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3.1. Zwicky's Heresy

The mass of Coma was first estimated by Zwicky [162], [163] to be (4) M > 5 × 1014 Msun, using the virial theorem. This estimation was based on a value [165] of 1200 km/s for the radial velocity dispersion of the cluster galaxies, sigmav, not too far from current estimates (e.g. Colless & Dunn [27]).

The corresponding mass-to-light ratio was large, M/L > 50Msun / Lsun, and a form of unvisible matter seemed needed. Zwicky suggested that this dark matter could be detected as diffuse IC light.

Zwicky's hypothesis of some form of dark matter dominating the cluster dynamics, was not accepted by his contemporaries. Holmberg [70] considered it "an unlikely assumption", and his scepticism was still shared by the Burbidges [20] and de Vaucouleurs [36] 20 years later! However, the alternative hypothesis, clusters being unbound and expanding systems, would imply a very short timescale for disruption. This was found to be incompatible with the large number of galaxy clusters in the sky, and the similarity of nearby and distant (z appeq 0.2) clusters (Zwicky [169], Limber [85]).

Had Zwicky grossly overestimated the total cluster mass? Zwicky [165] himself pointed out that the application of the virial theorem may be of only limited validity when the system has an irregular distribution of galaxies, implicitely questioning the results obtained by Smith [126] on the Virgo cluster, and anticipating recent results on clusters affected by substructures. The problem of outliers in the velocity distribution was first considered by Schwarschild [120]. The lower limit he set to the velocity dispersion of Coma, sigmav > 630 km/s, was still too high to get rid of the dark matter problem.

4 Throughout this paper I use H0 = 50 km/s/Mpc, and scale all H0-dependent quantities accordingly. Note that the recent measurement of the Sunyaev-Zel'dovich effect in Coma (Herbig et al. [68]) implies H0 = 71+30-25 km/s/Mpc, consistent with the value adopted here. Back

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