The Butcher-Oemler Effect in Abell 2317

This paper presents deep narrow band photometry of the cluster A2317 (z=0.211) carried out using KPNO 4 m and Steward 2.3 m telescopes. Using rest frame Stromgren photometry, it is determined that A2317 has an unusually high fraction of blue galaxies (the Butcher-Oemler effect) for its redshift (f_B=0.35). We demonstrate that the ratio of blue to red galaxies has a strong dependence on absolute magnitude such that blue galaxies dominate the top of the luminosity function. Spectrophotometric classification shows that a majority of the red galaxies are E/S0's, with a small number of reddened starburst galaxies. Butcher-Oemler galaxies are shown to be galaxies with star formation rates typical of late-type spirals and irregular. Starburst systems were typically found to be on the lower end of the cluster luminosity function. In addition, blue galaxies are preferentially found in the outer edges of the cluster, whereas the red galaxies are concentrated in the cluster core.Comment: 23 pages including 1 table and 6 figures, AASTeX v4.0. Accepted by Ap.J. Data, referee report and response are avaliable from http://zebu.uoregon.edu/~j

Cluster Populations in A115 and A2283

This paper presents four color narrow-band photometry of clusters A115 ($z=0.191$) and A2283 ($z=0.182$) in order to follow the star formation history of various galaxy types. Although located at similar redshifts, the two clusters display very different fractions of blue galaxies (i.e. the Butcher-Oemler effect, $f_B = 0.13$ for A115, $f_B = 0.30$ for A2283). A system of photometric classification is applied to the cluster members that divides the cluster population into four classes based on their recent levels of star formation. It is shown that the blue population of each cluster is primarily composed of normal starforming (SFR < 1 M_{\sun} yrs$^{-1}$) galaxies at the high luminosity end, but with an increasing contribution from a dwarf starburst population below $M_{5500}= -20$. This dwarf starburst population appears to be the same population of low mass galaxies identified in recent HST imaging (Koo et al 1997), possible progenitors to present-day cluster dwarf ellipticals, irregulars and BCD's. Deviations in the color-magnitude relationship for the red galaxies in each cluster suggest that a population of blue S0's is evolving into present-day S0 colors at this epoch. The radial distribution of the blue population supports the prediction of galaxy harassment mechanisms for tidally induced star formation operating on an infalling set of gas-rich galaxies.Comment: 28 pages including 2 tables and 9 figures, AASTeX v4.0. Accepted by Ap.J. Data, referee report and response are avaliable from http://zebu.uoregon.edu/~j

Ages and Metallicities of Cluster Galaxies in a779 Using Modified Strömgren Photometry

In the quest for the formation and evolution of galaxy clusters, Rakos and co-workers introduced a spectrophotometric method using modified Strömgren photometry, but with the considerable debate toward the project's abilities, we re-introduce the system by testing for the repeatability of the modified Strömgren colors and compare them with the Strömgren colors, and check for the reproducibility of the ages and metallicities (using the Principle Component Analysis (PCA) technique and the GALEV models) for the six common galaxies in all three A779 data sets. As a result, a fair agreement between two filter systems was found to produce similar colors (with a precision of 0.09 mag in (uz - vz), 0.02 mag in (bz - yz), and 0.03 mag in (vz - vz)) and the generated ages and metallicities are also similar (with an uncertainty of 0.36 Gyr and 0.04 dex from PCA and 0.44 Gyr and 0.2 dex using the GALEV models). We infer that the technique is able to relieve the age-metallicity degeneracy by separating the age effects from the metallicity effects, but it is still unable to completely eliminate it.We further extend this paper to re-study the evolution of galaxies in the low mass, dynamically poor A779 cluster (as it was not elaborately analyzed by Rakos and co-workers in their previous work) by correlating the luminosity (mass), density, and radial distance with the estimated age, metallicity, and the star formation history. Our results distinctly show the bimodality of the young, low-mass, metal-poor population with a mean age of 6.7 Gyr (± 0.5 Gyr) and the old, high-mass, metal-rich galaxies with a mean age of 9 Gyr (± 0.5 Gyr). The method also observes the color evolution of the blue cluster galaxies to red (Butcher-Oemler phenomenon), and the downsizing phenomenon. Our analysis shows that modified Strömgren photometry is very well suited for studying low- and intermediate-z clusters, as it is capable of observing deeper with better spatial resolution at spectroscopic redshift limits, and the narrow-band filters estimate the age and metallicity with fewer uncertainties compared to other methods that study stellar population scenarios

