362 research outputs found
Probing the Shape of the Galactic Halo with Hyper-Velocity Stars
Precise proper motion measurements (sigma_mu ~ 10 mkas/yr) of the recently
discovered hyper-velocity star (HVS) SDSS J090745.0+024507 would yield
significant constraints on the axis ratios and orientation of a triaxial model
for the Galactic halo. Triaxiality of dark matter halos is predicted by Cold
Dark Matter models of galaxy formation and may be used to probe the nature of
dark matter. However, unless the distance to this star is determined to better
than 10%, these constraints suffer from one-dimensional degeneracies, which we
quantify. We show how proper motion measurements of several HVSs could
simultaneously resolve the distance degeneracies of all such stars and produce
a detailed picture of the triaxial halo. Additional HVSs may be found from
radial velocity surveys or from parallax/proper-motion data derived from GAIA.
High-precision proper-motion measurements of these stars using the Space
Interferometry Mission (SIM PlanetQuest) would substantially tighten the
constraints they yield on the Galactic potential.Comment: 7 pages, matches printed versio
Dark halo baryons not in ancient halo white dwarfs
Having ruled out the possibility that stellar objects are the main
contributor of the dark matter embedding galaxies, microlensing experiments
cannot exclude the hypothesis that a significant fraction of the Milky Way dark
halo might be made of MACHOs with masses in the range 0.5-0.8 \msun. Ancient
white dwarfs are generally considered the most plausible candidates for such
MACHOs. We report the results of a search for such white dwarfs in a proper
motion survey covering a 0.16 sqd field at three epochs at high galactic
latitude, and 0.938 sqd at two epochs at intermediate galactic latitude (VIRMOS
survey), using the CFH telescope. Both surveys are complete to I = 23, with
detection efficiency fading to 0 at I = 24.2. Proper motion data are suitable
to separate unambiguously halo white dwarfs identified by belonging to a non
rotating system. No candidates were found within the colour-magnitude-proper
motion volume where such objects can be safely discriminated from any standard
population as well as from possible artefacts. In the same volume, we estimate
the maximum white dwarf halo fraction compatible with this observation at
different significance levels if the halo is at least 14 gigayears old and
under different ad hoc initial mass functions. Our data alone rules out a halo
fraction greater than 14% at 95% confidence level. Combined with two previous
investigations exploring comparable volumes pushes the limit below 4 % (95%
confidence level) or below 1.3% (64% confidence), this implies that if baryonic
dark matter is present in galaxy halos, it is not, or it is only marginally in
the form of faint hydrogen white dwarfs.Comment: accepted in Astronomy and Astrophysics (19-05-2004
Does M31 result from an ancient major merger?
The numerous streams in the M31 halo are currently assumed to be due to
multiple minor mergers. Here we use the GADGET2 simulation code to test whether
M31 could have experienced a major merger in its past history. It results that
a 3+/-0.5:1 gaseous rich merger with r(per)=25+/-5 kpc and a polar orbit can
explain many properties of M31 and of its halo. The interaction and the fusion
may have begun 8.75+/-0.35 Gyr and 5.5 +/-0.5 Gyr ago, respectively. With an
almost quiescent star formation history before the fusion we retrieve fractions
of bulge, thin and thick disks as well as relative fractions of intermediate
age and old stars in both the thick disk and the Giant Stream. The Giant Stream
is caused by returning stars from a tidal tail previously stripped from the
satellite prior to the fusion. These returning stars are trapped into
elliptical orbits or loops for almost a Hubble time period. Large loops are
also predicted and they scale rather well with the recently discovered features
in the M31 outskirts. We demonstrate that a single merger could explain
first-order (intensity and size), morphological and kinematical properties of
the disk, thick disk, bulge and streams in the halo of M31, as well as the
distribution of stellar ages, and perhaps metallicities. It challenges
scenarios assuming one minor merger per feature in the disk (10 kpc ring) or at
the outskirts (numerous streams & thick disk). Further constraints will help to
properly evaluate the impact of such a major event to the Local Group.Comment: accepted in Astrophysical Journal, 29 September, 2010 ; proof-edited
