397 research outputs found
Planes of satellite galaxies: when exceptions are the rule
The detection of planar structures within the satellite systems of both the Milky Way (MW) and Andromeda (M31) has been reported as being in stark contradiction to the predictions of the standard cosmological model (Λ cold dark matter – ΛCDM). Given the ambiguity in defining a planar configuration, it is unclear how to interpret the low incidence of the MW and M31 planes in ΛCDM. We investigate the prevalence of satellite planes around galactic mass haloes identified in high-resolution cosmological simulations. We find that planar structures are very common, and that ∼10 per cent of ΛCDM haloes have even more prominent planes than those present in the Local Group. While ubiquitous, the planes of satellite galaxies show a large diversity in their properties. This precludes using one or two systems as small-scale probes of cosmology, since a large sample of satellite systems is needed to obtain a good measure of the object-to-object variation. This very diversity has been misinterpreted as a discrepancy between the satellite planes observed in the Local Group and ΛCDM predictions. In fact, ∼10 per cent of ΛCDM galactic haloes have planes of satellites that are as infrequent as the MW and M31 planes. The look-elsewhere effect plays an important role in assessing the detection significance of satellite planes and accounting for it leads to overestimating the significance level by a factor of 30 and 100 for the MW and M31 systems, respectively
Kinematics of Metal-Poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Non-Kinematically Selected Stars
(Abbreviated) We present a detailed analysis of the space motions of 1203
solar-neighborhood stars with metal abundances [Fe/H] <= -0.6, on the basis of
a recently revised and supplemented catalog of metal-poor stars selected
without kinematic bias (Beers et al. 2000). This sample, having available
proper motions, radial velocities, and distance estimates for stars with a wide
range of metal abundances, is by far the largest such catalog to be assembled
to date. Unlike essentially all previous kinematically selected catalogs, the
metal-poor stars in our sample exhibit a diverse distribution of orbital
eccentricities, e, with no apparent correlation between [Fe/H] and e. This
demonstrates, clearly and convincingly, that the evidence offered by Eggen,
Lynden-Bell, and Sandage (1962) for a rapid collapse of the Galaxy, an apparent
correlation between the orbital eccentricity of halo stars with metallicity, is
basically the result of their proper-motion selection bias. However, even in
our non-kinematically selected sample, we have identified a small concentration
of high-e stars at [Fe/H] = -1.7, which may originate, in part, from infalling
gas during the early formation of the Galaxy. The implications of our results
for the formation of the Galaxy are also discussed, in particular in the
context of the currently favored CDM theory of hierarchical galaxy formation.Comment: 51 pages, including 17 figures, to appear in AJ (June 2000), full
paper with all figures embedded available at
http://pluto.mtk.nao.ac.jp/people/chiba/preprint/halo5
Chemo-dynamical evolution of Globular Cluster Systems
We studied the relation between the ratio of rotational velocity to velocity
dispersion and the metallicity (/\sigma_{v}-metallicity relation) of
globular cluster systems (GCS) of disk galaxies by comparing the relation
predicted from simple chemo-dynamical models for the formation and evolution of
disk galaxies with the observed kinematical and chemical properties of their
GCSs. We conclude that proto disk galaxies underwent a slow initial collapse
that was followed by a rapid contraction and derive that the ratio of the
initial collapse time scale to the active star formation time scale is \sim 6
for our Galaxy and \sim 15 for M31. The fundamental formation process of disk
galaxies was simulated based on simple chemo-dynamical models assuming the
conservation of their angular momentum. We suggest that there is a typical
universal pattern in the /\sigma_{v}-metallicity relation of the GCS
of disk galaxies. This picture is supported by the observed properties of GCSs
in the Galaxy and in M31. This relation would deviate from the universal
pattern, however, if large-scale merging events took major role in
chemo-dynamical evolution of galaxies and will reflect the epoch of such
merging events. We discuss the properties of the GCS of M81 and suggest the
presence of past major merging event.Comment: 25 pages, 8 figures, Accepted for publication in the Astrophysical
Journa
Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy
HIRES on the Keck I telescope has been used to measure the first radial
velocities for stars belonging to eleven, heavily-reddened globular clusters in
the direction of the inner Galaxy. The question of kinematic substructuring
among the Galactic globular cluster system is investigated using an updated
catalog of globular cluster distances, metallicities and velocities. It is
found that the population of metal-rich globular clusters shows significant
rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc
of the Galactic center, the measured rotation velocity and line-of-sight
velocity dispersion are similar to those of bulge field stars. We investigate
claims that the metal-rich clusters are associated with the central Galactic
bar by comparing the kinematics of the innermost clusters to that of the atomic
hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich
and metal-poor clusters bears a remarkable similarity to that of the gas,
including the same non-circular motions which have traditionally been
interpreted as evidence for a Galactic bar, or, alternatively, a
non-axisymmetric bulge. However, uncertainties in the existing
three-dimensional Galactocentric positions for most of the clusters do not yet
allow an unambiguous discrimination between the competing scenarios of
membership in a rigidly rotating bar, or in a bulge which is an oblate
isotropic rotator. We conclude that the majority of metal-rich clusters within
the central 4 kpc of the Galaxy are probably associated with the bulge/bar, and
not the thick disk. (ABRIDGED)Comment: 18 pages, including 7 of 13 postscript figures. Figures 1-6 available
at http://astro.caltech.edu/~pc. Accepted for publication in the Astronomical
Journa
The search for decaying Dark Matter
We propose an X-ray mission called Xenia to search for decaying superweakly
interacting Dark Matter particles (super-WIMP) with a mass in the keV range.
