1,366 research outputs found
A monolithic collapse origin for the thin/thick disc structure of ESO 243-49
ESO 243-49 is a high-mass (circular velocity ) edge-on S0 galaxy in the Abell 2877 cluster at a distance of
. To elucidate the origin of its thick disc, we use MUSE
science verification data to study its kinematics and stellar populations. The
thick disc emits of the light at heights in excess of
(). The rotation velocities of its stars
lag by compared to those in the thin disc, which is
compatible with the asymmetric drift. The thick disc is found to be more
metal-poor than the thin disc, but both discs have old ages. We suggest an
internal origin for the thick disc stars in high-mass galaxies. We propose that
the thick disc formed either first in a turbulent phase with a high
star formation rate and that a thin disc formed shortly afterwards, or because of the dynamical heating of a thin pre-existing component. Either
way, the star formation in ESO 243-49 was quenched just a few Gyrs after the
galaxy was born and the formation of a thin and a thick disc must have occurred
before the galaxy stopped forming stars. The formation of the discs was so fast
that it could be described as a monolithic collapse where several generations
of stars formed in a rapid succession.Comment: Accepted for publication in A&A. The reduced data-cube as well as the
data necessary to build the kinematic and stellar population maps are
available at
https://etsin.avointiede.fi/dataset/urn-nbn-fi-csc-kata2016092414291163237
Exponential instability in the fractional Calder\'on problem
In this note we prove the exponential instability of the fractional
Calder\'on problem and thus prove the optimality of the logarithmic stability
estimate from \cite{RS17}. In order to infer this result, we follow the
strategy introduced by Mandache in \cite{M01} for the standard Calder\'on
problem. Here we exploit a close relation between the fractional Calder\'on
problem and the classical Poisson operator. Moreover, using the construction of
a suitable orthonormal basis, we also prove (almost) optimality of the Runge
approximation result for the fractional Laplacian, which was derived in
\cite{RS17}. Finally, in one dimension, we show a close relation between the
fractional Calder\'on problem and the truncated Hilbert transform.Comment: 17 page
Frequency-dependent current correlation functions from scattering theory
We present a general formalism based on scattering theory to calculate quantum correlation functions involving several time-dependent current operators. A key ingredient is the causality of the scattering matrix, which allows one to deal with arbitrary correlation functions. The formalism proves useful, e.g., in view of recent developments in full counting statistics of charge transfer, where detecting schemes have been proposed for measurement of frequency dependent spectra of higher moments. Some of these schemes are different from the well-known fictitious spin detector and therefore generally involve calculation of non-Keldysh-contour-ordered correlation functions. As an illustration of the approach we consider various third order correlation functions of current, including the usual third cumulant of current statistics. We investigate the frequency dependence of these correlation functions explicitly in the case of energy-independent scattering. The results can easily be generalized to the calculation of arbitrary nth order correlation functions, or to include the effect of interactions.Peer reviewe
Matter X waves
We predict that an ultra-cold Bose gas in an optical lattice can give rise to
a new form of condensation, namely matter X waves. These are non-spreading 3D
wave-packets which reflect the symmetry of the Laplacian with a negative
effective mass along the lattice direction, and are allowed to exist in the
absence of any trapping potential even in the limit of non-interacting atoms.
This result has also strong implications for optical propagation in periodic
structuresComment: 5 pages, 2 figure
REBOUND: An open-source multi-purpose N-body code for collisional dynamics
REBOUND is a new multi-purpose N-body code which is freely available under an
open-source license. It was designed for collisional dynamics such as planetary
rings but can also solve the classical N-body problem. It is highly modular and
can be customized easily to work on a wide variety of different problems in
astrophysics and beyond.
REBOUND comes with three symplectic integrators: leap-frog, the symplectic
epicycle integrator (SEI) and a Wisdom-Holman mapping (WH). It supports open,
periodic and shearing-sheet boundary conditions. REBOUND can use a Barnes-Hut
tree to calculate both self-gravity and collisions. These modules are fully
parallelized with MPI as well as OpenMP. The former makes use of a static
domain decomposition and a distributed essential tree. Two new collision
detection modules based on a plane-sweep algorithm are also implemented. The
performance of the plane-sweep algorithm is superior to a tree code for
simulations in which one dimension is much longer than the other two and in
simulations which are quasi-two dimensional with less than one million
particles.
In this work, we discuss the different algorithms implemented in REBOUND, the
philosophy behind the code's structure as well as implementation specific
details of the different modules. We present results of accuracy and scaling
tests which show that the code can run efficiently on both desktop machines and
large computing clusters.Comment: 10 pages, 9 figures, accepted by A&A, source code available at
https://github.com/hannorein/reboun
Quantification and physical analysis of nanoparticle emissions from a marine engine using different fuels and a laboratory wet scrubber
A marine test-bed diesel engine was used to study how international fuel sulfur content (FSC) regulations and wet scrubbing can affect physical properties of submicron exhaust particles. Particle size distributions, particle number and mass emission factors as well as effective densities of particulate emissions were measured for three distillate fuels of varying FSC and a laboratory wet scrubber. While particle number concentrations were reduced by up to 9% when switching to low FSC fuels, wet scrubbing led to increased ultrafine particulate emissions (<30 nm). Exhaust processed through the scrubber was also found to have particles with greater effective densities, a result that directly contradicts the particulate characteristics of low FSC fuel emissions. The results demonstrate that alternative pathways to comply with marine FSC regulations can have opposing effects and thus may have very different implications for important atmospheric processes. The relevance for air quality, and the potential implications for cloud and climate interactions are discussed
Thick discs in galaxies were most likely not accreted
The origin of thick discs in galaxies remains shrouded in mystery. A variety of formation scenarios has been proposed. Here we aim to test one such scenario where the thick disc stars are proposed to be accreted from satellite galaxies. In this scenario, in at least some galaxies a fraction of thick disc stars would rotate in a retrograde way, which would cause a large thick disc velocity lag. Here, we compare the rotation curves of the thin and the thick discs of eight edge-on galaxies observed with MUSE at the VLT. We find that the velocity lags of the thick discs are compatible with those expected from asymmetric drift. If we consider the galaxies with thick disc rotation curves in the literature, only one in about fifteen shows clear signs of an accreted thick disc. Based on simulations in the literature we estimate that if thick discs were accreted, at least one in six would show clear signs of retrograde material. Thus, there is a growing tension between the observations and the hypothesis that thick discs are made of accreted stars
Comparison of bar strengths in active and non-active galaxies
Bar strengths are compared between active and non-active galaxies for a
sample of 43 barred galaxies. The relative bar torques are determined using a
new technique (Buta and Block 2001), where maximum tangential forces are
calculated in the bar region, normalized to the axisymmetric radial force
field. We use JHK images of the 2 Micron All Sky Survey. We show a first clear
empirical indication that the ellipticies of bars are correlated with the
non-axisymmetric forces in the bar regions. We found that nuclear activity
appears preferentially in those early type galaxies in which the maximum bar
torques are weak and appear at quite large distances from the galactic center.
Most suprisingly the galaxies with the strongest bars are non-active. Our
results imply that the bulges may be important for the onset of nuclear
activity, but that the correlation between the nuclear activity and the early
type galaxies is not straightforward.Comment: MNRAS macro in tex format, 9 pages, 10 figure
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