Morphological Dependence of Star Formation Properties for the Galaxies in the Merging Galaxy Cluster A2255

The merging cluster of galaxies A2255 is covered by the Sloan Digital Sky Survey (SDSS) survey. The physical parameters of 184 bright member galaxies derived from the SDSS data analyses by Brinchmann et al. allow a detailed study of the star formation properties of galaxies within a merging cluster at intermediate redshift. In this paper we perform a morphological classification on the basis of the SDSS imaging and spectral data and investigate the morphological dependence of the star formation rates (SFRs) for these member galaxies. As we expect, a tight correlation between the SFR normalized by stellar mass (SFR/M*) and the Hα equivalent width is found for the late-type galaxies in A2255. The correlation of SFR/M* with the continuum break strength at 4000 Å is also confirmed. A SFR/M*-M* correlation is found for both early- and late-type galaxies, indicating that the star formation activity tends to be suppressed when the assembled stellar mass (M*) increases, and this correlation is tighter and steeper for the late-type cluster galaxies. Compared with the mass range of field spiral galaxies, only two massive late-type galaxies with M* > 1011 M⊙ have survived in A2255, suggesting that the gas disks of massive spiral galaxies could have been tidally stripped during cluster formation. In addition, the SFR variation with the projected radial distance is found to be heavily dependent on galaxy morphology: the early-type galaxies have a very weak inner decrease in SFR/M*, while the inner late-type galaxies tend to have higher SFR/M* values than the outer late-type galaxies. This may suggest that the galaxy-scale turbulence stimulated by the merging of subclusters might have played different roles in early- and late-type galaxies, which leads to a suppression of the star formation activity for E/S0 galaxies and an SFR enhancement for spiral and irregular galaxies.