45 research outputs found
Spurious heating of stellar motions in simulated galactic discs by dark matter halo particles
Galaxie
The origin of the mass discrepancy-acceleration relation in ΛCDM
We examine the origin of the mass discrepancy–radial acceleration relation (MDAR) of disc galaxies. This is a tight empirical correlation between the disc centripetal acceleration and that expected from the baryonic component. The MDAR holds for most radii probed by disc kinematic tracers, regardless of galaxy mass or surface brightness. The relation has two characteristic accelerations: a0, above which all galaxies are baryon dominated, and amin, an effective minimum acceleration probed by kinematic tracers in isolated galaxies. We use a simple model to show that these trends arise naturally in Λ cold dark matter (ΛCDM). This is because (i) disc galaxies in ΛCDM form at the centre of dark matter haloes spanning a relatively narrow range of virial mass; (ii) cold dark matter halo acceleration profiles are self-similar and have a broad maximum at the centre, reaching values bracketed precisely by amin and a0 in that mass range and (iii) halo mass and galaxy size scale relatively tightly with the baryonic mass of a galaxy in any successful ΛCDM galaxy formation model. Explaining the MDAR in ΛCDM does not require modifications to the cuspy inner mass profiles of dark haloes, although these may help to understand the detailed rotation curves of some dwarf galaxies and the origin of extreme outliers from the main relation. The MDAR is just a reflection of the self-similar nature of cold dark matter haloes and of the physical scales introduced by the galaxy formation process
A high stability semiconductor laser system for a Sr-based optical lattice clock
We describe a frequency stabilized diode laser at 698 nm used for high
resolution spectroscopy of the 1S0-3P0 strontium clock transition. For the
laser stabilization we use state-of-the-art symmetrically suspended optical
cavities optimized for very low thermal noise at room temperature. Two-stage
frequency stabilization to high finesse optical cavities results in measured
laser frequency noise about a factor of three above the cavity thermal noise
between 2 Hz and 11 Hz. With this system, we demonstrate high resolution remote
spectroscopy on the 88Sr clock transition by transferring the laser output over
a phase-noise-compensated 200 m-long fiber link between two separated
laboratories. Our dedicated fiber link ensures a transfer of the optical
carrier with frequency stability of 7 \cdot 10^{-18} after 100 s integration
time, which could enable the observation of the strontium clock transition with
an atomic Q of 10^{14}. Furthermore, with an eye towards the development of
transportable optical clocks, we investigate how the complete laser system
(laser+optics+cavity) can be influenced by environmental disturbances in terms
of both short- and long-term frequency stability.Comment: 9 pages, 9 figures, submitted to Appl. Phys.
The impact of stochastic modelling on the predictive power of galaxy formation simulations
Galaxie
Amplitude to phase conversion of InGaAs pin photo-diodes for femtosecond lasers microwave signal generation
When a photo-diode is illuminated by a pulse train from a femtosecond laser,
it generates microwaves components at the harmonics of the repetition rate
within its bandwidth. The phase of these components (relative to the optical
pulse train) is known to be dependent on the optical energy per pulse. We
present an experimental study of this dependence in InGaAs pin photo-diodes
illuminated with ultra-short pulses generated by an Erbium-doped fiber based
femtosecond laser. The energy to phase dependence is measured over a large
range of impinging pulse energies near and above saturation for two typical
detectors, commonly used in optical frequency metrology with femtosecond laser
based optical frequency combs. When scanning the optical pulse energy, the
coefficient which relates phase variations to energy variations is found to
alternate between positive and negative values, with many (for high harmonics
of the repetition rate) vanishing points. By operating the system near one of
these vanishing points, the typical amplitude noise level of commercial-core
fiber-based femtosecond lasers is sufficiently low to generate state-of-the-art
ultra-low phase noise microwave signals, virtually immune to amplitude to phase
conversion related noise.Comment: 7 pages, 6 figures, submitted to Applied Physics
Quantum Computing and Quantum Simulation with Group-II Atoms
Recent experimental progress in controlling neutral group-II atoms for
optical clocks, and in the production of degenerate gases with group-II atoms
has given rise to novel opportunities to address challenges in quantum
computing and quantum simulation. In these systems, it is possible to encode
qubits in nuclear spin states, which are decoupled from the electronic state in
the S ground state and the long-lived P metastable state on the
clock transition. This leads to quantum computing scenarios where qubits are
stored in long lived nuclear spin states, while electronic states can be
accessed independently, for cooling of the atoms, as well as manipulation and
readout of the qubits. The high nuclear spin in some fermionic isotopes also
offers opportunities for the encoding of multiple qubits on a single atom, as
well as providing an opportunity for studying many-body physics in systems with
a high spin symmetry. Here we review recent experimental and theoretical
progress in these areas, and summarise the advantages and challenges for
quantum computing and quantum simulation with group-II atoms.Comment: 11 pages, 7 figures, review for special issue of "Quantum Information
Processing" on "Quantum Information with Neutral Particles
Non--power law behavior of the radial profile of phase--space density of halos
We study the pseudo phase-space density, , of
CDM dark matter halos with and without baryons (baryons+DM, and pure
DM), by using the model introduced in Del Popolo (2009), which takes into
account the effect of dynamical friction, ordered and random angular momentum,
baryons adiabatic contraction and dark matter baryons interplay. We examine the
radial dependence of over 9 orders of magnitude in radius
for structures on galactic and cluster of galaxies scales. We find that
is approximately a power-law only in the range of halo
radius resolved by current simulations (down to 0.1% of the virial radius)
while it has a non-power law behavior below the quoted scale, with inner
profiles changing with mass. The non-power-law behavior is more evident for
halos constituted both of dark matter and baryons while halos constituted just
of dark matter and with angular momentum chosen to reproduce a
Navarro-Frenk-White (NFW) density profile, are characterized by an
approximately power-law behavior. The results of the present paper lead to
conclude that density profiles of the NFW type are compatible with a power-law
behavior of , while those flattening to the halo center,
like those found in Del Popolo (2009) or the Einasto profile, or the Burkert
profile, cannot produce radial profile of the pseudo-phase-space density that
are power-laws at all radii. The results argue against universality of the
pseudo phase-space density and as a consequence argue against universality of
density profiles constituted by dark matter and baryons as also discussed in
Del Popolo (2009).Comment: 20 pages; 7 figs. arXiv admin note: substantial text overlap with
arXiv:0906.4447 and arXiv:1012.432
Testing General Relativity with Atomic Clocks
We discuss perspectives for new tests of general relativity which are based
on recent technological developments as well as new ideas. We focus our
attention on tests performed with atomic clocks and do not repeat arguments
present in the other contributions to the present volume. In particular, we
present the scientific motivations of the space projects ACES and SAGAS.Comment: Contribution for "The Nature of Gravity" (eds. F. Everitt et al