Age and Metallicity Estimations in Old Stellar Populations from Stromgren Photometry

We present a new technique to determine age and metallicity of old stellar populations (globular clusters and elliptical galaxies) using an iterative principal component analysis on narrow band (Str\"omgren) colors. Our technique is capable of reproducing globular cluster [Fe/H] values to 0.02 dex and CMD ages to 1.0 Gyrs. We also present preliminary results on the application of our technique to a sample of high mass, field ellipticals and low mass, cluster dwarf ellipticals. We confirm the results of earlier studies which find that globular clusters increase in metallicity with age and that age and metallicity increase with galaxy mass. However, we find that dwarf ellipticals deviate from the elliptical sequence by having little to no correlation between age and metallicity.Comment: 8 pages IAU LaTeX, 5 figures, contributing talk IAU #198 Near-Field Cosmology with Dwarf Elliptical Galaxie

Gas-Rich Dwarf Galaxies from the PSS-II --- II. Optical Properties

We describe the optical properties of a sample of 101 gas-rich field dwarf galaxies found on PSS-II (Second Palomar Sky Survey) plates, most newly discovered as part of a survey to investigate the clustering properties of dwarf galaxies relative to giants. These galaxies have low surface brightnesses and are relatively distant, with recession velocities ranging up to 10,000 km/s. They have bluer V-I colors (median value of 0.75) than either actively star-forming giant galaxies or low metallicity globular clusters, implying that these dwarfs have both low metallicities and little past star formation. These galaxies are also extremely gas rich, with a median HI mass to V luminosity ratio of approximately 2 in solar units. We divide the sample into two groups: true dwarfs with diameters (at 25 I mag arcsec^-2) less than 7.5 kpc and Magellanic dwarfs with diameters greater than that value. The true dwarfs have greater HI mass to V luminosity ratios and slightly bluer V-I colors than the Magellanic dwarfs. Overall, the optical properties of our sample of dwarf galaxies point towards their being quiescent objects that have undergone little star formation over the age of the universe. They are not faded objects, but instead may be going through one of their first periods of weak star formation.Comment: 27 pages, to appear in 20 May 1997 ApJ, paper also available at http://www.astro.nwu.edu/astro/pildis/dwarfphot.html and http://zebu.uoregon.edu/~js/dwarf.htm

Nearby Gas-Rich Low Surface Brightness Galaxies

We examine the Fisher-Tully cz<1000 km/s galaxy sample to determine whether it is a complete and representative sample of all galaxy types, including low surface brightness populations, as has been recently claimed. We find that the sample is progressively more incomplete for galaxies with (1) smaller physical diameters at a fixed isophote and (2) lower HI masses. This is likely to lead to a significant undercounting of nearby gas-rich low surface brightness galaxies. However, through comparisons to other samples we can understand how the nearby galaxy counts need to be corrected, and we see some indications of environmental effects that probably result from the local high density of galaxies.Comment: 12 page, 2 figures, to appear in Ap

Age and Metallicities of Cluster Galaxies: A1185 and Coma

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