Published in ARKIV FOR ASTRONOMI Band 5 nr 20, 1969
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This paper presents the results of an investigation of 174 physical groups of galaxies, most of them presumably comparable to the Milky Way and M31 groups (= Local Group), and the M81 group. The groups selected are centered on prominent spiral galaxies, for which the distance moduli can. be estimated. The survey work has been based on the Palomar Sky Atlas, the prints being evaluated down to the practical limit, as regards galaxies; the limiting diameter of the group members is 0.6 kpc, and the limiting absolute pg magnitude about -10.6. On account of the large disturbances from the background-foreground fields it has- been necessary to restrict the survey to circular areas with a radius of 50 kpc around the central spiral galaxies. Areas of this size probably include about 30% of all the satellites. A summary of the observational data is given in Table 7.
In the case of spiral galaxies with an edgewise orientation, the physical satellites have a peculiar distribution; most of them are found along the elongation of the minor axis, and they thus seem to favor high local latitudes (sect. 7). The number of satellites seems to be larger for spirals with exceptionally blue nuclei (such nuclei often appear to be abnormal) and for spirals with large hydrogen masses (sect. 8 - 9). The results favor the hypothesis that the satellites are produced by matter ejected from the nuclear regions of the spiral systems. The statistical evidence is however not conclusive.
The distribution of the log. abs. diameters of all the members of the physical groups can be transformed into a distribution of abs. magnitudes by means of the correlation diagram of Fig. 9. Luminosity functions have been determined for the E-So-Ir and Sa-Sb-Sc groups separately. The curves of Figs. 8 and 10 are based on a total of about 370 galaxies, and extend from M = -10.6 to M = -22. The log. distribution referring to the E-So-Ir group is a straight line with an inclination of 0.2; the Sa-Sb-Se spirals can be represented by a normal error-curve, extending from M = -15 to M = -22. The results are supported by luminosity curves derived from redshifts.
The smoothed-out space density of galaxies (outside the big clusters) has been determined from magnitudes, diameters, and redshifts; the result is 0.17 per Mpc3 (M < -15.0). Reasonable assumptions, as regards the mass/lum. ratios for different types of galaxies, lead to a mass density of 2.5 × 10-31 gr / cm3. The statistical distribution of the log. masses, referring to the members of the physical groups, is reproduced in Fig. 12. The distribution covers an interval of log. mass (solar units) from 8 to 12.
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