2.2. The Coma cluster of nebulæ

In historic times, Herschel [69] was the first to notice the concentration of nebulæ in the constellation of Coma Berenices. A more rigorous catalogation of nebulæ in the Coma region was done by Wolf [159], [160] in the early years of this century (following up an earlier work of D'Arrest [32]). Wolf counted 108 nebulæ in a circle of 30' diameter. The number of catalogued nebulæ in the Coma region increased to more than 300 with the observations of Curtis [31]. Wolf's map of the density of nebulæ in the Coma region (see Fig. 2), already shows the elongated shape of the cluster in the south-west direction, where a secondary density concentration appears to lie (the south-west subcluster, see Section 7.1).

Figure 2. The density of nebulae in the region of Coma, according to Wolf. Note the south-western extension (north is up, east is to the left). Every grid element is 28' × 60'.

Hubble & Humason [71] first measured the velocities of a few cluster nebulæ, ranging from 5100 km/s to 8500 km/s (an interval quite close to the real full extension of Coma in velocity space - see, e.g. Gavazzi et al. [54]).