2,059 research outputs found
A Virial Core in the Sculptor Dwarf Spheroidal Galaxy
The projected virial theorem is applied to the case of multiple stellar
populations in the nearby dwarf spheroidal galaxies. As each population must
reside in the same gravitational potential, this provides strong constraints on
the nature of the dark matter halo. We derive necessary conditions for two
populations with Plummer or exponential surface brightnesses to reside in a
cusped Navarro-Frenk-White (NFW) halo. We apply our methods to the Sculptor
dwarf spheroidal, and show that there is no NFW halo compatible with the
energetics of the two populations. The dark halo must possess a core radius of
~ 120 pc for the virial solutions for the two populations to be consistent.
This conclusion remains true, even if the effects of flattening or self-gravity
of the stellar populations are included.Comment: 4 pages, 2 figures, ApJL, submitte
Clean Kinematic Samples in Dwarf Spheroidals: An Algorithm for Evaluating Membership and Estimating Distribution Parameters When Contamination is Present
(abridged) We develop an algorithm for estimating parameters of a
distribution sampled with contamination, employing a statistical technique
known as ``expectation maximization'' (EM). Given models for both member and
contaminant populations, the EM algorithm iteratively evaluates the membership
probability of each discrete data point, then uses those probabilities to
update parameter estimates for member and contaminant distributions. The EM
approach has wide applicability to the analysis of astronomical data. Here we
tailor an EM algorithm to operate on spectroscopic samples obtained with the
Michigan-MIKE Fiber System (MMFS) as part of our Magellan survey of stellar
radial velocities in nearby dwarf spheroidal (dSph) galaxies. These samples are
presented in a companion paper and contain discrete measurements of
line-of-sight velocity, projected position, and Mg index for ~1000 - 2500 stars
per dSph, including some fraction of contamination by foreground Milky Way
stars. The EM algorithm quantifies both dSph and contaminant distributions,
returning maximum-likelihood estimates of the means and variances, as well as
the probability that each star is a dSph member. Applied to our MMFS data, the
EM algorithm identifies more than 5000 probable dSph members. We test the
performance of the EM algorithm on simulated data sets that represent a range
of sample size, level of contamination, and amount of overlap between dSph and
contaminant velocity distributions. The simulations establish that for samples
ranging from large (N ~3000) to small (N~30), the EM algorithm distinguishes
members from contaminants and returns accurate parameter estimates much more
reliably than conventional methods of contaminant removal (e.g., sigma
clipping).Comment: Accepted for publication in The Astronomical Journal. Download pdf
with full-resolution figures from
http://www.ast.cam.ac.uk/~walker/dsph_em.pd
Unambiguous quasar microlensing
Microlensing studies of quasars can reveal dark matter lumps over a broad
mass spectrum; we highlight the importance of monitoring quasars which are seen
through the halos of low-redshift galaxies. For these configurations
microlensing by planetary-mass objects will manifest itself as isolated events
which are only weakly chromatic. Statistical comparison of the observed optical
depths with their theoretical counterparts provides a strong test for a
microlensing origin of such events. If microlensing is detected, the
light-curves can reveal not only the characteristic microlens masses, and their
corresponding contribution to dark halos, but also how compact the individual
objects are. In this way we can decisively test the possibility that the dark
matter associated with galaxies is composed principally of planetary-mass gas
clouds.Comment: Revised version; 5 pages, 4 figures, to appear in MNRA
Probing non-spherical dark halos in the Galactic dwarf galaxies
We construct axisymmetric mass models for dwarf spheroidal (dSph) galaxies in
the Milky Way to obtain plausible limits on the non-spherical structure of
their dark halos. This is motivated by the fact that the observed luminous
parts of the dSphs are actually non-spherical and Cold Dark Matter (CDM) models
predict non-spherical virialized dark halos. Our models consider velocity
anisotropy of stars , which can vary with the
adopted cylindrical coordinates under the assumption
for simplicity, and also include an inclination of the system as a fitting
parameter to explain the observed line-of-sight velocity dispersion profile.
Applying these models to six of the bright dSphs in the Milky Way, we find that
the best-fitting cases for most of the dSphs yield oblate and flattened dark
halos, irrespective of assumed density profiles in their central parts. We also
find that the total mass of the dSphs enclosed within a spheroid with
major-axis length of 300 pc varies from to ,
contrary to the conclusion from spherical models. This suggests the importance
of considering shapes of dark halos in mass models of the dSphs. It is also
found that dark halos of the Galactic dSphs may be more flattened than N-body
predictions, thereby implying our yet incomplete understanding of baryonic
and/or non-baryonic dark matter physics in dwarf galaxy scales.Comment: 13 pages, 9 figures, Accepted for publication in Ap
Study of the Sextans dwarf spheroidal galaxy from the DART CaII triplet survey
We use VLT/FLAMES intermediate resolution (R~6500) spectra of individual red
giant branch stars in the near-infrared CaII triplet (CaT) region to
investigate the wide-area metallicity properties and internal kinematics of the
Sextans dwarf spheroidal galaxy (dSph). Our final sample consists of 174
probable members of Sextans with accurate line-of-sight velocities (+- 2 km/s)
and CaT [Fe/H] measurements (+- 0.2 dex). We use the MgI line at 8806.8 \AA\,
as an empirical discriminator for distinguishing between probable members of
the dSph (giant stars) and probable Galactic contaminants (dwarf stars).
