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6.2. Spirals in Coma?

The fact that nebulæ near the centre of concentrated clusters are predominantly of the elliptic type, whereas spirals are relatively more numerous on the outskirts of clusters ...

...was already well known at the time Zwicky [164] was writing these lines, even if, in 1962, Neyman et al. [100] maintained that most (if not all) of this effect was due to an observational bias. Although Andreon [7] has recently shown they were not completely wrong, morphological segregation is real, and it can be seen in Coma as in (almost) any other cluster (being particularly evident when galaxies are selected in the UV, see Donas et al. [38]).

In this respect, what distinguishes Coma from most other clusters, is the almost complete absence of spirals. Abell [4] maintained there are no spirals in Coma at all, in contrast with Rood [111] and Rood et al. who found that some peculiar spirals do belong to Coma, and that at least 16 of the spectroscopically confirmed members of Coma were spirals or irregulars. Faced to the evidence that some spirals have velocities close to the mean cluster velocity, Abell [5] made the hypothesis that these spirals are members not of the cluster but of the Coma supercluster.

Sullivan & Johnson [129] observed three spirals in Coma and found that they had a surprisingly low HI abundance for their luminosity, when compared to similar spirals in the field, i.e. they were "HI-deficient". The authors concluded that these spirals have passed through Coma and have been stripped of part of their gas. Following studies (Sullivan et al. [128], Chincarini et al. [26], Bothun et al. [18], Gavazzi et al. [53]) not only confirmed these results, but also showed that the HI-deficiency mostly concerns spirals in the core of Coma, and not spirals in the Coma supercluster. This definitely proved the existence of a population of cluster spirals (note however that Coma spirals are not H2-deficient, see Boselli, these proceedings).

Doi et al. [37], via automatic classification of galaxy types, have recently concluded that the spiral fraction in Coma was previously underestimated (see also the contribution of Andreon in these proceedings).

Note that, even if spirals are cluster members, dwarf irregulars are not (Thompson & Gregory).

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