Short-exposure photographs of the central regions of galaxies usually show a bright spot which is star-like in appearance, or just resolved (one second of arc diameter which, although frequently absent in some small galaxies, is a characteristic feature of the larger ones. The angular diameter of one second corresponds to a linear size of 3.3pc at M 31, 16pc at M 81 arid 03 pc at M 87. Because the size of the nuclear image is usually determined by atmospheric seeing, these values are upper limits to the true sizes of the nuclei. Hence one can safely say that a galaxy’s nucleus is small compared with its overall size.

Like a galaxy itself, the nucleus usually emits a black-body continuum with an effective temperature around 5000K. This suggests that it consists of stars very much like those that the rest of a galaxy contains. However, the greater brightness of the nucleus leads to the conclusion that its star density must be about a million times the average density in a galaxy. Population models of nuclei are made in the same way as they are of galaxies. A complication arises because the nucleus has such a faint appearance, even though it is one of the brightest spots in the galaxy, that the region to be observed must have a diameter above 10 seconds of arc in order to collect enough light for photometry. This means that the sample thus obtained contains a considerable amount of light from stars well outside the nucleus. The populations deduced from these measurements resemble those of the underlying galaxy, so that galaxies which have many ionized hydrogen regions tend to have blue nuclei due to the presence of young massive stars, whereas galaxies without such a strong young component tend to have redder nuclei in which the bulk of the light comes from G-, K- or M -type giants. There is a clear tendency for the nuclei to be somewhat redder than their galaxies, and it seems fairly certain that this is so because the nuclei contain a larger amount of elements heavier than helium.

Notwithstanding these general properties of a galaxy’s central parts (in the range 10-60 arc seconds apparent diameter), the inner¬most, and usually unresolved, part of the nucleus is often less predictable. Such a semi-stellar nucleus may be rather different in colour from its surroundings, either redder or bluer. It is often a fairly strong radio source, as in our Galaxy (the Sagittarius A radio source), sometimes it is weaker, as in M 31, or even absent, as m M 33. Some nuclei emit infrared radiation in the region near 300 micron wavelength. This radiation cannot be ascribed to fluorescence of dust grains near giant stars, for most of this emission occurs at about 2 or 3 microns. It appears therefore to be certain that many galaxies contain in their nuclei one or more sources of radiation that are not stars. In active galaxies, nuclear sources are usually very prominent. What these sources are is one of the most important unsolved astronomical problems.

The nucleus of a galaxy is often seen to emit a spectral line at 372.7 nm, due to a, forbidden transition in singly ionized oxygen. The apparent presence of ionized oxygen serves to indicate the existence of gas in the nucleus, and also the presence of some ionizing agent. The amounts of ionized gas are relatively small, of the order of 100 000 solar masses (i.e. less than one per cent of the total nuclear mass). Spectroscopic studies of some nearby nuclei show that the gas is probably ionized by a few very hot stars (50 stars with an effective temperature of 40 000 K are sufficient) or perhaps by a weak non-thermal ultraviolet source. Also, it is not excluded that some of the ionization is due to collisions with fast electrons.

The emission of infrared radiation with a wavelength of a microns indicates the presence of dust grains which intercept the light of stars in the nucleus and re-emit this at a wavelength approximately equal to the grain size. Where dust is present, one expects to find other cool components of the interstellar matter as well. Possibly the observed radio emission line of carbon monoxide is also emitted near the nucleus. In addition, the 21-cm emission line of neutral hydrogen has sometimes been observed. The problem is, however, that many nuclei are continuum radio sources, and it is very difficult to separate this continuum from the emission lines. Radio absorption lines due to hydroxyl and to formaldehyde have been observed against nuclear sources. It is inferred that nuclei contain at least as much neutral as ionized gas, and possibly up to 10 times more.

We have seen that the contents of other galaxies are very much like those of our Galaxy. Yet it is not obvious that matter in the Universe should appear to be the same everywhere; the observation that it is apparently the same, is extremely important. The fact that galaxies throughout the Universe are made of about the same kind of stars and interstellar matter indicates that the formation of these objects was a natural consequence of the properties of matter and of the way these arose hi the early Universe.

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