622 research outputs found
Structure and Dynamics of the Globular Cluster Palomar 13
We present Keck/DEIMOS spectroscopy and Canada-France-Hawaii Telescope/MegaCam photometry for the Milky Way globular cluster Palomar 13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar 13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/High Resolution Echelle Spectrometer 21 star velocity dispersion of Ï = 2.2 ± 0.4 km s^(â1). We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61 stars, the velocity dispersion is Ï = 0.7^(+0.6)_(â0.5) km s^(â1). Combining our DEIMOS data with literature values, our final velocity dispersion is Ï = 0.4^(+0.4)_( â0.3) km s^(â1). We determine a spectroscopic metallicity of [Fe/H] = â1.6 ± 0.1 dex, placing a 1Ï upper limit of Ï_([Fe/H]) ~ 0.2 dex on any internal metallicity spread. We determine Palomar 13's total luminosity to be M_V = â2.8 ± 0.4, making it among the least luminous known globular clusters. The photometric isophotes are regular out to the half-light radius and mildly irregular outside this radius. The outer surface brightness profile slope is shallower than typical globular clusters (ÎŁ α r^η, η = â2.8 ± 0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar 13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of M_(1/2) = 1.3^(+2:7)_(â1.3) Ă 10^3 M_â and a mass-to-light ratio of M/L_V = 2.4^(+5.0)_(â2.4) M_â/L_â. Within our measurement errors, the mass-to-light ratio agrees with the theoretical predictions for a single stellar population. We conclude that, while there is some evidence for tidal stripping at large radius, the dynamical mass of Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics
Darwin Tames an Andromeda Dwarf: Unraveling the Orbit of NGC 205 Using a Genetic Algorithm
NGC 205, a close satellite of the M31 galaxy, is our nearest example of a
dwarf elliptical galaxy. Photometric and kinematic observations suggest that
NGC 205 is undergoing tidal distortion from its interaction with M31. Despite
earlier attempts, the orbit and progenitor properties of NGC 205 are not well
known. We perform an optimized search for these unknowns by combining a genetic
algorithm with restricted N-body simulations of the interaction. This approach,
coupled with photometric and kinematic observations as constraints, allows for
an effective exploration of the parameter space. We represent NGC 205 as a
static Hernquist potential with embedded massless test particles that serve as
tracers of surface brightness. We explore 3 distinct, initially stable
configurations of test particles: cold rotating disk, warm rotating disk, and
hot, pressure-supported spheroid. Each model reproduces some, but not all, of
the observed features of NGC 205, leading us to speculate that a rotating
progenitor with substantial pressure support could match all of the
observables. Furthermore, plausible combinations of mass and scale length for
the pressure-supported spheroid progenitor model reproduce the observed
velocity dispersion profile. For all 3 models, orbits that best match the
observables place the satellite 11+/-9 kpc behind M31 moving at very large
velocities: 300-500 km/s on primarily radial orbits. Given that the observed
radial component is only 54 km/s, this implies a large tangential motion for
NGC 205, moving from the NW to the SE. These results suggest NGC 205 is not
associated with the stellar arc observed to the NE of NGC 205. Furthermore, NGC
205's velocity appears to be near or greater than its escape velocity,
signifying that the satellite is likely on its first M31 passage.Comment: 34 pages, 20 figures, accepted for publication in the Astrophysical
Journal, A pdf version with high-resolution figures may be obtained from
http://www.ucolick.org/~kirsten/ms.pd
A MegaCam Survey of Outer Halo Satellites. VI: The Spatially Resolved Star Formation History of the Carina Dwarf Spheroidal Galaxy
We present the spatially resolved star formation history (SFH) of the Carina
dwarf spheroidal galaxy, obtained from deep, wide-field g,r imaging and a
metallicity distribution from the literature. Our photometry covers
deg, reaching up to times the half-light radius of Carina with a
completeness higher than at , more than one magnitude fainter
than the oldest turnoff. This is the first time a combination of depth and
coverage of this quality has been used to derive the SFH of Carina, enabling us
to trace its different populations with unprecedented accuracy. We find that
Carina's SFH consists of two episodes well separated by a star formation
temporal gap. These episodes occurred at old ( Gyr) and intermediate
(- Gyr) ages. Our measurements show that the old episode comprises the
majority of the population, accounting for of the stellar mass
within times the King tidal radius, while the total stellar mass derived
for Carina is , and the stellar
mass-to-light ratio . The SFH derived is consistent with no recent
star formation which hints that the observed blue plume is due to blue
stragglers. We conclude that the SFH of Carina evolved independently of the
tidal field of the Milky Way, since the frequency and duration of its star
formation events do not correlate with its orbital parameters. This result is
supported by the age/metallicity relation observed in Carina, and the gradients
calculated indicating that outer regions are older and more metal poor.Comment: Accepted in ApJ (22 pages, 13 figures
A SAURON study of dwarf elliptical galaxies in the Virgo Cluster: kinematics and stellar populations
Dwarf elliptical galaxies (dEs) are the most common galaxy type in nearby
galaxy clusters; even so, many of their basic properties have yet to be
quantified. Here we present the results of our study of 4 Virgo dwarf
ellipticals obtained with the SAURON integral field unit on the William
Herschel Telescope (La Palma, Spain). While traditional long-slit observations
are likely to miss more complicated kinematic features, with SAURON we are able
to study both kinematics and stellar populations in two dimensions, obtaining a
much more detailed view of the mass distribution and star formation histories.
