5,787 research outputs found
Strong magnetic fields and large rotation measures in protogalaxies by supernova seeding
We present a model for the seeding and evolution of magnetic fields in
protogalaxies. Supernova (SN) explosions during the assembly of a protogalaxy
provide magnetic seed fields, which are subsequently amplified by compression,
shear flows and random motions. We implement the model into the MHD version of
the cosmological N-body / SPH simulation code GADGET and we couple the magnetic
seeding directly to the underlying multi-phase description of star formation.
We perform simulations of Milky Way-like galactic halo formation using a
standard LCDM cosmology and analyse the strength and distribution of the
subsequent evolving magnetic field. A dipole-shape divergence-free magnetic
field is injected at a rate of 10^{-9}G / Gyr within starforming regions, given
typical dimensions and magnetic field strengths in canonical SN remnants.
Subsequently, the magnetic field strength increases exponentially on timescales
of a few ten million years. At redshift z=0, the entire galactic halo is
magnetized and the field amplitude is of the order of a few G in the
center of the halo, and 10^{-9} G at the virial radius. Additionally, we
analyse the intrinsic rotation measure (RM) of the forming galactic halo over
redshift. The mean halo intrinsic RM peaks between redshifts z=4 and z=2 and
reaches absolute values around 1000 rad m^{-2}. While the halo virializes
towards redshift z=0, the intrinsic RM values decline to a mean value below 10
rad m^{-2}. At high redshifts, the distribution of individual starforming, and
thus magnetized regions is widespread. In our model for the evolution of
galactic magnetic fields, the seed magnetic field amplitude and distribution is
no longer a free parameter, but determined self-consistently by the star
formation process occuring during the formation of cosmic structures.Comment: 13 pages, 14 figures, accepted to MNRAS after moderate revisio
Synthetic X-ray and radio maps for two different models of Stephan's Quintet
We present simulations of the compact galaxy group Stephan's Quintet (SQ)
including magnetic fields, performed with the N-body/smoothed particle
hydrodynamics (SPH) code \textsc{Gadget}. The simulations include radiative
cooling, star formation and supernova feedback. Magnetohydrodynamics (MHD) is
implemented using the standard smoothed particle magnetohydrodynamics (SPMHD)
method. We adapt two different initial models for SQ based on Renaud et al. and
Hwang et al., both including four galaxies (NGC 7319, NGC 7320c, NGC 7318a and
NGC 7318b). Additionally, the galaxies are embedded in a magnetized, low
density intergalactic medium (IGM). The ambient IGM has an initial magnetic
field of G and the four progenitor discs have initial magnetic fields
of G. We investigate the morphology, regions of star
formation, temperature, X-ray emission, magnetic field structure and radio
emission within the two different SQ models. In general, the enhancement and
propagation of the studied gaseous properties (temperature, X-ray emission,
magnetic field strength and synchrotron intensity) is more efficient for the SQ
model based on Renaud et al., whose galaxies are more massive, whereas the less
massive SQ model based on Hwang et al. shows generally similar effects but with
smaller efficiency. We show that the large shock found in observations of SQ is
most likely the result of a collision of the galaxy NGC 7318b with the IGM.
This large group-wide shock is clearly visible in the X-ray emission and
synchrotron intensity within the simulations of both SQ models. The order of
magnitude of the observed synchrotron emission within the shock front is
slightly better reproduced by the SQ model based on Renaud et al., whereas the
distribution and structure of the synchrotron emission is better reproduced by
the SQ model based on Hwang et al..Comment: 20 pages, 15 figures, accepted to MNRA
Computer model calibration with large non-stationary spatial outputs: application to the calibration of a climate model
Bayesian calibration of computer models tunes unknown input parameters by
comparing outputs with observations. For model outputs that are distributed
over space, this becomes computationally expensive because of the output size.
