2,109 research outputs found
Schwarzschild models of the Sculptor dSph galaxy
We have developed a spherically symmetric dynamical model of a dwarf
spheroidal galaxy using the Schwarzschild method. This type of modelling yields
constraints both on the total mass distribution (e.g. enclosed mass and scale
radius) as well as on the orbital structure of the system modelled (e.g.
velocity anisotropy). Therefore not only can we derive the dark matter content
of these systems, but also explore possible formation scenarios. Here we
present preliminary results for the Sculptor dSph. We find that the mass of
Sculptor within 1kpc is 8.5\times10^(7\pm0.05) M\odot, its anisotropy profile
is tangentially biased and slightly more isotropic near the center. For an NFW
profile, the preferred concentration (~15) is compatible with cosmological
models. Very cuspy density profiles (steeper than NFW) are strongly disfavoured
for Sculptor.Comment: 2 pages, 4 figures, to appear in the proceedings of "Assembling the
Puzzle of the Milky Way", Le Grand Bornand (Apr. 17-22, 2011
Orbit-based dynamical models of the Sculptor dSph galaxy
We have developed spherically symmetric dynamical models of dwarf spheroidal
galaxies using Schwarzschild's orbit superposition method. This type of
modelling yields constraints both on the total mass distribution (e.g. enclosed
mass and scale radius) as well as on the orbital structure of the system (e.g.
velocity anisotropy). This method is thus less prone to biases introduced by
assumptions in comparison to the more commonly used Jeans modelling, and it
allows us to derive the dark matter content in a robust way. Here we present
our results for the Sculptor dwarf spheroidal galaxy, after testing our methods
on mock data sets. We fit both the second and fourth velocity moment profile to
break the mass-anisotropy degeneracy. For an NFW dark matter halo profile, we
find that the mass of Sculptor within 1 kpc is M_1kpc = 1.03 \pm 0.07 x 10^8
Msol, and that its velocity anisotropy profile is tangentially biased and
nearly constant with radius. The preferred concentration (c ~ 15) is low for
its dark matter mass but consistent within the scatter found in N-body
cosmological simulations. When we let the value of the central logarithmic
slope \alpha{} vary, we find that the best-fit model has \alpha{} = 0, although
an NFW cusp or shallower is consistent at 1 \sigma{} confidence level. On the
other hand, very cuspy density profiles with logarithmic central slopes
\alpha{} < -1.5 are strongly disfavoured for Sculptor.Comment: 18 pages, 12 figures, accepted by MNRAS
http://mnras.oxfordjournals.org/content/433/4/317
Mass growth and mergers: direct observations of the luminosity function of LRG satellite galaxies out to z=0.7 from SDSS and BOSS images
We present a statistical study of the luminosity functions of galaxies
surrounding luminous red galaxies (LRGs) at average redshifts =0.34 and
=0.65. The luminosity functions are derived by extracting source photometry
around more than 40,000 LRGs and subtracting foreground and background
contamination using randomly selected control fields. We show that at both
studied redshifts the average luminosity functions of the LRGs and their
satellite galaxies are poorly fitted by a Schechter function due to a
luminosity gap between the centrals and their most luminous satellites. We
utilize a two-component fit of a Schechter function plus a log-normal
distribution to demonstrate that LRGs are typically brighter than their most
luminous satellite by roughly 1.3 magnitudes. This luminosity gap implies that
interactions within LRG environments are typically restricted to minor mergers
with mass ratios of 1:4 or lower. The luminosity functions further imply that
roughly 35% of the mass in the environment is locked in the LRG itself,
supporting the idea that mass growth through major mergers within the
environment is unlikely. Lastly, we show that the luminosity gap may be at
least partially explained by the selection of LRGs as the gap can be reproduced
by sparsely sampling a Schechter function. In that case LRGs may represent only
a small fraction of central galaxies in similar mass halos.Comment: ApJ accepted versio
Tracking Down a Critical Halo Mass for Killing Galaxies through the Growth of the Red-Sequence
Red-sequence galaxies record the history of terminated star-formation in the
Universe and can thus provide important clues to the mechanisms responsible for
this termination. We construct composite samples of published cluster and field
galaxy photometry in order to study the build-up of galaxies on the
red-sequence, as parameterised by the dwarf-to-giant ratio (DGR). We find that
the DGR in clusters is higher than that of the field at all redshifts, implying
that the faint end of the red-sequence was established first in clusters. We
find that the DGR evolves with redshift for both samples, consistent with the
``down-sizing'' picture of star formation. We examine the predictions of
semi-analytic models for the DGR and find that neither the magnitude of its
environmental dependence nor its evolution is correctly predicted in the
models. Red-sequence DGRs are consistently too high in the models, the most
likely explanation being that the strangulation mechanism used to remove hot
gas from satellite galaxies is too efficient. Finally we present a simple toy
model including a threshold mass, below which galaxies are not strangled, and
show that this can predict the observed evolution of the field DGR.Comment: MNRAS letters accepted. 5 pages, 1 figur
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Evolutionary bi-stability in pathogen transmission mode
