Stellar evolution calculations predict the flux-weighted gravity g/T 4 eff and absolute bolometric magnitude of blue supergiants to be strongly correlated. We use medium resolution multi-object spectroscopy of late B and early A supergiants in two spiral galaxies, NGC 300 and NGC 3621, to demonstrate the existence of such a relationship, which proves to be surprisingly tight. An analysis of hig...

We derive stellar metallicities, light-weighted ages and stellar masses for a magnitudelimited sample of 175,128 galaxies drawn from the Sloan Digital Sky Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of these parameters using a large library of model spectra at medium-high resolution, covering a comprehensive range of star formation histories. The constraints we de...

Stellar streams in galaxy halos are the natural consequence of a history of merging and accretion. We present evidence for a blue tidal stream of young stars in the nearest giant elliptical galaxy, NGC 5128 (Centaurus A). Using optical UBVR color maps, unsharp masking, and adaptive histogram equalization, we detect a blue arc in the northwest portion of the galaxy that traces a partial ellipse ...

We present evidence for the existence of an old stellar halo in the dwarf irregular galaxy WLM, an isolated member of the Local Group. The halo consists of population-II stars, with low metallicities and age ≥ 10yr. The finding of a halo in a dwarf irregular galaxy argues for a generic mode of galaxy formation that requires a halo in the presence of a disk, regardless of galaxy size. This impli...

We present N-body simulations of elliptical galaxy encounters into dry mergers to study the resulting unbound intergalactic stellar population, in particular that of the post-Main Sequence stars. The systems studied are pairs of spherical galaxies without dark halos. The stellar content of the model galaxies is distributed into mass-bins representing lowand intermediate-mass stars (0.85 – 8 M⊙)...

We have obtained Keck Low Resolution Imaging Spectrograph images in V and I of the newly discovered Local Group dwarf galaxies PegasusdSph and Cassiopeia dSph and their surrounding field. The first stellar luminosity functions and color-magnitude diagrams are presented for stars with V ∼< 25 and I ∼< 24. The distances to the new dwarfs are estimated from the apparent I mag of the tip of the red...

We present a study of the stellar populations of galaxies in the cluster RXJ0152.7–1357 at a redshift of 0.83. The study is based on new high signal-to-noise spectroscopy of 29 cluster members covering the wavelength range 5000-10000Å as well as riz photometry of the cluster. We use scaling relations between the central velocity dispersions of the galaxies and their luminosities, Balmer line st...

Single-star stellar population (ssSSP) models are usually used for spectral stellar population studies. However, more than 50% of stars are in binaries and evolve differently from single stars. This suggests that the effects of binary interactions should be considered when modeling the stellar populations of galaxies and star clusters. Via a rapid spectral stellar population synthesis (RPS) mod...

We present the evolution of the stellar mass function (SMF) of galaxies from z = 4.0 to z = 1.3 measured from a sample constructed from the deep NIR MUSYC, the FIRES, and the GOODS-CDFS surveys, all having very high-quality optical to mid-infrared data. This sample, unique in that it combines data from surveys with a large range of depths and areas in a self-consistent way, allowed us to 1) min...

Context. The ages and metallicities of the dominating stellar populations of galaxies without current star formation (e.g., early type galaxies) are usually estimated by comparing the observed absorption line indices or colors to predictions of some simple stellar population models. However, some studies show that there is recent star formation in galaxies, including early type galaxies. This s...