The Space Density of Quasars

It is known that the surface density of quasars increases as one goes to fainter magnitudes. The available data suggests (Wills, 1978) that a conservative estimate will be 5 quasars per square degree to B less than 20th magnitude. This corresponds to 200,000 quasars over the whole sky. There does not seem to be any statistical data on the surface density of planetary nuclei available. We can, however, look at the problem in another way. The total number of planetary nebulae in our galaxy has been estimated (Cahn and Wyatt) by a number of authors, from as low as 10,000 to as high as half a million. However the favored value seems to be in the neighbourhood of 50,000. We have noted above that the total number of quasars visible from the earth (B < 20) is estimated to be 200,000, that would mean that the total number of quasars present in our galaxy is much greater than this number and this would imply that the space density of quasars is greater than that of planetary nuclei.

Elsewhere (Varshni, 1980) we have discussed the best available data (Cudworth, 1974) on the proper motions of central stars of planetary nebulae and have given a diagram showing the proper motion against distance for 60 planetary nebulae. The nearest planetary NGC 7293 has the largest proper motion and it stands out from the rest. However, its proper motion is only 0.040±0.003 arcsec/yr and it is an isolated case. Proper motions for all other planetary nebulae (in Cudworth's list) are smaller than 0.024 arcsec/yr, with considerable uncertainty in many cases. A great majority of planetary nuclei have proper motions less than 0.015 arcsec/yr. If our hypothesis concerning the relationship between planetary nuclei and quasars is correct, it would be reasonable to expect that the proper motions of quasars will have the same sort of values as those of planetary nuclei. Also, if the space density of quasars is indeed greater than than that of planetary nuclei, one would expect that for similar value of the proper motion, the number of quasars should be greater than that of planetary nuclei.

Next Section: Results from the conventional photographic method.