22 research outputs found
EzGal: A Flexible Interface for Stellar Population Synthesis Models
We present EzGal, a flexible python program designed to easily generate
observable parameters (magnitudes, colors, mass-to-light ratios) for any
stellar population synthesis (SPS) model. As has been demonstrated by various
authors, the choice of input SPS models can be a significant source of
systematic uncertainty. A key strength of EzGal is that it enables simple,
direct comparison of different models sets. EzGal is also capable of generating
composite stellar population models (CSPs) and can interpolate between
metallicities for a given model set. We have created a web interface to run
EzGal and generate observables for a variety of star formation histories and
model sets. We make many commonly used SPS models available from this
interface; the BC03 models, an updated version of these models, the Maraston
models, the BaSTI models, and finally the FSPS models. We use EzGal to compare
magnitude predictions for the model sets as a function of wavelength, age,
metallicity, and star formation history. We recover the well-known result that
the models agree best in the optical for old, solar metallicity models, with
differences at the ~0.1 magnitude level. The most problematic regime for SPS
modeling is for young ages (7500
Angstroms) where scatter between models can vary from 0.3 mags (Sloan i) to 0.7
mags (Ks). We find that these differences are best understood as general
uncertainties in SPS modeling. Finally we explore a more physically motivated
example by generating CSPs with a star formation history matching the global
star formation history of the universe. We demonstrate that the wavelength and
age dependence of SPS model uncertainty translates into a redshift dependent
model uncertainty, highlighting the importance of a quantitative understanding
of model differences when comparing observations to models as a function of
redshift.Comment: 10 pages, 5 figures, accepted for publication in PAS
RR Lyrae Variables in Two Fields in the Spheroid of M31
We present Hubble Space Telescope observations taken with the Advanced Camera for Surveys Wide Field Channel of two fields near M32—between 4 and 6 kpc from the center of M31. The data cover a time baseline sufficient for the identification and characterization of 681 RR Lyrae variables of which 555 are ab-type and 126 are c-type. The mean magnitude of these stars is = 25.29 ± 0.05, where the uncertainty combines both the random and systematic errors. The location of the stars in the Bailey diagram and the ratio of c-type RR Lyraes to all types are both closer to RR Lyraes in Oosterhoff type I globular clusters in the Milky Way as compared with Oosterhoff II clusters. The mean periods of the ab-type and c-type RR Lyraes are = 0.557 ± 0.003 and = 0.327 ± 0.003, respectively, where the uncertainties in each case represent the standard error of the mean. When the periods and amplitudes of the ab-type RR Lyraes in our sample are interpreted in terms of metallicity, we find the metallicity distribution function to be indistinguishable from a Gaussian with a peak at = –1.50 ± 0.02, where the quoted uncertainty is the standard error of the mean. Using a relation between RR Lyrae luminosity and metallicity along with a reddening of E(B – V) = 0.08 ± 0.03, we find a distance modulus of (m – M)_0 = 24.46 ± 0.11 for M31. We examine the radial metallicity gradient in the environs of M31 using published values for the bulge and halo of M31 as well as the abundances of its dwarf spheroidal companions and globular clusters. In this context, we conclude that the RR Lyraes in our two fields are more likely to be halo objects rather than associated with the bulge or disk of M31, in spite of the fact that they are located at 4-6 kpc in projected distance from the center
Stellar Populations in the Andromeda V Dwarf Spheroidal Galaxy
Using archival imaging from the Wide Field Planetary Camera 2 aboard the
Hubble Space Telescope, we investigate the stellar populations of the Local
Group dwarf spheroidal Andromeda V - a companion satellite galaxy of M31. The
color-magnitude diagram (CMD) extends from above the first ascent red giant
branch (RGB) tip to approximately one magnitude below the horizontal branch
(HB). The steep well-defined RGB is indicative of a metal-poor system while the
HB is populated predominantly redward of the RR Lyrae instability strip.
Utilizing Galactic globular cluster fiducial sequences as a reference, we
calculate a mean metallicity of [Fe/H] = -2.20 +/- 0.15 and a distance of
(m-M)0 = 24.57 +/- 0.04 after adopting a reddening of E(B-V) = 0.16. This metal
abundance places And V squarely in the absolute magnitude - metallicity diagram
for dwarf spheroidal galaxies. In addition, if we attribute the entire
error-corrected color spread of the RGB stars to an abundance spread, we
estimate a range of ~0.5 dex in the metallicities of And V stars. Our analysis
of the variable star population of And V reveals the presence of 28 potential
variables. Of these, at least 10 are almost certainly RR Lyrae stars based on
their time sequence photometry.Comment: 4 pages, 6 figures Accepted to the Astronomical Journa
Distances to Populous Clusters in the LMC via the K-Band Luminosity of the Red Clump
We present results from a study of the distances and distribution of a sample
of intermediate-age clusters in the Large Magellanic Cloud. Using deep
near-infrared photometry obtained with ISPI on the CTIO 4m, we have measured
the apparent K-band magnitude of the core helium burning red clump stars in 17
LMC clusters. We combine cluster ages and metallicities with the work of
Grocholski & Sarajedini to predict each cluster's absolute K-band red clump
magnitude, and thereby calculate absolute cluster distances. An analysis of
these data shows that the cluster distribution is in good agreement with the
thick, inclined disk geometry of the LMC, as defined by its field stars. We
also find that the old globular clusters follow the same distribution,
suggesting that the LMC's disk formed at about the same time as the globular
clusters, ~ 13 Gyr ago. Finally, we have used our cluster distances in
conjunction with the disk geometry to calculate the distance to the LMC center,
for which we find (m-M)o = 18.40 +/- 0.04_{ran} +/- 0.08_{sys}, or Do = 47.9
+/- 0.9 +/- 1.8 kpc.Comment: 31 pages including 5 figures and 7 tables. Accepted for publication
in the August 2007 issue of A