Structures of Central Bulges and Nuclei of Galaxies

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Abstract
The isophotal surfaces of central bulges of normal spirals, often described as ‘spheroidal’, are shown to depart from ellipsoids of revolution and to resemble MacLaurin or Jacobi spheroids with zonal distortion; two typical examples, NGC 4565, 5746, are illustrated. Weaker examples are NGC 2683, 4594 and 891. Stronger examples are NGC 7332 and 128.The true nucleus of an ordinary spiral is at most a very small object (~ 10 pc) resolved only in the nearest galaxies. The apparently larger nuclei seen in more distant spirals and lenticulars, in particular the so-called N-types, are probably artifacts due to insufficient resolution. It is possible that even in M31 the small ‘nucleus’ is merely the seeing-convolved image of the central peak in the r1/4 luminosity distribution of the spheroidal component.In the nearest barred lenticulars and early spirals each large ellipsoidal ‘nucleus’ (~ 1.0 × 1.5 kpc) in the centre of the bar or central lens is found to include a second bar and inner nucleus on a smaller scale, in the approximate ratio of the corresponding Jeans' lengths. This is illustrated by high resolution photographs of NGC 1291; another example is NGC 1326.

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