Tammy A. Smecker-Hane

The Average Mass Profile of Galaxy Clusters

The average mass density profile measured in the Canadian Network for Observational Cosmology cluster survey is well described with the analytic form ρ(r) = Ar-1(r + aρ)-2, as advocated on the basis of n-body simulations by Navarro, Frenk, & White. The predicted core radii are aρ = 0.20 (in units of the radius where the mean interior density is 200 times the critical density) for an Ω = 0.2 open cold dark matter model and aρ = 0.26 for a flat Ω = 0.2 model, with little dependence on other cosmological parameters for simulations normalized to the observed cluster abundance. The dynamically derived local mass-to-light ratio, which has little radial variation, converts the observed light profile to a mass profile. We find that the scale radius of the mass distribution, 0.20 ≤ aρ ≤ 0.30 (depending on modeling details, with a 95% confidence range of 0.12-0.50), is completely consistent with the predicted values. Moreover, the profiles and total masses of the clusters as individuals can be acceptably predicted from the cluster rms line-of-sight velocity dispersion alone. This is strong support for the hierarchical clustering theory for the formation of galaxy clusters in a cool, collisionless, dark-matter-dominated universe.

The stellar populations of the Carina dwarf spheroidal Galaxy. 1: A new color-magnitude diagram for the giant and horizontal branches

We report on the first in a series of studies of the Carina dwarf spheroidal galaxy, a nearby satellite of our Galaxy. Our two major results are: 1) precise BI photometry (

The Star Formation History of the Large Magellanic Cloud

\sigma_{B-I} \simlt 0.05

Homogeneous Photometry for Star Clusters and Resolved Galaxies. I. A Survey of Bright Stars in the Fornax Dwarf Spheroidal Galaxy

for

The effects of age on red giant metallicities derived from the near-infrared Ca ii triplet

V \simlt 22

The Dynamical Equilibrium of Galaxy Clusters

) for 11,489 stars in the Carina field, and 2) the detection of two, morphologically distinct, horizontal branches, which confirms that star formation in Carina occurred in two well-separated episodes. The old horizontal branch and RR Lyrae instability strip belong to a > 10 Gyr stellar population, while the populous red-clump horizontal branch belongs to an approximately 6 Gyr stellar population. We derive a distance modulus

The Star Clusters in the Starburst Irregular Galaxy NGC 1569

(m-M)_0=20.09 \pm 0.06

Constraints on the Origin of Manganese from the Composition of the Sagittarius Dwarf Spheroidal Galaxy and the Galactic Bulge

for Carina from the apparent magnitudes of the old horizontal branch and the tip of the red giant branch, and discuss modifications to the previously estimated distance, total magnitude, and stellar ages. Using the color of the red giant branch, we estimate the metallicities of the younger and older populations to be [Fe/H] = -2.0 and -2.2, respectively.

Faint stars in the Ursa Minor dwarf spheroidal galaxy: implications for the low-mass stellar initial mass function at high redshift

Using WFPC2 on board the Hubble Space Telescope, we have created deep color-magnitude diagrams in the V and I passbands for approximately 105 stars in a field at the center of the LMC bar and another in the disk. The main-sequence luminosity functions (LFs) from 19V23.5, the red clump and horizontal branch morphologies, and the differential Hess diagram of the two fields all strongly imply that the disk and bar have significantly different star formation histories (SFHs). The disks SFH has been relatively smooth and continuous over the last 15 Gyr, while the bars SFH was dominated by star formation episodes at intermediate ages. Comparison of the LF against predictions based on Padova theoretical stellar evolution models and an assumed age-metallicity relationship allows us to identify the dominant stellar populations in the bar with episodes of star formation that occurred from 4 to 6 and 1 to 2 Gyr ago. These events accounted for 25% and 15%, respectively, of its stellar mass. The disk field may share a mild enhancement in SF for the younger episode, and thus we identify the 4 to 6 Gyr episode with the formation of the LMC bar.

The Unusual Abundance of Copper in the Sagittarius Dwarf Spheroidal Galaxy and Implications for the Origin of ω Centauri

We present accurate photometry on the Johnson B, Kron-Cousins R photometric system for approximately 100,000 stars in a 1/3 deg2 field centered on the dwarf spheroidal galaxy in Fornax. We identify numerous probable short-period variable stars, blue stars that appear to be the main sequence of a small population with an age of order 108 yr, and two distinct types of luminous red star: an extended sequence of primarily carbon stars and a clump of mostly M giants slightly more luminous than the giant-branch tip. The spatial distribution of each of these subpopulations within the Fornax dwarf galaxy is considered.