648 research outputs found
Building Merger Trees from Cosmological N-body Simulations
Although a fair amount of work has been devoted to growing Monte-Carlo merger
trees which resemble those built from an N-body simulation, comparatively
little effort has been invested in quantifying the caveats one necessarily
encounters when one extracts trees directly from such a simulation. To somewhat
revert the tide, this paper seeks to provide its reader with a comprehensive
study of the problems one faces when following this route. The first step to
building merger histories of dark matter haloes and their subhaloes is to
identify these structures in each of the time outputs (snapshots) produced by
the simulation. Even though we discuss a particular implementation of such an
algorithm (called AdaptaHOP) in this paper, we believe that our results do not
depend on the exact details of the implementation but extend to most if not all
(sub)structure finders. We then highlight different ways to build merger
histories from AdaptaHOP haloes and subhaloes, contrasting their various
advantages and drawbacks. We find that the best approach to (sub)halo merging
histories is through an analysis that goes back and forth between
identification and tree building rather than one which conducts a
straightforward sequential treatment of these two steps. This is rooted in the
complexity of the merging trees which have to depict an inherently dynamical
process from the partial temporal information contained in the collection of
instantaneous snapshots available from the N-body simulation.Comment: 19 pages, 28 figure
Bewaarproeven op voorverpakte verse haring (<i>Clupea harengus</i> L.) onder gesimuleerde verkoopsomstandigheden
Prepackaged herring was stored under simulated selling conditions for up to 5 days: by day (from 9 to 21 h) at 0°, 4° and 8°C and by night at 0°C. Samples withdrawn every day were analysed by sensory, chemical and bacteriological quality determination methods. Gram-negative bacteria were indentified according to the Shewan scheme. The results indicated that the herring had a shelf life of > 5 days at 0°C, 5 days at 4°-2°C and 4 days at 8°C- 0°C. The results indicated that the herring had a total-shelf life of > 5 days revealed by the counts of total aerobic bacteria and by the Enterobacteriaceae. At the end of the storage period the Pseudomonas and the Acinetobacter group represented {approx} 80% of total flora. Furthermore the results showed that Pseudomonas spoilers have a more psychrophilic character than the Acinetobacter group
Cloud Dispersal in Turbulent Flows
Cold clouds embedded in warm media are very common objects in astrophysics.
Their disruption timescale depends strongly on the dynamical configuration. We
discuss the evolution of an initially homogeneous cold cloud embedded in warm
turbulent gas. Within a couple of dynamical timescales, the filling factor of
the cold gas within the original cloud radius drops below 50%. Turbulent
diffusivities estimated from the time evolution of radial filling factor
profiles are not constant with time. Cold and warm gas are bodily transported
by turbulence and mixed. This is only mildly indicated by column density maps.
The radiation field within the cloud, however, increases by several orders of
magnitudes due to the mixing, with possible consequences for cloud chemistry
and evolution within a few dynamical timescales.Comment: 11 pages, 12 figures, accepted by MNRA
On the filamentary environment of galaxies
The correlation between the large-scale distribution of galaxies and their
spectroscopic properties at z=1.5 is investigated using the Horizon MareNostrum
cosmological run.
We have extracted a large sample of 10^5 galaxies from this large
hydrodynamical simulation featuring standard galaxy formation physics. Spectral
synthesis is applied to these single stellar populations to generate spectra
and colours for all galaxies. We use the skeleton as a tracer of the cosmic web
and study how our galaxy catalogue depends on the distance to the skeleton. We
show that galaxies closer to the skeleton tend to be redder, but that the
effect is mostly due to the proximity of large haloes at the nodes of the
skeleton, rather than the filaments themselves.
This effects translate into a bimodality in the colour distribution of our
sample. The origin of this bimodality is investigated and seems to follow from
the ram pressure stripping of satellite galaxies within the more massive
clusters of the simulation.
The virtual catalogues (spectroscopical properties of the MareNostrum
galaxies at various redshifts) are available online at
http://www.iap.fr/users/pichon/MareNostrum/cataloguesComment: 18 pages, 27 figures, accepted for publication in MNRA
Magnetized Non-linear Thin Shell Instability: Numerical Studies in 2D
We revisit the analysis of the Non-linear Thin Shell Instability (NTSI)
numerically, including magnetic fields. The magnetic tension force is expected
to work against the main driver of the NTSI -- namely transverse momentum
transport. However, depending on the field strength and orientation, the
instability may grow. For fields aligned with the inflow, we find that the NTSI
is suppressed only when the Alfv\'en speed surpasses the (supersonic)
velocities generated along the collision interface. Even for fields
perpendicular to the inflow, which are the most effective at preventing the
NTSI from developing, internal structures form within the expanding slab
interface, probably leading to fragmentation in the presence of self-gravity or
thermal instabilities. High Reynolds numbers result in local turbulence within
the perturbed slab, which in turn triggers reconnection and dissipation of the
excess magnetic flux. We find that when the magnetic field is initially aligned
with the flow, there exists a (weak) correlation between field strength and gas
density. However, for transverse fields, this correlation essentially vanishes.
In light of these results, our general conclusion is that instabilities are
unlikely to be erased unless the magnetic energy in clouds is much larger than
the turbulent energy. Finally, while our study is motivated by the scenario of
molecular cloud formation in colliding flows, our results span a larger range
of applicability, from supernovae shells to colliding stellar winds.Comment: 12 pages, 17 figures, some of them at low resolution. Submitted to
ApJ, comments welcom
Reionization history constraints from neural network based predictions of high-redshift quasar continua
Observations of the early Universe suggest that reionization was complete by
, however, the exact history of this process is still unknown. One
method for measuring the evolution of the neutral fraction throughout this
epoch is via observing the Ly damping wings of high-redshift quasars.
