In view of the current discussion on the possibility of explosive events in the nuclear parts of spiral systems, resulting in the ejection of matter, it seemed important to look for indications in this direction in the analysis of the present material. Since calibrated pg and pv plates are available for most of the spiral galaxies of Table 7, we may as a starting-point try to collect some information regarding the nuclei of these systems. On account of the difficulty of defining the extension of a nucleus, no attempts will be made to determine apparent diameter or magnitude. The color index of the central part of the nucleus can however be easily measured.
Nuclear colors have been determined for altogether 172 spiral galaxies listed in my previous catalogue (Holmberg 1958). As a rule, 2 pg and -2 pv plates are available, taken with the Mount Wilson 60-inch or 100-inch telescope; the plates are calibrated by extrafocal exposures of stars. A number of galaxies could not be included, due to the fact that the nucleus is wholly or partly covered by absorption lanes; in a few cases, the central part of the nucleus is over-exposed. The measurements refer to a central cross-section along the minor axis of each galaxy, the derived color index being a mean result, corresponding to three points on either side of the center of the nucleus at distances of ± 2", ± 4" and ± 6"; in the case of large nuclei, the distances have been increased to 4", 6" and 8". The colors have been corrected for selective galactic absorption by means of a relation derived by the writer in the above-mentioned paper, and for redshift effect (-0m.15 per 103 km / sec); the latter correction is in the present case rather small. The mean error of a final color index (2 + 2 plates) amounts to 0m.045, as found from the internal agreement.
The nuclear part of a spiral system is apparently free from absorbing matter, the color index showing no systematic dependence on the apparent diameter ratio as long as this ratio is larger than 0.52, corresponding to an inclination of about 30° of the principal plane to the line of sight (absorbing matter could however be present, if it is assumed to have a spherical distribution). For diameter ratios smaller than 0.52 the color index increases, indicating that the nuclear part, although no absorption lanes are visible across the nucleus, is being covered by obscuring matter located in the outer parts of the system.
Table 3 gives the mean pg-pv colors for different types of galaxies, and the dispersions of the individual values around the means; the results are based on galaxies with diameter ratios larger than 0.52. As the type changes from Sc+ to So, the color index gradually increases from +0m.38 to +0m.77; the latter color happens to be the same as the mean integrated color previously derived by the writer (1958) for elliptical galaxies.
| Type | Number |  |
Disp. |
| Sc + | 21 | +0m.38 | 0m.093 |
| Sc- | 45 | 0.62 | 0.108 |
| Sb + | 21 | 0.74 | 0.093 |
| Sb -, Sa | 20 | 0.76 | 0.092 |
| So | 19 | 0.77 | 0.064 |
In Table 7 the nuclear color
excess,
CN,
or the
difference between observed color
and mean color, as given in Table 3, is listed
for 110 galaxies. For those objects that
have diameter ratios 
0.52,
a correction for internal absorption
(as derived from the material) has been applied; in a few exceptional
cases this correction may be as large as -0m.15.
It is naturally of the greatest interest to study the nuclear colors in
those cases in which the spectrum indicates an unusual activity in the
nuclear region or in which the nucleus in other ways appears to be
peculiar. Unfortunately, the present material includes only two Seyfert
galaxies, NGC 4051 and 4258; they both have large negative color
excesses, -0m.11 and -0m.09, respectively. From an
inspection of the Rubble plate collection,
Sérsic and Pastoriza
(1967)
have selected 20 galaxies that appear to
have abnormal nuclei; it is a very interesting fact that the list
comprises only barred spirals. The present material includes eight of
these galaxies, their mean nuclear color
excess amounting to -0m.06. The presence of the emission line
3727 (0 II) in the
spectrum also seems to affect the nuclear color. When this line is
present in measurable strength in the nuclear region, as indicated in
the redshift catalogue by
Humason et al. (1956),
the mean color excess is -0m.03; when the line is not
present, the mean excess is +0m.03.
In conclusion, it should be remarked that there is a fairly pronounced correlation between the nuclear color index and the spectral class, as listed by Humason in the redshift catalogue. In those cases in which the spectrograph slit has been put across the nucleus, the spectral classification apparently refers to the nuclear part of the galaxy. The linear regression line gives a mean color index of 0m.44 for F0, 0m.57 for F5, 0m.70 for G0, and 0m.82 for G5, the dispersion in the residuals being 0m.087. There is also a correlation between the nuclear color excess, as defined here, and the deviation of the observed spectrum from the mean spectral class corresponding to the morphological type. The good agreement indicates that the data available on nuclear Colors and spectra form a consistent system.