7,221 research outputs found
Quantifying stellar radial migration in an N-body simulation: blurring, churning, and the outer regions of galaxy discs
Radial stellar migration in galactic discs has received much attention in
studies of galactic dynamics and chemical evolution, but remains a dynamical
phenomenon that needs to be fully quantified. In this work, using a Tree-SPH
simulation of an Sb-type disc galaxy, we quantify the effects of blurring
(epicyclic excursions) and churning (change of guiding radius). We quantify
migration (either blurring or churning) both in terms of flux (the number of
migrators passing at a given radius), and by estimating the population of
migrators at a given radius at the end of the simulation compared to
non-migrators, but also by giving the distance over which the migration is
effective at all radii. We confirm that the corotation of the bar is the main
source of migrators by churning in a bar-dominated galaxy, its intensity being
directly linked to the episode of a strong bar, in the first 1-3 Gyr of the
simulation. We show that within the outer Lindblad resonance (OLR), migration
is strongly dominated by churning, while blurring gains progressively more
importance towards the outer disc and at later times. Most importantly, we show
that the OLR limits the exchange of angular momentum, separating the disc in
two distinct parts with minimal or null exchange, except in the transition
zone, which is delimited by the position of the OLR at the epoch of the
formation of the bar, and at the final epoch. We discuss the consequences of
these findings for our understanding of the structure of the Milky Way disc.
Because the Sun is situated slightly outside the OLR, we suggest that the solar
vicinity may have experienced very limited churning from the inner disc.Comment: Accepted for publication in Astronomy and Astrophysics (acceptance
date: 27/04/15), 24 pages, 24 figure
Old stellar counter-rotating components in early-type galaxies from elliptical-spiral mergers
We investigate, by means of numerical simulations, the possibility of forming
counter-rotating old stellar components by major mergers between an elliptical
and a spiral galaxy. We show that counter-rotation can appear both in
dissipative and dissipationless retrograde mergers, and it is mostly associated
to the presence of a disk component, which preserves part of its initial spin.
In turn, the external regions of the two interacting galaxies acquire part of
the orbital angular momentum, due to the action of tidal forces exerted on each
galaxy by the companion.Comment: 6 pages, 15 figures. Accepted on Astronomy & Astrophysic
Star formation efficiency in galaxy interactions and mergers: a statistical study
We investigate the enhancement of star formation efficiency in galaxy
interactions and mergers, by numerical simulations of several hundred galaxy
collisions. All morphological types along the Hubble sequence are considered in
the initial conditions of the two colliding galaxies, with varying
bulge-to-disk ratios and gas mass fractions. Different types of orbits are
simulated, direct and retrograde, according to the initial relative energy and
impact parameter, and the resulting star formation history is compared to that
occuring in the two galaxies when they are isolated. Our principal results are:
(1) retrograde encounters have a larger star formation efficiency (SFE) than
direct encounters; (2) the amount of gas available in the galaxy is not the
main parameter governing the SFE in the burst phase; (3) there is an
anticorrelation between the amplitude of the star forming burst and the tidal
forces exerted per unit of time, which is due to the large amount of gas
dragged outside the galaxy by tidal tails in strong interactions; (4) globally,
the Kennicutt-Schmidt law is retrieved statistically for isolated galaxies,
interacting pairs and mergers; (5) the enhanced star formation is essentially
occurring in nuclear starbursts, triggered by inward gas flows driven by
non-axisymmetries in the galaxy disks. Direct encounters develop more
pronounced asymmetries than retrograde ones. Based on these statistical
results, we derive general laws for the enhancement of star formation in galaxy
interactions and mergers, as a function of the main parameters of the
encounter.Comment: 22 pages, 37 figures, 4 tables. Accepted on Astronomy & Astrophysic
Effects of diversification among assets in an agent-based market model
We extend to the multi-asset case the framework of a discrete time model of a
single asset financial market developed in Ghoulmie et al (2005). In
particular, we focus on adaptive agents with threshold behavior allocating
their resources among two assets. We explore numerically the effect of this
diversification as an additional source of complexity in the financial market
and we discuss its destabilizing role. We also point out the relevance of these
studies for financial decision making.Comment: 12 pages, 5 figures, accepted for publication in the Proceedings of
the Complex Systems II Conference at the Australian National University, 4-7
December 2007, Canberra, ACT Australi
The Power-law Tail Exponent of Income Distributions
In this paper we tackle the problem of estimating the power-law tail exponent
of income distributions by using the Hill's estimator. A subsample
semi-parametric bootstrap procedure minimising the mean squared error is used
to choose the power-law cutoff value optimally. This technique is applied to
personal income data for Australia and Italy.Comment: Latex2e v1.6; 8 pages with 3 figures; in press (Physica A
On the nature of the magnetic ground-state wave function of V_2O_3
After a brief historical introduction, we dwell on two recent experiments in
the low-temperature, monoclinic phase of V_2O_3: K-edge resonant x-ray
scattering and non-reciprocal linear dichroism, whose interpretations are in
conflict, as they require incompatible magnetic space groups. Such a conflict
is critically reviewed, in the light of the present literature, and new
experimental tests are suggested, in order to determine unambiguously the
magnetic group. We then focus on the correlated, non-local nature of the
ground-state wave function, that is at the basis of some drawbacks of the LDA+U
approach: we singled out the physical mechanism that makes LDA+U unreliable,
and indicate the way out for a possible remedy. Finally we explain, by means of
a symmetry argument related to the molecular wave function, why the magnetic
moment lies in the glide plane, even in the absence of any local symmetry at
vanadium sites.Comment: 7 pages, 1 figur
Propagation of UHECRs in cosmological backgrounds: some results from SimProp
Ultra-High-Energy Cosmic Ray (UHECR) nuclei propagating in cosmological
radiation backgrounds produce secondary particles detectable at Earth. SimProp
is a one dimensional code for extragalactic propagation of UHECR nuclei,
inspired by the kinetic approach of Aloisio et al. As in this approach, only a
subset of nuclei and nuclear channels are used as representative. We discuss
the validation of the code and present applications to UHECR experimental
results. In particular we present the expected fluxes of neutrinos produced in
some astrophysical scenario.Comment: Poster presented by A. Di Matteo at the 33rd International Cosmic Ray
Conference, Rio De Janeiro (Brasil) July 2-9 201
Cosmogenic neutrinos and ultra-high energy cosmic ray models
We use an updated version of {\it SimProp}, a Monte Carlo simulation scheme
for the propagation of ultra-high energy cosmic rays, to compute cosmogenic
neutrino fluxes expected on Earth in various scenarios. These fluxes are
compared with the newly detected IceCube events at PeV energies and with recent
experimental limits at EeV energies of the Pierre Auger Observatory. This
comparison allows us to draw some interesting conclusions about the source
models for ultra-high energy cosmic rays. We will show how the available
experimental observations are almost at the level of constraining such models,
mainly in terms of the injected chemical composition and cosmological evolution
of sources. The results presented here will also be important in the evaluation
of the discovery capabilities of the future planned ultra-high energy cosmic
ray and neutrino observatories.Comment: 15 pages, 8 figures, some reference added, version accepted for
publication in JCA
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