FOCA 0728-331 , the SIMBAD biblio

In a field of 1deg radius centered in the Coma cluster of galaxies, UV (λ=2000A) observations with a 40-cm balloon-borne imaging telescope (FOCA) have provided a list of 442 UV sources brighter than m_UV=18, which are identified in the Godwin et al. (1983) catalogue. 254 are identified as galaxies, 178 as star-like objects and 10 as galaxy-star pairs, unresolved in the UV image. The 254 galaxies fall into two sub-groups. The galaxies brighter than b=17 define a sequence in the (b-r,m_UV-b) color-color diagram which is well fitted by the models of Bruzual & Charlot (1993); most are cluster members. The galaxies fainter than b=17 show distinctly (m_UV-b) colors bluer than the former group at the same (b-r); most of them are likely background late-type galaxies. Notably, a significant fraction of the 178 star-like objects have UV magnitudes and colors similar to that of the second group of galaxies; ground-based spectroscopy is required to determine their nature and/or membership. In the UV sample, 61 galaxies have radial velocities consistent with cluster membership. Among them, nearly half are blue (m_UV-b<1), star-forming galaxies. Their cumulated UV fluxes lead to a star formation rate of ∼23h^–2M_☉/yr. Their velocity distribution shows an unexpected concentration near 7500 km/s and a void in the 6000-7000km/s range. The other half contains red galaxies (m_UV-b>1) with little or no recent star formation. All early-type galaxies brighter than b=14.5 are detected in UV and have a median color (m_UV-b)=3.0 in agreement with the representative spectral distribution of an elliptical compiled from IUE. 17 early-types galaxies which have spectroscopic data in the sample of Caldwell et al. (1993) show a correlation between the (m_UV-b) color and the Hδ (absorption line) equivalent width; some, with (m_UV-b)<1, would imply that the last starburst event is younger than 1 Gyr or that a residual star-formation is still present. The median UV flux and the fraction of blue star-forming galaxies which are cluster members show an enhancement at ∼25'(0.5h^–1Mpc) from the cluster center, suggesting that a global physical process might induce star formation in a rich cluster such as Coma.