version; 1st column of Table 3 correcte
Multiple Stellar Populations in the Globular Cluster omega Centauri as Tracers of a Merger Event
The discovery of the Sagittarius dwarf galaxy, which is being tidally
disrupted by and merging with the Milky Way, supports the view that the halo of
the Galaxy has been built up at least partially by the accretion of similar
dwarf systems. The Sagittarius dwarf contains several distinct populations of
stars, and includes M54 as its nucleus, which is the second most massive
globular cluster associated with the Milky Way. The most massive globular
cluster is omega Centauri, and here we report that omega Centauri also has
several distinct stellar populations, as traced by red-giant-branch stars. The
most metal-rich red-giant-branch stars are about 2 Gyr younger than the
dominant metal-poor component, indicating that omega Centauri was enriched over
this timescale. The presence of more than one epoch of star formation in a
globular cluster is quite surprising, and suggests that omega Centauri was once
part of a more massive system that merged with the Milky Way, as the
Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were
much more frequent in the early history of the Galaxy and omega Centauri
appears to be a relict of this era.Comment: 7 pages, 3 figures, Latex+nature.sty (included), To appear in
November 4th issue of Natur
NGC 770: A Counter-Rotating Core in a Low-Luminosity Elliptical Galaxy
We present evidence for a counter-rotating core in the low-luminosity (M_B =
-18.2) elliptical galaxy NGC 770 based on internal stellar kinematic data. This
counter-rotating core is unusual as NGC 770 is not the primary galaxy in the
region and it lies in an environment with evidence of on-going tidal
interactions. We discovered the counter-rotating core via single-slit Keck/ESI
echelle spectroscopy; subsequent integral field spectroscopy was obtained with
the Gemini/GMOS IFU. The counter-rotating region has a peak rotation velocity
of 21 km/s as compared to the main galaxy's rotation speed of greater than 45
km/s in the opposite direction. The counter-rotating region extends to a radius
of 4'' (0.6 kpc), slightly smaller than the half-light radius of the galaxy
which is 5.3'' (0.8 kpc) and is confined to a disk whose scale height is less
than 0.8'' (0.1 kpc). We compute an age and metallicity of the inner
counter-rotating region of 3 +/- 0.5 Gyr and [Fe/H] = 0.2 +/- 0.2 dex, based on
Lick absorption-line indices. The lack of other large galaxies in this region
limits possible scenarios for the formation of the counter-rotating core. We
discuss several scenarios and favor one in which NGC 770 accreted a small
gas-rich dwarf galaxy during a very minor merging event. If this scenario is
correct, it represents one of the few known examples of merging between two
dwarf-sized galaxies.Comment: 26 pages, 9 figures. Accepted to AJ. See this
http://www.ociw.edu/~mgeha/geha.ps.gz for version with high resolution
figure
The Anisotropic Distribution of Galactic Satellites
We present a study of the spatial distribution of subhalos in galactic dark
matter halos using dissipationless cosmological simulations of the concordance
LCDM model. We find that subhalos are distributed anisotropically and are
preferentially located along the major axes of the triaxial mass distributions
of their hosts. The Kolmogorov-Smirnov probability for drawing our simulated
subhalo sample from an isotropic distribution is P_KS \simeq 1.5 \times
10^{-4}. An isotropic distribution of subhalos is thus not the correct null
hypothesis for testing the CDM paradigm. The nearly planar distribution of
observed Milky Way (MW) satellites is marginally consistent (probability \simeq
0.02) with being drawn randomly from the subhalo distribution in our
simulations. Furthermore, if we select the subhalos likely to be luminous, we
find a distribution that is consistent with the observed MW satellites. In
fact, we show that subsamples of the subhalo population with a
centrally-concentrated radial distribution, similar to that of the MW dwarfs,
typically exhibit a comparable degree of planarity. We explore the origin of
the observed subhalo anisotropy and conclude that it is likely due to (1)
preferential accretion of subhalos along filaments, often closely aligned with
the major axis of the host halo, and (2) evolution of satellite orbits within
the prolate, triaxial potentials typical of CDM halos. Agreement between
