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

The trend of [Mn/Fe] in the Galactic bulge follows the solar-neighborhood relation, but most stars in the Sagittarius dwarf spheroidal galaxy show [Mn/Fe] deficient by approximately 0.2 dex. This leads us to conclude that the Mn yields from both type Ia and type II SNe are metallicity-dependent. Our observations militate against the idea, suggested by Gratton, that Mn is over-produced by type Ia SNe, relative to type II SNe. We predict Mn/Fe ratios, lower than the solar neighborhood relation, for the younger populations of nearly all dwarf galaxies, and that Mn/Fe ratios may be useful for tracing the accretion of low-mass satellites into the Milky Way.Comment: 10 pages, 3 figures, accepted for publication in ApJ

Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy: Implications for the Low-Mass Stellar Initial Mass Function at High Redshift

The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population - all of the stars being old and metal-poor - similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi Sph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to 0.3 solar masses. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way.Comment: Accepted by New Astronomy. 64 pages, including 9 embedded postscript tables and 20 embedded postscript figures, plus 14 separate jpeg figures. Postscript versions of the jpeg figures and a complete version of the paper with all figures embedded can be found at http://tarkus.pha.jhu.edu/~mlh

The Star Formation History of the Large Magellanic Cloud

Using WFPC2 aboard the Hubble Space Telescope, we have created deep color-magnitude diagrams in the V and I passbands for approximately 100,000 stars in a field at the center of the LMC bar and another in the disk. The main--sequence luminosity functions (LFs) from 19 mag < V < 23.5 mag, 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 disk's SFH has been relatively smooth and continuous over the last 15 Gyr while the bar's 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, respectively, for approximately 25% and 15% 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.Comment: 14 pages, 5 figures, Latex, also available at http://www.ps.uci.edu/physics/smeckerhane.html. Accepted for publication in Ap

The Three-Dimensional Mass Distribution in NGC 1700

A variety of modeling techniques is used with surface photometry from the literature and recently acquired high-accuracy stellar kinematic data to constrain the three-dimensional mass distribution in the luminous cuspy elliptical galaxy NGC 1700. First, we model the radial velocity field and photometry, and, using a Bayesian technique, estimate the triaxiality T and short-to-long axis ratio c in five concentric annuli between approximately 1 and 3 effective radii. The results are completely consistent with T being constant inside about 2.5 r_e (36 arcsec; 6.7/h kpc). Adding an assumption of constant T as prior information gives an upper limit of T < 0.16 (95% confidence); this relaxes to T < 0.22 if it is also assumed that there is perfect alignment between the angular momentum and the galaxy's intrinsic short axis. Near axisymmetry permits us then to use axisymmetric models to constrain the radial mass profile. Using the Jeans (moment) equations, we demonstrate that 2-integral, constant-M/L models cannot fit the data; but a 2-integral model in which the cumulative enclosed M/L increases by a factor of roughly 2 from the center out to 12/h kpc can. Three-integral models constructed by quadratic programming show that, in fact, no constant-M/L model is consistent with the kinematics. Anisotropic 3-integral models with variable M/L, while not uniquely establishing a minimum acceptable halo mass, imply, as do the moment models, a cumulative M/L_B approximately 10 h at 12/h kpc. We conclude that NGC 1700 represents the best stellar dynamical evidence to date for dark matter in elliptical galaxies.Comment: 26 pages, Latex, AASTeX v4.0, with 11 eps figures. To appear in The Astronomical Journal, January 1999. Figures 1 and 3 are color but are readable in b/

The Stellar Kinematic Fields of NGC 3379

We have measured the stellar kinematic profiles of NGC 3379 along four position angles using the MMT. The data extend 90" from the center, at essentially seeing-limited resolution out to 17". The mean velocities and dispersions have total errors better than 10 km/s (frequently better than 5 km/s) out to 55". We find very weak (3 km/s) rotation on the minor axis interior to 12", and no detectable rotation above 6 km/s from 12" to 50" or above 16 km/s out to 90" (95% confidence). However, a Fourier reconstruction of the mean velocity field from all 4 sampled PAs does indicate a 5 degree twist of the kinematic major axis, opposite to the known isophotal twist. The h_3 and h_4 parameters are small over the entire observed region. The azimuthally-averaged dispersion profile joins smoothly at large radii with the dispersions of planetary nebulae. Unexpectedly, we find sharp bends in the major-axis rotation curve, also visible (though less pronounced) on the diagonal position angles. The outermost bend coincides in position with other sharp kinematic features: an abrupt flattening of the dispersion profile, and local peaks in h_3 and h_4. All of these features are in a region where the surface brightness profile departs significantly from a de Vaucouleurs law. Features such as these are not generally known in ellipticals owing to a lack of data at comparable resolution; however, very similar behavior is seen the kinematics of the edge-on S0 NGC 3115. We discuss the suggestion that NGC 3379 could be a misclassified S0; preliminary results from dynamical modeling indicate that it may be a flattened, weakly triaxial system seen in an orientation that makes it appear round.Comment: 31 pages incl. 4 tables, Latex, AASTeX v4.0, with 17 eps figures. To appear in The Astronomical Journal, February 199