Age and Metallicities of Cluster Galaxies: A1185 and Coma

Abstract

We present age and metallicities determinations based on narrow band continuum colors for the galaxies in the rich clusters A1185 and Coma. Using a new technique to extract luminosity-weighted age and [Fe/H] values for non-star-forming galaxies, we find that both clusters have two separate populations based on these parameters. One population is old ($\tau >$ 11 Gyrs) with a distinct mass-metallicity relation. The second population is slightly younger ($\tau \approx$ 9 Gyrs) with lower metallicities and lower stellar masses. We find detectable correlations between age and galaxy mass in both populations such that older galaxies are more massive and have higher mean metallicities, confirming previous work with line indices for the same type of galaxies in other clusters (Kelson et al 2006, Thomas et al 2005). Our results imply shorter durations for higher mass galaxies, in contradiction to the predictions of classic galactic wind models. Since we also find a clear mass-metallicity relation for these galaxies, then we conclude that star formation was more efficient for higher mass galaxies, a scenario described under the inverse wind models (Matteucci 1994). With respect to cluster environmental effects, we find there is a significant correlation between galaxy mean age and distance from the cluster center, such that older galaxies inhabit the core. This relationship would nominally support hierarchical scenarios of galaxy formation (younger age in lower density regions); however, environmental effects probably have larger signature in the sample and present-day galaxies are remnants from an epoch of quenching of initial star formation, which would result in the same age gradients.Comment: 27 pages, 8 figures, accepted to ApJ, AAS LaTe

Galaxy properties in clusters

“…If morphological types are considered, we find that this effect corresponds to LT galaxies. These results are consistent with the age gradients found in clusters by Rakos et al (2007). Smith et al (2012) found that the age gradient depends on the mass of galaxy being lower for the most massive objects.…”

“…The implementation of these codes allows the estimation of the stellar masses, the mean ages (τ), and the metallicities (Z) of the stellar population of galaxies. Rakos et al (2007) analysed galaxies in Coma and Abell 1185, finding a clear correlation between τ and Z with both the stellar mass of galaxies and the environment. They found a significant correlation between galaxy mean age and clustocentric distance, such that older galaxies inhabit the core.…”

The Age of Cluster Galaxies from Continuum Colors

“…In order of increasing redshift, they are Fornax (z ¼ 0:0046), Coma (z ¼ 0:0231), A119 (z ¼ 0:0442), A400 (z ¼ 0:0244), A539 (z ¼ 0:0284), A779 (z ¼ 0:0225), A1185 (z ¼ 0:0325), and A2218 (z ¼ 0:175). Four clusters (Fornax, Coma, A1185, and A2218) were published in our earlier studies (Rakos et al 2007;Rakos & Schombert 2005;Odell et al 2002. The remaining four (A119, A400, A539, and A779) were obtained in the last few observing seasons using the 2.3 m Bok Telescope of the Steward Observatory located at Kitt Peak, Arizona.…”

Comparative Studies of Population Synthesis Models in the Framework of Modified Strömgren Filters

“…The idea is to look for the change in colour of the RGB, that acts as a metallicity indicator, and similarly change in colour, produced by the shifting the turnoff points, reflects the age (Tinsley et al 1980) in galaxies. Since broadband colours are not well suited for separating age effects from metallicity effects, Rakos et al (1995Rakos et al ( , 2005Rakos et al ( , 2007 attempted to solve the age-metallicity degeneracy problem uniquely by using narrow-band colours with the PCA technique. The PC analysis is a threedimensional multicolour space defined by these modified Strömgren colours and is formed by three PC equations (refer S01), where different Hubble type galaxies take selected places in that cluster-box.…”