Stromgren Photometry from z=0 to z~1. The Method

We use rest-frame Stromgren photometry to observe clusters of galaxies in a self-consistent manner from z=0 to z=0.8. Stromgren photometry of galaxies is an efficient compromise between standard broad-band photometry and spectroscopy, in the sense that it is more sensitive to subtle variations in spectral energy distributions than the former, yet much less time-consuming than the latter. Principal Component Analysis (PCA) is used to extract maximum information from the Stromgren data. By calibrating the Principal Components using well-studied galaxies (and stellar population models), we develop a purely empirical method to detect, and subsequently classify, cluster galaxies at all redshifts smaller than 0.8. Interlopers are discarded with unprecedented efficiency (up to 100%). The first Principal Component essentially reproduces the Hubble Sequence, and can thus be used to determine the global star formation history of cluster members. The (PC2, PC3) plane allows us to identify Seyfert galaxies (and distinguish them from starbursts) based on photometric colors alone. In the case of E/S0 galaxies with known redshift, we are able to resolve the age-dust- metallicity degeneracy, albeit at the accuracy limit of our present observations. This technique will allow us to probe galaxy clusters well beyond their cores and to fainter magnitudes than spectroscopy can achieve. We are able to directly compare these data over the entire redshift range without a priori assumptions because our observations do not require k-corrections. The compilation of such data for different cluster types over a wide redshift range is likely to set important constraints on the evolution of galaxies and on the clustering process.Comment: 35 pages, 18 figures, accepted by ApJ

Ages and Metallicities of Fornax Dwarf Ellipticals

Narrow band photometry is presented on 27 dwarf ellipticals in the Fornax cluster. Calibrated with Galactic globular cluster data and spectrophotometric population models, the colors indicated that dwarf ellipticals have a mean [Fe/H] of -1.00+/-0.28 ranging from -1.6 to -0.4. The mean age of dwarf ellipticals, also determined photometrically, is estimated at 10+/-1 Gyrs compared to 13 Gyrs for bright Fornax ellipticals. Comparison of our metallicity color and Mg_2 indices demonstrates that the [Mg/Fe] ratio is lower in dwarf ellipticals than their more massive cousins, which is consistent with a longer duration of initial star formation to explain their younger ages. There is a increase in dwarf metallicity with distance from the Fornax cluster center where core galaxies are, on average, 0.5 dex more metal-poor than halo dwarfs. In addition, we find the halo dwarfs are younger in mean age compared to core dwarfs. One possible explanation is that the intracluster medium ram pressure strips the gas from dwarf ellipticals halting star formation (old age) and stopping enrichment (low metallicity) as they enter the core.Comment: 40 pages AAS LaTeX, 14 figures, accepted for publication in A

Narrow-band photometry and the stellar populations in elliptical galaxies

The modified Strömgren uvby filter system, which has been used to investigate the spectrophotometric evolution of elliptical galaxies, is also found to be a powerful tool for the determination of stellar parameters in elliptical galaxies, especially metal content and the stellar type of the dominant population. We present an introduction to the behaviour of the filter system based on comparison to stellar libraries and then explore the results with respect to composite systems: globular clusters, spirals, and ellipticals.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41874/1/10509_2004_Article_BF00652674.pd