Star Formation History Of The Galaxy Cluster Member Galaxies

To study the star-formation properties of galaxies has become one of the most popular fields of extragalactic astrophysics in this decade. For a better understanding of formation and evolutionary history of galaxies and the whole universe, it is important to investigate the physical properties and their dependences upon morphology, environment, and other physical quantities.Multi-band observational data show that the cD galaxy cluster A85 (z=0.055) in southern hemisphere is undergoing a merger. Based on the spectra of 242 member galaxies provided by the Sloan Digital Sky Survey (SDSS), we use the spectral synthesis code, STARLIGHT, to derive the star formation history (SFH) of these galaxies, and to analyze the relation between the star-formation properties and local density. It is found that the galaxies in high-density areas have higher metallicities and older average stellar ages. During the whole SFH, higher star formation rates (SFRs) are found in the galaxies with higher local densities. However, within recent period of 24.5 Myr, the galaxies in low-density regions are found to have higher SFRs, and no sign of star formation is found in the galaxies in high-density regions. Higher specific SFRs are found the galaxies in low-density regions. For the massive galaxies in high-density regions, their specific SFRs within recent 24.5 Myr are rather small.This thesis also presents a comparative study of the star-formation properties of the galaxies in A2255. The STARLIGHT code based on the continuum fitting is taken to derive star-formation properties, and then compare these parameters with those released by the MPA/JHU working group. Following results are obtained:(1) Both methods produce very similar stellar mass; (2) The specific SFRs within recent period of 24.5 Myr, given by the STARLIGHT fitting, are basically in agreement with the specific SFRs given by MPA/JHU; (3) The variations of both specific SFRs with projected distances are compared, and a consistent environmental effect with previous work has been achieved; (4) The light-weighted average age given by the STARLIGHT correlates strongly with the continuum parameter Dn(4000); (5) Using the STARLIGHT-given metallicities and the MPA/JHU-given stellar masses, a tight metallicity-mass correlation is confirmed.