What is visible even in such a small sample is that dEs are not a uniform
group, not only morphologically, but also as far as their kinematic and stellar
population properties are concerned. We find the presence of substructures,
varying degrees of flattening and of rotation, as well as differences in age
and metallicity gradients. We confirm that two of our galaxies are
significantly flattened, yet non-rotating objects, which makes them likely
triaxial systems. The comparison between the dwarf and the giant groups shows
that dEs could be a low-mass extension of Es in the sense that they do seem to
follow the same trends with mass. However, dEs as progenitors of Es seem less
likely as we have seen that dEs have much lower abundance ratios.Comment: 8 pages, 6 figures; to appear in the proceedings of the JENAM 2010
Symposium on Dwarf Galaxies (Lisbon, September 9-10, 2010); minor edits and
references adde
The MACHO Project: Microlensing Detection Efficiency
The MACHO project is a search for dark matter in the form of massive compact
halo objects (MACHOs). The project has photometrically monitored tens of
millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud
(SMC), and Galactic bulge in search of rare gravitational microlensing events
caused by these otherwise invisible objects. In 5.7 years of observations
toward the LMC some 13-17 microlensing events have been observed by the MACHO
survey, allowing powerful statements to be made about the nature of the dark
population in the halo of our Galaxy. A critical component of these statements
is an accurate determination of the survey's detection efficiency. The
detection efficiency is a complicated function of temporal sampling, stellar
crowding (the luminosity function), image quality, photometry, time-series
analysis, and criteria used to select the microlensing candidates. Such a
complex interdependence is most naturally solved using a Monte Carlo approach.
Here we describe the details of the Monte Carlo used to calculate the
efficiency presented in the MACHO 5.7-year LMC results. Here we correct several
shortcomings of past determinations, including (1) adding fainter source stars
(2.5 magnitudes below our faintest detected "stars"), (2) an up-to-date
luminosity function for the LMC, (3) better sampling of real images in both
stellar density and observing conditions, (4) an improved scheme for adding
artificial microlensing onto a random sample of real lightcurves, and many
other improvements. [Abridged]Comment: 32 pages, Latex with 16 postscript figures, submitted to ApJ
The Primeval Populations of the Ultra-Faint Dwarf Galaxies
We present new constraints on the star formation histories of the ultra-faint
dwarf (UFD) galaxies, using deep photometry obtained with the Hubble Space
Telescope (HST). A galaxy class recently discovered in the Sloan Digital Sky
Survey, the UFDs appear to be an extension of the classical dwarf spheroidals
to low luminosities, offering a new front in efforts to understand the missing
satellite problem. They are the least luminous, most dark-matter dominated, and
least chemically-evolved galaxies known. Our HST survey of six UFDs seeks to
determine if these galaxies are true fossils from the early universe. We
present here the preliminary analysis of three UFD galaxies: Hercules, Leo IV,
and Ursa Major I. Classical dwarf spheroidals of the Local Group exhibit
extended star formation histories, but these three Milky Way satellites are at
least as old as the ancient globular cluster M92, with no evidence for
intermediate-age populations. Their ages also appear to be synchronized to
within ~1 Gyr of each other, as might be expected if their star formation was
truncated by a global event, such as reionization.Comment: Accepted for publication in The Astrophysical Journal Letters. Latex,
5 pages, 2 color figures, 1 tabl
Blue Variable Stars from the MACHO database I: Photometry and Spectroscopy of the LMC sample
We present the photometric properties of 1279 blue variable stars within the
LMC. Photometry is derived from the MACHO database. The lightcurves of the
sample exhibit a variety of quasi-periodic and aperiodic outburst behavior. A
characteristic feature of the photometric variation is that the objects are
reddest when at maximum outburst. A subset of 102 objects were examined
spectroscopically. Within this subset, 91% exhibited Balmer emission in at
least one epoch, in some cases with spectacular spectral variability. The
variability observed in the sample is consistent with the establishment and
maintenance of the Be phenomenon.Comment: 19 pages, AJ accepte
- âŠ