To overcome this challenge, we employ a basis representation of the model
outputs and observations: we match these decompositions to carry out the
calibration efficiently. In the second step, we incorporate the non-stationary
behaviour, in terms of spatial variations of both variance and correlations, in
the calibration. We insert two integrated nested Laplace
approximation-stochastic partial differential equation parameters into the
calibration. A synthetic example and a climate model illustration highlight the
benefits of our approach
Roots of Ehrhart Polynomials of Smooth Fano Polytopes
V. Golyshev conjectured that for any smooth polytope P of dimension at most
five, the roots z\in\C of the Ehrhart polynomial for P have real part equal
to -1/2. An elementary proof is given, and in each dimension the roots are
described explicitly. We also present examples which demonstrate that this
result cannot be extended to dimension six.Comment: 10 page
Using meta-patterns to construct patterns
International audienceThe pattern notion defines techniques allowing the existing knowledge reuse. Usually, the knowledge encapsulated in these patterns is stored in classic library repositories that quickly become overcrowded. To solve this problem, [1] proposes the use of process maps in order to organize and select them. But the completeness of the maps is a very important problem that has to be solved in order to offer a useful guidance to the method engineer. This paper proposes a guideline pattern construction technique guiding engineers when creating the maps
An integrated approach to modelling the fluid-structure interaction of a collapsible tube
The well known collapsible tube experiment was conducted to obtain flow, pressure and materials property data for steady state conditions. These were then used as the boundary conditions for a fully coupled fluid-structure interaction (FSI) model using a propriety computer code, LS-DYNA. The shape profiles for the tube were also recorded. In order to obtain similar collapse modes to the experiment, it was necessary to model the tube flat, and then inflate it into a circular profile, leaving residual stresses in the walls. The profile shape then agreed well with the experimental ones. Two departures from the physical properties were required to reduce computer time to an acceptable level. One of these was the lowering of the speed of sound by two orders of magnitude which, due to the low velocities involved, still left the mach number below 0.2. The other was to increase the thickness of the tube to prevent the numerical collapse of elements. A compensation for this was made by lowering the Young's modulus for the tube material. Overall the results are qualitatively good. They give an indication of the power of the current FSI algorithms and the need to combine experiment and computer models in order to maximise the information that can be extracted both in terms of quantity and quality
On the effect of galactic outflows in cosmological simulations of disc galaxies
We investigate the impact of galactic outflow modelling on the formation and evolution of a disc galaxy, by performing a suite of cosmological simulations with zoomed-in initial conditions of a Milky Way-sized halo. We verify how sensitive the general properties of the simulated galaxy are to the way in which stellar feedback triggered outflows are implemented, keeping initial conditions, simulation code and star formation (SF) model all fixed. We present simulations that are based on a version of the GADGET3 code where our sub-resolution model is coupled with an advanced implementation of Smoothed Particle Hydrodynamics that ensures a more accurate fluid sampling and an improved description of gas mixing and hydrodynamical instabilities. We quantify the strong interplay between the adopted hydrodynamic scheme and the sub-resolution model describing SF and feedback. We consider four different galactic outflow models, including the one introduced by Dalla Vecchia and Schaye (2012) and a scheme that is inspired by the Springel and Hernquist (2003) model. We find that the sub-resolution prescriptions adopted to generate galactic outflows are the main shaping factor of the stellar disc component at low redshift. The key requirement that a feedback model must have to be successful in producing a disc-dominated galaxy is the ability to regulate the high-redshift SF (responsible for the formation of the bulge component), the cosmological infall of gas from the large-scale environment, and gas fall-back within the galactic radius at low redshift, in order to avoid a too high SF rate at z=0
Spatially resolved H2 emission from the disk around T Tau N
We report the detection of quiescent H2 emission in a spatially resolved
ring-like structure within 100 AU of T Tau N. We present evidence to show that
the emission most likely arises from shocks in the atmosphere of a nearly
face-on disk around T Tau N. Using high spatial resolution 3D spectroscopic
K-band data, we trace the spatial distribution of several H2 NIR rovibrational
lines in the vicinity of T Tau N. We detect weak H2 emission from the v=1-0
S(0), S(1), Q(1) lines and the v=2-1 S(1) line in a ring-like structure around
T Tau N between 0.1'' (~15 AU) and 0.7'' (~100AU) from the star. The v=1-0 S(0)
and v=2-1 S(1) lines are detected only in the outer parts of the ring
structure. Closer to the star, the strong continuum limits our sensitivity to
these lines. The total flux of the v=1-0 S(1) line is 1.8 *10^{-14} ergs
s^{-1}cm^{-2}, similar to previous measurements of H2 in circumstellar disks.
The velocity of the H2 emitting gas around T Tau N is consistent with the rest
velocity of the star, and the H2 does not seem to be part of a collimated
outflow. Both shocks impinging on the surface of a disk and irradiation of a
disk by UV-photons and X-rays from the central star are plausible candidates
for the H2 excitation mechanism. However, irradiation should not create a large
degree of excitation at radii larger than 20 AU. Most likely the H2 emission
arises in the atmosphere of a flared disk with radius 85-100 AU and mass
0.005-0.5Msun, where the gas is excited by shocks created when a wide-angle
wind impinges on the disk. The H2 emission could also originate from shock
excitation in the cavity walls of an envelope, but this requires an unusually
high velocity of the wide-angle wind from T Tau N.Comment: Accepted by A&
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