The central stellar populations of brightest cluster galaxies

Abstract

The majority of galaxy clusters contain a massive galaxy in the centre of the clusters that are far more luminous and massive than the other galaxies in the cluster. These galaxies are called the brightest galaxy clusters (BCGs) and the formation and evolution of these BCGs are intimately related to the formation of the host clusters. In this project, the star formation histories (SFHs) of 51 galaxies (49 BCGs and two ellipticals) were determined by using high signal–to–noise ratio, long–slit spectra. The spectra of the galaxies were fitted against the software package ULySS which is a stellar population synthesis code. Two stellar population models, the Pegase.HR (P.HR) and the Vazdekis/MILES were used to determine the SFHs of the galaxies, more specifically determine whether a single stellar population (SSP) or composite stellar population (CSP) provided the most probable representation of the SFHs. Additional parameters, such as the velocity dispersions of the galaxies, the redshifts, the error spectra and the wavelength range were defined to extend these models. The observed spectra were then respectively fitted against a SSP and CSP. A series of 500 Monte–Carlo simulations were then preformed to asses the relevance of the solutions and aided in the selection of the most probable SFHs of the BCGs. The c2 maps were then drawn to assist in the understanding of the structure of the parameter space. The SFHs of the galaxies were given in the form of stellar components characterised by the derived ages and metallicities ([Fe/H]). The derived parameters were then compared against those derived with the LICK Indices to determine whether these approaches produced consistent results. Lastly, the derived parameters were tested against the internal galaxy properties (the velocity dispersions and absolute K–band magnitudes) and the properties of the host cluster environment (the X–ray temperatures, luminosities, offsets and the presence of cooling flows (CFs)) to determine whether any correlations could be derived to shed light on the formation and evolution of the BCGs. The results indicate that the P.HR model gave the most probable representation of the SFHs of the sample. Although 55% of the sample could be represented by a single star formation epoch, the remaining 45% had a more complex SFH. The ages, derived by the P.HR and LICK Indices showed significant consistency when compared but the [Fe/H] did not because the current P.HR model does not include a enhancements. 14 galaxies contained CFs. No correlations could be found between the internal properties (velocity dispersion and the absolute K–band magnitudes) and the ages/[Fe/H] but it was found that clusters containing CFs were located at higher luminosities than those without CFs. No correlations could be found between the ages/[Fe/H] and the X–ray temperatures. The intermediate aged galaxies with CFs were located closer to the centre than the old aged galaxies with CFs. These results indicated that at least some of the galaxies in the sample had a more complex SFH than first assumed and the presence of the CFs could account for some, but not all of the star formation activities in the clusters.