2MASX J17134043+6411510 , the SIMBAD biblio

We present our optical multicolor photometry for the nearby cluster of galaxies A2255 with 13 intermediate filters in the Beijing-Arizona-Taiwan-Connecticut (BATC) system, which cover an optical wavelength range from 3000 to 10000 Å. The spectral energy distributions (SEDs) in the optical band for more than 7000 sources are achieved down to V∼20 in a field of 58'x58' centered on this rich cluster. A2255 has been recently observed by the Sloan Digital Sky Survey (SDSS) down to V∼17.5 spectroscopically and r'∼22.0 photometrically. A method of combining the SDSS photometric data in five broad bands and the BATC SEDs is then explored. A sample of 254 galaxies with known redshifts in the region of A2255 is constructed for testing the reliability of the method of combining SEDs. Our application of the technique of photometric redshift on this sample shows that the combined SEDs with higher resolution could lead to a more accurate estimate of photometric redshift. Based on 214 spectroscopically confirmed member galaxies, spatial and dynamical properties of this cluster are studied. Bimodality and large dispersion in the velocity distribution indicate that A2255 is an unrelaxed system. A tight color-magnitude correlation for 188 known early-type cluster galaxies is found. After an exclusion of 254 extragalactic sources with known redshifts, the combined SEDs for 2522 galaxies allow a further membership selection by the photometric redshift technique and color-magnitude correlation. As a result, 313 galaxies with photometric redshifts between 0.068 and 0.090 are selected as new cluster members. The combined SEDs and estimated redshifts for previously known and newly selected member galaxies are cataloged. On the basis of the enlarged sample of member galaxies, the spatial distribution, localized velocity structure, color-magnitude relation, and luminosity function of cluster galaxies are discussed. The reverse peculiar velocities are found for two satellite subclusters located on opposite sides of the central concentration, which supports an ongoing merger in A2255.