SPECTROSCOPY AND MULTICOLOR PHOTOMETRY OF THE MEDIUM REDSHIFT SOUTHERN CLUSTER OF GALAXIES A3639

We present the results of the spectroscopic and photometric observations of the Abell cluster of galaxies 3639. We obtained 15 redshifts in the core of the cluster: apart 1 object which is a star, of the 14 galaxies one is a foreground and three are background objects. Ten galaxies are cluster members, yielding [z] = 0.1496 +/- 0.0027. The estimated velocity dispersion is sigma = 538 ( + 212; -128) kms s-1. Photometry has been obtained in the three Gunn filters g, r, and i in different areas (a NE and a central field) during two different observing runs. Independent automatic classification of stars and galaxies in the three filters allows a construction of a catalog of galaxies detected in all filters containing 138 objects with m(r) < 22 in the central area and 43 objects with m(r) < 20.18 in the NE field. The total are investigated is 39.46 arcmin2. We estimate that this catalog is at least 90% complete and that the stellar contamination can be approximately 3%. The study of the surface-brightness distribution of the dominant galaxy could be carried out to a limiting isophotal magnitude m(r) approximately 26 mag arcsec-2, and the galaxy does not show any evidence of an extended envelope typical of cDs. Effective surface brightness and radius are fully consistent with the values expected for brightest cluster members. The two-color diagrams for the galaxies show differences both with different magnitude ranges (a higher percentage of red galaxies at fainter magnitudes) and with distance from the cluster center (a higher percentage of blue galaxies farther away from the BCG). These effects could be due, respectively, to field contamination and morphological segregation. The luminosity function of the cluster galaxies could be studied over a dynamical range of more than 5 mag (down to M(r) = - 18) in an area of approximately 0.5 Mpc radius (H(o) = 50 km s-1 Mpc-1 and q(o) = 0.5). Background galaxies have been excluded on the basis of their position in the color-color diagram. The total number of galaxies in the sample is thus 105. The maximum likelihood method has been used to estimate M * and alpha of the Schechter luminosity function. The results are comparable to those of similar studies but the allowed range of values is large when confidence levels are taken into account.