The mission and its observation plan are capable of providing a major break
through in our understanding of the nature of Dark Matter (DM). It will
confirm, or reject, predictions of a number of particle physics models by
increasing the sensitivity of the search for decaying DM by about two orders of
magnitude through a wide-field imaging X-ray spectrometer in combination with a
dedicated observation program.
The proposed mission will provide unique limits on the mixing angle and mass
of neutral leptons, right handed partners of neutrinos, which are important
Dark Matter candidates. The existence of these particles is strongly motivated
by observed neutrino flavor oscillations and the problem of baryon asymmetry of
the Universe.
In super-WIMP models, the details of the formation of the cosmic web are
different from those of LambdaCDM. The proposed mission will, in addition to
the search for decaying Dark Matter, provide crucial insight into the nature of
DM by studying the structure of the "cosmic web". This will be done by
searching for missing baryons in emission, and by using gamma-ray bursts as
backlight to observe the warm-hot intergalactic media in absorption.Comment: A white paper submitted in response to the Fundamental Physics
Roadmap Advisory Team (FPR-AT) Call for White Paper
General models in min-max continous location
In this paper, a class of min-max continuous location problems is discussed. After giving a complete characterization of th stationary points, we propose a simple central and deep-cut ellipsoid algorithm to solve these problems for the quasiconvex case. Moreover, an elementary convergence proof of this algorithm and some computational results are presented
The asymptotic behaviour of a distributive sorting method
In the distributive sorting method of Dobosiewicz, both the interval between the minimum and the median of the numbers to be sorted and the interval between the median and the maximum are partitioned inton/2 subintervals of equal length; the procedure is then applied recursively on each subinterval containing more than three numbers. We refine and extend previous analyses of this method, e.g., by establishing its asymptotic linear behaviour under various probabilistic assumptions
The Dispersion Velocity of Galactic Dark Matter Particles
The self-consistent spatial distribution of particles of Galactic dark matter
is derived including their own gravitational potential, as also that of the
visible matter of the Galaxy. In order to reproduce the observed rotation curve
of the Galaxy the value of the dispersion velocity of the dark matter
particles, \rmsveldm, should be \sim 600\kmps or larger.Comment: RevTex, 4 pages, 1 ps figure, accepted for publication in Physical
Review Letter
The Star Clusters of the Small Magellanic Cloud: Structural Parameters
We present structural parameters for 204 stellar clusters in the Small
Magellanic Cloud derived from fitting King and Elson, Fall, & Freeman model
profiles to the V-band surface brightness profiles as measured from the
Magellanic Clouds Photometric Survey images. Both King and EFF profiles are
satisfactory fits to the majority of the profiles although King profiles are
generally slightly superior to the softened power-law profiles of Elson, Fall,
and Freeman and provide statistically acceptable fits to ~90% of the sample. We
find no correlation between the preferred model and cluster age. The only
systematic deviation in the surface brightness profiles that we identify is a
lack of a central concentration in a subsample of clusters, which we designate
as "ring" clusters. In agreement with previous studies, we find that the
clusters in the SMC are significantly more elliptical than those in the Milky
Way. However, given the mean age difference and the rapid destruction of these
systems, the comparison between SMC and MW should not directly be interpreted
as either a difference in the initial cluster properties or their subsequent
evolution. We find that cluster ellipticity correlates with cluster mass more
strongly than with cluster age. We identify several other correlations (central
surface brightness vs. local background density, core radius vs. tidal force,
size vs. distance) that can be used to constrain models of cluster evolution in
the SMC.Comment: 14 pgs., accepted for publication in A
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