3.1. Zwicky's Heresy
The mass of Coma was first estimated
by Zwicky [162],
[163]
to be (4)
M > 5 × 1014
M
, 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,
v,
not too far from current estimates (e.g. Colless & Dunn
[27]).
The corresponding mass-to-light ratio was large, M/L >
50M /
L, 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
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,
v > 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.
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