Multicolor Photometry of the Galaxies in Abell 1775: Substructures, Luminosity Functions, and Star-Formation Properties

An optical photometric observation in 15 bands was carried out for nearby galaxy cluster Abell 1775 by the Beijing-Arizona-Taiwan-Connecticut (BATC) multi-color system. Over 5000 sources' spectral energy distributions (SEDs) were obtained. Since this cluster has also been observed by the Sloan Digital Sky Survey (SDSS), the BATC SEDs were combined with the SDSS five-band photometric data. Using the combined SEDs, 146 faint galaxies were selected as new member galaxies by the photometric redshift technique. Based on the positions, redshifts and 20-band SEDs of member galaxies, dynamical substructures and luminosity functions (LFs) of A1775 were investigated. The previous reported bimodal structure of A1775 has been confirmed: a poor subcluster with lower redshift, A1775A, is located ~14' southeast to the main concentration A1775B. After taking into account the new supplemented member galaxies, a new subcluster A1775C was found along the aligned direction of A1775A and A1775B. The different LF faint ends of the two subclusters indicate that A1775B is a more dynamically evolved system, while A1775A is still dynamically young. By the STARLIGHT spectral synthesis code, the star- formation histories of the member galaxies were studied. The dependence of the mean stellar ages upon the Hubble type was confirmed, and the environmental effect on star-formation activities for galaxies in A1775B has been explored.