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Description
Using the prime focus of the 6-meter telescope, we have searched for low surface brightness dwarf galaxies in two regions in the Coma cluster: one in the center of the cluster and the other one 40arcmin to the west. The observations were carried out with a total exposure time 4x5x600sec and with a 1.2arcsec seeing. Our survey is complete down to V=25.5 and R=25.0mag. The central field shows an excess of faint galaxies (121 versus 104 in the reference field) significant at a one sigma level only. However, the central-field excess is mainly caused by the presence of faint (V=24/25mag), neutral colour (V-R=0.3/0.4) objects of low surface brightness, SB=26.0/26.5mag/arcsec^2^. These characteristics are quite consistent with those of the dwSph's in the Local Group which have typically M_v_~10mag and A~1kpc. The observed excess corresponds to a surface number density of 4gal/arcmin^2^ in the center of Coma, thus implying a total population of cluster dwarfs of ~4000 down to the limiting absolute magnitude -10mag. The number ratio of LSBD's to normal galaxies (with M<-18mag for normals) in Coma turns out to be 20:1. Such ratio is comparable to that observed in other rich clusters: A3574 (14:1) and A1367 (6:1), but is much larger than the 2:1 ratio obtained for moderate nearby clusters such as Virgo, Fornax, Antlia.
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