predictions and observations requires the major axis of the outer dark matter
halo of the Milky Way to be nearly perpendicular to the disk. We discuss
possible observational tests of such disk-halo alignment with current large
galaxy surveys.Comment: 14 pages (including appendix), 9 figures. Accepted for Publication in
ApJ. Minor changes to reflect referee's comment
Triaxial Modeling of Halo Density Profiles with High-resolution N-body Simulations
We present a detailed non-spherical modeling of dark matter halos on the
basis of a combined analysis of the high-resolution halo simulations (12 halos
with particles within their virial radius) and the large
cosmological simulations (5 realizations with particles in a
Mpc boxsize). The density profiles of those simulated halos are well
approximated by a sequence of the concentric triaxial distribution with their
axis directions being fairly aligned. We characterize the triaxial model
quantitatively by generalizing the universal density profile which has
previously been discussed only in the framework of the spherical model. We
obtain a series of practically useful fitting formulae in applying the triaxial
model; the mass and redshift dependence of the axis ratio, the mean of the
concentration parameter, and the probability distribution functions of the the
axis ratio and the concentration parameter. These accurate fitting formulae
form a complete description of the triaxial density profiles of halos in Cold
Dark Matter models. Our current description of the dark halos will be
particularly useful in predicting a variety of nonsphericity effects, to a
reasonably reliable degree, including the weak and strong lens statistics, the
orbital evolution of galactic satellites and triaxiality of galactic halos, and
the non-linear clustering of dark matter. In addition, this provides a useful
framework for the non-spherical modeling of the intra-cluster gas, which is
crucial in discussing the gas and temperature profiles of X-ray clusters and
the Hubble constant estimated via the Sunyaev -- Zel'dovich effect.Comment: 39 pages with 19 figures; final version match the publication in ApJ
(Vol 574, 538, 2002
Abnormal expansion of naĂŻve B lymphocytes after unrelated cord blood transplantation â a case report
A 33-year-old woman underwent unrelated cord blood transplantation (U-CBT) for myelodysplastic syndrome (MDS)-related secondary AML. She showed impressive increases in the number of CD19(+) B cells in bone marrow and CD19(+)27(â)IgD(+) B cells in peripheral blood from about 1 month to 3 months after U-CBT. The serum level of IL-6 temporarily increased after transplantation, and this increase seemed to be correlated with the expansion of CD19(+) B cells. Although, compared with BMT, little is known about the kinetics of hematological and immunological reconstitution in U-CBT, there was initial B-cell recovery after CBT as some described. This B cell recovery may be associated with a high number of B-cell precursors present in cord blood (CB). The phenomenon of naĂŻve B lymphocyte expansion that we found might be associated with a high number of B-cell precursors present in CB
Orbital Instabilities in a Triaxial Cusp Potential
This paper constructs an analytic form for a triaxial potential that
describes the dynamics of a wide variety of astrophysical systems, including
the inner portions of dark matter halos, the central regions of galactic
bulges, and young embedded star clusters. Specifically, this potential results
from a density profile of the form , where the radial
coordinate is generalized to triaxial form so that . Using the resulting analytic form of the potential, and the
corresponding force laws, we construct orbit solutions and show that a robust
orbit instability exists in these systems. For orbits initially confined to any
of the three principal planes, the motion in the perpendicular direction can be
unstable. We discuss the range of parameter space for which these orbits are
unstable, find the growth rates and saturation levels of the instability, and
develop a set of analytic model equations that elucidate the essential physics
of the instability mechanism. This orbit instability has a large number of
astrophysical implications and applications, including understanding the
formation of dark matter halos, the structure of galactic bulges, the survival
of tidal streams, and the early evolution of embedded star clusters.Comment: 50 pages, accepted for publication in Ap
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