Sextans shows a similar chemo-dynamical behaviour to other Milky Way dSphs,
with its central regions being more metal rich than the outer parts and with
the more metal-rich stars displaying colder kinematics than the more metal-poor
stars. Hints of a velocity gradient are found along the projected major axis
and along an axis at P.A.=191 deg, however a larger and more spatially extended
sample may be necessary to pin down the amplitude and direction of this
gradient. We detect a cold kinematic substructure at the centre of Sextans,
consistent with being the remnant of a disrupted very metal poor stellar
cluster. We derive the most extended line-of-sight velocity dispersion profile
for Sextans, out to a projected radius of 1.6 deg. From Jeans modelling of the
observed line-of-sight velocity dispersion profile we find that this is
consistent with both a cored dark matter halo with large core radius and cuspy
halo with low concentration. The mass within the last measured point is in the
range 2-4 x 10^8 M_sun, giving very large mass-to-light ratios, from 460 to 920
(M/L)_(V,sun).Comment: 30 pages, 17 figures; 4 tables. Accepted for publication in MNRAS.
Table 4 will appear as online material in the published version of the
manuscript. Typo correcte
The Velocity Dispersion Profile of the Remote Dwarf Spheroidal Galaxy Leo I: A Tidal Hit and Run?
(abridged) We present kinematic results for a sample of 387 stars located
near Leo I based on spectra obtained with the MMT's Hectochelle spectrograph
near the MgI/Mgb lines. We estimate the mean velocity error of our sample to be
2.4 km/s, with a systematic error of < 1 km/s. We produce a final sample of 328
Leo I red giant members, from which we measure a mean heliocentric radial
velocity of 282.9 +/- 0.5 km/s, and a mean radial velocity dispersion of 9.2
+/- 0.4 km/s for Leo I. The dispersion profile of Leo I is flat out to beyond
its classical `tidal' radius. We fit the profile to a variety of equilibrium
dynamical models and can strongly rule out models where mass follows light.
Two-component Sersic+NFW models with tangentially anisotropic velocity
distributions fit the dispersion profile well, with isotropic models ruled out
at a 95% confidence level. The mass and V-band mass-to-light ratio of Leo I
estimated from equilibrium models are in the ranges 5-7 x 10^7 M_sun and 9-14
(solar units), respectively, out to 1 kpc from the galaxy center. Leo I members
located outside a `break radius' (about 400 arcsec = 500 pc) exhibit
significant velocity anisotropy, whereas stars interior appear to have
isotropic kinematics. We propose the break radius represents the location of
the tidal radius of Leo I at perigalacticon of a highly elliptical orbit. Our
scenario can account for the complex star formation history of Leo I, the
presence of population segregation within the galaxy, and Leo I's large outward
velocity from the Milky Way. The lack of extended tidal arms in Leo I suggests
the galaxy has experienced only one perigalactic passage with the Milky Way,
implying that Leo I may have been injected into its present orbit by a third
body a few Gyr before perigalacticon.Comment: ApJ accepted, 23 figures, access paper as a pdf file at
http://www.astro.lsa.umich.edu/~mmateo/research.htm
Proper Motion of Milky Way Dwarf Spheroidals from Line-of-Sight Velocities
Proper motions for several Milky Way dwarf spheroidal (dSph) galaxies have
been determined using both ground and space-based imaging. These measurements
require long baselines and repeat observations and typical errors are of order
ten milli-arcseconds per century. In this paper, we utilize the effect of
"perspective rotation" to show that systematic proper motion of some dSphs can
be determined to a similar precision using only stellar line-of-sight
velocities. We show that including the effects of small intrinsic rotation in
dSphs increases the proper motion errors by about a factor of two. We provide
error projections for future data sets, and show that proposed thirty meter
class telescopes will measure the proper motion of a few dSphs with
milli-arcsecond per century precision.Comment: 4 pages, 1 figure. Minor changes to match published versio
On Kinematic Substructure in the Sextans Dwarf Spheroidal Galaxy
We present multifiber echelle radial velocity results for 551 stars in the
Sextans dwarf spheroidal galaxy and identify 294 stars as probable Sextans
members. The projected velocity dispersion profile of the binned data remains
flat to a maximum angular radius of . We introduce a nonparametric
technique for estimating the projected velocity dispersion surface, and use
this to search for kinematic substructure. Our data do not confirm previous
reports of a kinematically distinct stellar population at the Sextans center.
Instead we detect a region near the Sextans core radius that is kinematically
colder than the overall Sextans sample with 95% confidence.Comment: accepted for publication in ApJ Letters; 4 figures (2 color
Leo V: A Companion of a Companion of the Milky Way Galaxy
We report the discovery of a new Milky Way dwarf spheroidal galaxy in the
constellation of Leo identified in data from the Sloan Digital Sky Survey. Leo
V lies at a distance of about 180 kpc, and is separated by about 3 degrees from
another recent discovery, Leo IV. We present follow-up imaging from the Isaac
Newton Telescope and spectroscopy from the Hectochelle fiber spectrograph at
the Multiple Mirror Telescope. Leo V's heliocentric velocity is 173.4 km/s,
which is offset by about 40 km/s from that of Leo IV. A simple interpretation
of the kinematic data is that both objects may lie on the same stream, though
the implied orbit is only modestly eccentric (e = 0.2)Comment: Submitted to ApJ (Letters
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