stellar populations in early-type coma cluster galaxies

Abstract

A detailed modelling and statistical analysis is conducted of the stellar populations (which act as fossil records of galaxy formation and evolution) and the spectro-photometric relations of a sample of 87 bright early-type galaxies within the core of the rich Coma cluster (a diameter of 1 degree = 1.26h (^-1)Mpc) using a high quality, homogeneous data set with well characterised errors together with published Gunn r CCD surface photometry. The sample data set allows for the first time a new unbiased assessment of the Coma clusters' bright early-type galaxies' intrinsic properties and of the factors affecting their spectro-photometric relations, without any need to combine multiple data sets with the inherent systematic error problem that ensues. This work acts as an important baseline at z ~ 0 for studies of distant high redshift clusters, utilising the rich Coma cluster as a laboratory to explore hypotheses. It also expands the existing knowledge base of galaxy formation and evolution in rich clusters and provides further evidence for the usefulness of the fundamental plane as a distance indicator. The stellar populations show that the bright early-type galaxies within the Coma cluster core have a large metallicity distribution (with -0.55 ≤ [Fe/H] ≤ +0.92). Whilst it is more likely that there is also a small distribution in age, a single age of stellar population formation for the dominant group of galaxies is supportable. The bright early-type galaxies are found to have a luminosity-weighted mean age of 8Gyrs. There are in addition real differences between the elliptical and lenticular galaxy populations, with the elliptical stellar populations (mean age of 9Gyrs) on average 2 Gyrs older than those within the lenticulars (mean age of 7Gyrs). Modelling of the age distributions of the early-type galaxies shows that an age model of 8 Gyrs with a scatter of 0.300 dex is likely, with the ellipticals having a smaller age scatter of 9 Gyrs ± 0.275dex rather than the 7 Gyrs it 0.325dex of the lenticulars. The fundamental plane (FP) analysis shows that the FP relation is in general well behaved and common for ellipticals and lenticulars. The early-type galaxy sample is well fit by a fundamental plane of the form log(_10)r(_e) = 1.36(±0.07) log(_10)a - 0.78(±0.03) log(_10)(I)(_e) - 0.64(±0.16) for galaxies with velocity dispersions, a greater than 100 km s(^-1)This FP has a significant intrinsic rms thickness of 0.044 ± 0.005, implying that the scatter in the FP relation is not simply due to measurement errors. A detailed FP residual correlation analysis concludes that there are no additional terms for the existing FP relation and that previously suggested mechanisms are not responsible for the presence of an intrinsic scatter, suggesting that the real source lies in the underlying physics of the kinematics and dark matter structures. The evidence for real intrinsic differences between ellipticals and lenticulars is overwhelming, leading to the conclusion that they have differing: kinematics; dark matter fractions; formation mechanisms; and/or evolution histories. It is therefore important to no longer analyse conglomerate samples of early-type galaxies, and instead to consider separately elliptical and lenticular galaxies in rich galaxy clusters.