Many pathogens transmit to new hosts by both infection (horizontal transmission) and transfer to the
infected host's offspring (vertical transmission). These two transmission modes require speci®c adap-
tations of the pathogen that can be mutually exclusive, resulting in a trade-off between horizontal and
vertical transmission. We show that in mathematical models such trade-offs can lead to the simultaneous
existence of two evolutionary stable states (evolutionary bi-stability) of allocation of resources to the two
modes of transmission. We also show that jumping between evolutionary stable states can be induced by
gradual environmental changes. Using quantitative PCR-based estimates of abundance in seed and vege-
tative parts, we show that the pathogen of wheat, Phaeosphaeria nodorum, has jumped between two
distinct states of transmission mode twice in the past 160 years, which, based on published evidence,
we interpret as adaptation to environmental change. The ®nding of evolutionary bi-stability has impli-
cations for human, animal and other plant diseases. An ill-judged change in a disease control
programme could cause the pathogen to evolve a new, and possibly more damaging, combination of
transmission modes. Similarly, environmental changes can shift the balance between transmission
modes, with adverse effects on human, animal and plant health
The Chemical and Dynamical Evolution of Isolated Dwarf Galaxies
Using a suite of simulations (Governato et al. 2010) which successfully
produce bulgeless (dwarf) disk galaxies, we provide an analysis of their
associated cold interstellar media (ISM) and stellar chemical abundance
patterns. A preliminary comparison with observations is undertaken, in order to
assess whether the properties of the cold gas and chemistry of the stellar
components are recovered successfully. To this end, we have extracted the
radial and vertical gas density profiles, neutral hydrogen velocity dispersion,
and the power spectrum of structure within the ISM. We complement this analysis
of the cold gas with a brief examination of the simulations' metallicity
distribution functions and the distribution of alpha-elements-to-iron.Comment: To appear in the proceedings of the JENAM 2010 Symposium "Dwarf
Galaxies: Keys to Galaxy Formation and Evolution" (Lisbon, 9-10 September
2010), P. Papaderos, S. Recchi, G. Hensler (eds.), Springer Verlag (2011), in
pres
Galaxy Groups in the SDSS DR4: II. halo occupation statistics
We investigate various halo occupation statistics using a large galaxy group
catalogue constructed from the SDSS DR4 with an adaptive halo-based group
finder. The conditional luminosity function (CLF) is measured separately for
all, red and blue galaxies, as well as in terms of central and satellite
galaxies. The CLFs for central and satellite galaxies can be well modelled with
a log-normal distribution and a modified Schechter form, respectively. About
85% of the central galaxies and about 80% of the satellite galaxies in halos
with masses M_h\ga 10^{14}\msunh are red galaxies. These numbers decrease to
50% and 40%, respectively, in halos with M_h \sim 10^{12}\msunh. For halos of
a given mass, the distribution of the luminosities of central galaxies, ,
has a dispersion of about 0.15 dex. The mean luminosity (stellar mass) of the
central galaxies scales with halo mass as
() for halos with masses M\gg 10^{12.5}\msunh, and
both relations are significantly steeper for less massive halos. We also
measure the luminosity (stellar mass) gap between the first and second
brightest (most massive) member galaxies, (). These gap statistics, especially in halos with M_h \la
10^{14.0}\msunh, indicate that the luminosities of central galaxies are
clearly distinct from those of their satellites. The fraction of fossil groups,
defined as those groups with , ranges from for groups with M_h\sim 10^{14}\msunh to 18-60% for groups with
M_h\sim 10^{13}\msunh. Finally, we measure the fraction of satellites, which
changes from for galaxies with \rmag\sim -22.0 to for
galaxies with \rmag\sim -17.0. (abridged)Comment: 16 pages, 11 figures. Accepted for publication in Ap
Self-interacting dark matter and Higgs bosons in the SU(3)_C x SU(3)_L x U(1)_N model with right-handed neutrinos
We investigate the possibility that dark matter could be made from CP-even
and CP- odd Higgs bosons in the SU(3)_C X SU(3)_L X U(1)_N (3-3-1) model with
right-handed neutrinos. This self-interacting dark matters are stable without
imposing of new symmetry and should be weak-interacting.Comment: 7 pages, Latex, To appear in Europhys. Let
Topology of six degrees of freedom magnetic bearing
A novel magnetic topology has been designed for a six degrees of freedom, magnetically levitated and driven mirror, to be used in a three dimensional (3D) measurement system based on laser interferometry. The translations of the mirror are to be kept small, whereas the rotations are to be controlled over a large range with a high bandwidth and high accuracy. Finite element modelling (FEM) is used to analyze the proposed topology. For computational load reduction, a 2D FEM model has been derived from the actual 3D topology, which incorporates most of the magnetic subsystems. Simulations show that cross-influence between the actuators is small, that the forces and torques are proportional to the applied currents and that the angle of the rotor is of little influence. This allows the multiple in multiple out system to be regarded as multiple linear single in single out systems. ©2000 American Institute of Physics
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