Das Farben‐Durchmesser‐ und Farben‐Helligkeits‐Diagramm der Virgo‐Galaxien

Abstract

Holmberg's photometric data of the Virgo cluster members are used to construct a color‐diameter and color‐magnitude diagram (figure 3). These diagrams show some characteristic features: 1. The late type spirals Sc — Sm are concentrated near a line — the Y‐branch — for which diameter and magnitude increase with increasing color index. 2. The early type spirals Sa — Sbc form the “X‐branch”, for which diameter and magnitude decrease for increasing color index. 3. Elliptical and lenticular galaxies show a large scatter in a small interval of colors. 4. The types Sa — Sbc are clearly separated in two groups (X and X́ in fig. 3).Only the X‐ and Y‐branch probably belong to the main Virgo cloud (the mean velocity being the same), whereas the X́‐galaxies indicate the presence of a more distant cloud (the velocity is about 400 to 500 km/s larger). The distribution of the galaxies on the sky further shows some peculiarities of the X́‐galaxies (fig. 4). They are accumulated in two sectors around the cluster center and perhaps indicate a cloud expanding from the center or from the radio galaxy Virgo A. Without this cloud the plot of the remaining spiral galaxies reveals a color‐diameter and color‐magnitude diagram of relative small dispersion about the mean curves (fig. 5). It is suggested that other clusters may also possess such pronounced diagrams offering favourable possibilities to derive distance, structure and evolution effects. For the Virgo cluster the distance modulus is derived by means of Holmberg's data for M 31 (X‐galaxy) and M 33 (Y‐galaxy) using the distances of Sandage for the Local Group members. From both the diameter and magnitude the same value results: 31.15 ± 0·08. With the mean velocity of the investigated galaxies the Hubble parameter becomes H = (88 ± 9) km/s/Mpc.