5,386 research outputs found
Central dark matter content of early-type galaxies: scaling relations and connections with star formation histories
We examine correlations between masses, sizes and star formation histories for a large sample of low-redshift early-type galaxies, using a simple suite of dynamical and stellar population models. We confirm an anticorrelation between the size and stellar age and go on to survey for trends with the central content of dark matter (DM). An average relation between the central DM density and galaxy size of 〈ρDM〉∝R−2eff provides the first clear indication of cuspy DM haloes in these galaxies – akin to standard Λ cold dark matter haloes that have undergone adiabatic contraction. The DM density scales with galaxy mass as expected, deviating from suggestions of a universal halo profile for dwarf and late-type galaxies. We introduce a new fundamental constraint on galaxy formation by finding that the central DM fraction decreases with stellar age. This result is only partially explained by the size–age dependencies, and the residual trend is in the opposite direction to basic DM halo expectations. Therefore, we suggest that there may be a connection between age and halo contraction and that galaxies forming earlier had stronger baryonic feedback, which expanded their haloes, or lumpier baryonic accretion, which avoided halo contraction. An alternative explanation is a lighter initial mass function for older stellar populations
Protecting the operation from general and residual errors by continuous dynamical decoupling
We study the occurrence of errors in a continuously decoupled two-qubit state
during a quantum operation under decoherence. We consider a
realization of this quantum gate based on the Heisenberg exchange interaction,
which alone suffices for achieving universal quantum computation. Furthermore,
we introduce a continuous-dynamical-decoupling scheme that commutes with the
Heisenberg Hamiltonian to protect it from the amplitude damping and dephasing
errors caused by the system-environment interaction. We consider two
error-protection settings. One protects the qubits from both amplitude damping
and dephasing errors. The other features the amplitude damping as a residual
error and protects the qubits from dephasing errors only. In both settings, we
investigate the interaction of qubits with common and independent environments
separately. We study how errors affect the entanglement and fidelity for
different environmental spectral densities.Comment: Extended version of arXiv:1005.1666. To appear in PR
Evolution of central dark matter of early-type galaxies up to z ~ 0.8
We investigate the evolution of dark and luminous matter in the central
regions of early-type galaxies (ETGs) up to z ~ 0.8. We use a spectroscopically
selected sample of 154 cluster and field galaxies from the EDisCS survey,
covering a wide range in redshifts (z ~ 0.4-0.8), stellar masses ( ~ 10.5-11.5 dex) and velocity dispersions
( ~ 100-300 \, km/s). We obtain central dark matter (DM)
fractions by determining the dynamical masses from Jeans modelling of galaxy
aperture velocity dispersions and the from galaxy colours, and
compare the results with local samples. We discuss how the correlations of
central DM with galaxy size (i.e. the effective radius, ),
and evolve as a function of redshift, finding
clear indications that local galaxies are, on average, more DM dominated than
their counterparts at larger redshift. This DM fraction evolution with can
be only partially interpreted as a consequence of the size-redshift evolution.
We discuss our results within galaxy formation scenarios, and conclude that the
growth in size and DM content which we measure within the last 7 Gyr is
incompatible with passive evolution, while it is well reproduced in the
multiple minor merger scenario. We also discuss the impact of the IMF on our DM
inferences and argue that this can be non-universal with the lookback time. In
particular, we find the Salpeter IMF can be better accommodated by low redshift
systems, while producing stellar masses at high- which are unphysically
larger than the estimated dynamical masses (particularly for
lower- systems).Comment: 14 pages, 6 figures, 3 tables, MNRAS in pres
MOND and IMF variations in early-type galaxies from ATLAS3D
MOdified Newtonian dynamics (MOND) represents a phenomenological alternative
to dark matter (DM) for the missing mass problem in galaxies and clusters of
galaxies. We analyze the central regions of a local sample of
early-type galaxies from the survey, to see if the data can be
reproduced without recourse to DM. We estimate dynamical masses in the MOND
context through Jeans analysis, and compare to stellar masses
from stellar population synthesis. We find that the observed stellar
mass--velocity dispersion relation is steeper than expected assuming MOND with
a fixed stellar initial mass function (IMF) and a standard value for the
acceleration parameter . Turning from the space of observables to
model space, a) fixing the IMF, a universal value for cannot be
fitted, while, b) fixing and leaving the IMF free to vary, we find
that it is "lighter" (Chabrier-like) for low-dispersion galaxies, and "heavier"
(Salpeter-like) for high dispersions. This MOND-based trend matches inferences
from Newtonian dynamics with DM, and from detailed analysis of spectral
absorption lines, adding to the converging lines of evidence for a
systematically-varying IMF.Comment: 6 pages, 3 figures, accepted for publication on MNRAS Letters, typos
corrected and further references adde
Results from the LSND Neutrino Oscillation Search
The Liquid Scintillator Neutrino Detector (LSND) at the Los Alamos Meson
Physics Facility sets bounds on neutrino oscillations in the appearance channel
nu_mu_bar --> nu_e_bar by searching for the signature of the reaction nu_e_bar
p --> e^+ n: an e followed by a 2.2MeV gamma ray from neutron capture. Five
e^{+/-} -- gamma coincidences are observed in time with the LAMPF beam, with an
estimated background of 6.2 events. The 90\% confidence limits obtained are:
Delta (m^2) < 0.07eV^2 for sin^2 (2theta) = 1, and sin^2(2theta) < 6 10^{-3}
for Delta (m^2) > 20 eV^2.Comment: 10 pages, uses REVTeX and epsf macro
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