value was derived from its spectrum. Uncertain values, due to uncertain radio flux or H$\alpha$ flux values, are marked with a colon. PS stands for Point Source.

JaSt	Dist	z	R	$M_{\rm ion}$	E(B-V)	Comment
	kpc	pc	pc
1	14.6:	401.6:	0.20:	0.25:	2.5:
2	10.0	305.7	0.15	0.19	2.0
3	6.2	183.0	0.14	0.17	2.3
4	8.5	258.9	0.23	0.29	1.9
5	6.6	210.4	0.13	0.17	2.3
8	10.0	241.9	0.17	0.22	2.1
9	9.5	194.9	0.19	0.23	2.9
11	9.2	91.6	0.24	0.31	3.7
16	5.9	77.6	0.07	0.08	3.5
17	7.1	242.5	0.14	0.17	1.8
19	8.4	264.7	0.14	0.17	1.8
23					2.4	PS
24	6.6	31.4	0.09	0.11	X
26	5.6	49.7	0.15	0.19	X
27	10.8	246.2	0.23	0.29	2.3
31	6.4	91.2	0.14	0.17	3.7
34	13.2	392.9	0.23	0.29	2.1
36	6.1	120.2	0.05	0.06	2.7
37	6.1	-6.3	0.11	0.13	X
38	10.9:	251.7:	0.19:	0.23:	2.0:
41	6.3	150.4	0.10	0.12	2.1
42	7.4	207.0	0.09	0.11	2.1
44	9.6:	190.2:	0.20:	0.26:	1.9:
46	7.9	207.1	0.05	0.06	1.9
49	7.9	89.4	0.13	0.16	X
52	8.5	190.6	0.03	0.04	2.1
54	3.5	50.2	0.06	0.07	X
55	6.5	115.9	0.17	0.21	2.5
56	7.2	89.9	0.09	0.11	X
58	3.9	-77.8	0.15	0.18	4.1
60	16.2	-213.6	0.04	0.04	X
63	9.4	31.5	0.19	0.25	2.6
64	7.2	-216.3	0.04	0.05	2.6
65					1.3	PS
66	5.5	-111.1	0.03	0.04	2.8
67	7.9	-127.0	0.03	0.04	4.6: Z
68	16.7	-388.0	0.04	0.04	2.8
70	6.2	-82.0	0.13	0.16	X
71	5.5	-67.4	0.06	0.07	3.6
73	10.5	-261.7	0.04	0.05	1.9
74	6.2	-93.9	0.07	0.09	3.6
75	7.5	-157.5	0.05	0.06	2.7
76	16.8	-223.1	0.03	0.03	5.0: Z
77					2.4	PS
78	5.4	-87.5	0.19	0.24	3.4: Z
79					1.5	PS
81	8.2	-75.8	0.04	0.05	X
85	7.2:	-224.1	0.30	0.39:	1.2

 

Table 3: continued.

JaSt	Dist	z	R	$M_{\rm ion}$	E(B-V)	Comment
	kpc	pc	pc
86	6.6	-193.0	0.18	0.23	1.9
89	7.2	-138.3	0.15	0.19	2.1
90	17.2:	-303.3:	0.23:	0.29:	2.1
93	6.5:	-217.2:	0.27:	0.35:	1.9: Z
95	7.4:	-157.7:	0.23:	0.28:	1.3
97	5.9	-125.5	0.16	0.21	2.6
98					2.7 Z	PS

Figure 2: Histogram of the size of the known galactic bulge PNe with radio data and the new galactic bulge PNe (striped).