In order to constrain the neutral fraction from quasar observations, one needs
an accurate model of the quasar spectrum around Ly, after the spectrum
has been processed by its host galaxy but before it is altered by absorption
and damping in the intervening IGM. In this paper, we present a novel machine
learning approach, using artificial neural networks, to reconstruct quasar
continua around Ly. Our QSANNdRA algorithm improves the error in this
reconstruction compared to the state-of-the-art PCA-based model in the
literature by 14.2% on average, and provides an improvement of 6.1% on average
when compared to an extension thereof. In comparison with the extended PCA
model, QSANNdRA further achieves an improvement of 22.1% and 16.8% when
evaluated on low-redshift quasars most similar to the two high-redshift quasars
under consideration, ULAS J1120+0641 at and ULAS J1342+0928 at
, respectively. Using our more accurate reconstructions of these two
quasars, we estimate the neutral fraction of the IGM using a homogeneous
reionization model and find at
and at . Our
results are consistent with the literature and favour a rapid end to
reionization
Cooling, Gravity and Geometry: Flow-driven Massive Core Formation
We study numerically the formation of molecular clouds in large-scale
colliding flows including self-gravity. The models emphasize the competition
between the effects of gravity on global and local scales in an isolated cloud.
Global gravity builds up large-scale filaments, while local gravity --
triggered by a combination of strong thermal and dynamical instabilities --
causes cores to form. The dynamical instabilities give rise to a local focusing
of the colliding flows, facilitating the rapid formation of massive
protostellar cores of a few 100 M. The forming clouds do not reach an
equilibrium state, though the motions within the clouds appear comparable to
``virial''. The self-similar core mass distributions derived from models with
and without self-gravity indicate that the core mass distribution is set very
early on during the cloud formation process, predominantly by a combination of
thermal and dynamical instabilities rather than by self-gravity.Comment: 13 pages, 12 figures, accepted by Ap
Evidence for Multiple Mergers among Ultraluminous IR Galaxies (ULIRGs): Remnants of Compact Groups?
In a large sample of ULIRGs imaged with HST, we have identified a significant
subsample that shows evidence for multiple mergers. The evidence is seen among
two classes of ULIRGs: (1) those with multiple remnant nuclei in their core,
sometimes accompanied by a complex system of tidal tails; and (2) those that
are in fact dense groupings of interacting (soon-to-merge) galaxies. We
conservatively estimate that, in the redshift range 0.05<z<0.20, at least 20
(out of 99) ULIRGs satisfy one or both of these criteria. We present several
cases and discuss the possibility that the progenitors of ULIRGs may be the
more classical weakly interacting compact groups of galaxies (Hickson 1997). An
evolutionary progression is consistent with the results: from compact groups to
pairs to ULIRGs to ellipticals. The last step follows the blowout of gas and
dust from the ULIRG.Comment: 5 pages, including 1 color postscript figure. Published in the
Astrophysical Journal Letters (1 Feb 2000). Replaced with final edited
version, including corrected typos and additional references, plus the color
figure has been improved and is only available her
Prediction of dynamic strains on a monopile offshore wind turbine using virtual sensors
The monitoring of the condition of the offshore wind turbine during its operational states offers the possibility of performing accurate assessments of the remaining life-time as well as supporting maintenance decisions during its entire life. The efficacy of structural monitoring in the case of the offshore wind turbine, though, is undermined by the practical limitations connected to the measurement system in terms of cost, weight and feasibility of sensor mounting (e.g. at muddline level 30m below the water level). This limitation is overcome by reconstructing the full-field response of the structure based on the limited number of measured accelerations and a calibrated Finite Element Model of the system. A modal decomposition and expansion approach is used for reconstructing the responses at all degrees of freedom of the finite element model. The paper will demonstrate the possibility to predict dynamic strains from acceleration measurements based on the aforementioned methodology. These virtual dynamic strains will then be evaluated and validated based on actual strain measurements obtained from a monitoring campaign on an offshore Vestas V90 3 MW wind turbine on a monopile foundation
Simulations of Dust in Interacting Galaxies I: Dust Attenuation
A new Monte-Carlo radiative-transfer code, Sunrise, is used in conjunction
with hydrodynamic simulations of major galaxy mergers to calculate the effects
of dust in such systems. The simulations are in good agreement with
observations of dust absorption in starburst galaxies, and the dust has a
profound effect on their appearance. The dust attenuation increases with
luminosity such that at peak luminosities ~90% of the bolometric luminosity is
absorbed by dust. In general, the detailed appearance of the merging event
depends on the stage of the merger and the geometry of the encounter. The
fraction of bolometric energy absorbed by the dust, however, is a robust
quantity that can be predicted from the intrinsic properties bolometric
luminosity, baryonic mass, star-formation rate, and metallicity of the system.
This paper presents fitting formulae, valid over a wide range of masses and
metallicities, from which the absorbed fraction of luminosity (and consequently
also the infrared dust luminosity) can be predicted. The attenuation of the
luminosity at specific wavelengths can also be predicted, albeit with a larger
scatter due to the variation with viewing angle. These formulae for dust
attenuation appear to be valid for both isolated and interacting galaxies, are
consistent with earlier studies, and would be suitable for inclusion in
theoretical models, e.g. semi-analytic models of galaxy formation.Comment: 12 pages, 10 figures, submitted to Ap
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