3,636 research outputs found
Validating Semi-Analytic Models of High-Redshift Galaxy Formation using Radiation Hydrodynamical Simulations
We use a cosmological hydrodynamic simulation calculated with Enzo and the
semi-analytic galaxy formation model (SAM) GAMMA to address the chemical
evolution of dwarf galaxies in the early universe. The long-term goal of the
project is to better understand the origin of metal-poor stars and the
formation of dwarf galaxies and the Milky Way halo by cross-validating these
theoretical approaches. We combine GAMMA with the merger tree of the most
massive galaxy found in the hydrodynamic simulation and compare the star
formation rate, the metallicity distribution function (MDF), and the
age-metallicity relationship predicted by the two approaches. We found that the
SAM can reproduce the global trends of the hydrodynamic simulation. However,
there are degeneracies between the model parameters and more constraints (e.g.,
star formation efficiency, gas flows) need to be extracted from the simulation
to isolate the correct semi-analytic solution. Stochastic processes such as
bursty star formation histories and star formation triggered by supernova
explosions cannot be reproduced by the current version of GAMMA. Non-uniform
mixing in the galaxy's interstellar medium, coming primarily from
self-enrichment by local supernovae, causes a broadening in the MDF that can be
emulated in the SAM by convolving its predicted MDF with a Gaussian function
having a standard deviation of ~0.2 dex. We found that the most massive galaxy
in the simulation retains nearby 100% of its baryonic mass within its virial
radius, which is in agreement with what is needed in GAMMA to reproduce the
global trends of the simulation.Comment: 26 pages, 13 figures, 2 tables, submitted to ApJ (version 2
Short gamma-ray bursts from dynamically-assembled compact binaries in globular clusters: pathways, rates, hydrodynamics and cosmological setting
We present a detailed assessment of the dynamical pathways leading to the
coalescence of compact objects in Globular Clusters (GCs) and Short Gamma-Ray
Burst (SGRB) production. We consider primordial binaries, dynamically formed
binaries (through tidal two-body and three-body exchange interactions) and
direct impacts of compact objects (WD/NS/BH). We show that if the primordial
binary fraction is small, close encounters dominate the production rate of
coalescing compact systems. We find that the two dominant channels are the
interaction of field NSs with dynamically formed binaries, and two-body
encounters. We then estimate the redshift distribution and host galaxy
demographics of SGRB progenitors, and find that GCs can provide a significant
contribution to the overall observed rate.
We have carried out hydrodynamical modeling of evolution of close stellar
encounters with WD/NS/BH, and show that there is no problem in accounting for
the energy budget of a typical SGRB. The particulars of each encounter are
variable and lead to interesting diversity: the encounter characteristics are
dependent on the impact parameter, in contrast to the merger scenario; the
nature of the compact star itself can produce very different outcomes; the
presence of tidal tails in which material falls back onto the central object at
later times is a robust feature of these calculations, with the mass involved
being larger than for binary mergers. It is thus possible to account
generically in this scenario for a prompt episode of energy release, as well as
for activity many dynamical time scales later (abridged).Comment: Accepted for publication in ApJ (24 pages, 19 figures
Dynamical friction of massive objects in galactic centres
Dynamical friction leads to an orbital decay of massive objects like young
compact star clusters or Massive Black Holes in central regions of galaxies.
The dynamical friction force can be well approximated by Chandrasekhar's
standard formula, but recent investigations show, that corrections to the
Coulomb logarithm are necessary. With a large set of N-body simulations we show
that the improved formula for the Coulomb logarithm fits the orbital decay very
well for circular and eccentric orbits. The local scale-length of the
background density distribution serves as the maximum impact parameter for a
wide range of power-law indices of -1 ... -5. For each type of code the
numerical resolution must be compared to the effective minimum impact parameter
in order to determine the Coulomb logarithm. We also quantify the correction
factors by using self-consistent velocity distribution functions instead of the
standard Maxwellian often used. These factors enter directly the decay
timescale and cover a range of 0.5 ... 3 for typical orbits. The new Coulomb
logarithm combined with self-consistent velocity distribution functions in the
Chandrasekhar formula provides a significant improvement of orbital decay times
with correction up to one order of magnitude compared to the standard case. We
suggest the general use of the improved formula in parameter studies as well as
in special applications.Comment: 22 pages, 28 figures, accepted by MNRA
Evolution of shocks and turbulence in major cluster mergers
We performed a set of cosmological simulations of major mergers in galaxy
clusters to study the evolution of merger shocks and the subsequent injection
of turbulence in the post-shock region and in the intra-cluster medium (ICM).
The computations were done with the grid-based, adaptive mesh refinement hydro
code Enzo, using an especially designed refinement criteria for refining
turbulent flows in the vicinity of shocks. A substantial amount of turbulence
energy is injected in the ICM due to major merger. Our simulations show that
the shock launched after a major merger develops an ellipsoidal shape and gets
broken by the interaction with the filamentary cosmic web around the merging
cluster. The size of the post-shock region along the direction of shock
propagation is about 300 kpc h^-1, and the turbulent velocity dispersion in
this region is larger than 100 km s^-1. Scaling analysis of the turbulence
energy with the cluster mass within our cluster sample is consistent with
M^(5/3), i.e. the scaling law for the thermal energy in the self-similar
cluster model. This clearly indicates the close relation between virialization
and injection of turbulence in the cluster evolution. We found that the ratio
of the turbulent to total pressure in the cluster core within 2 Gyr after the
major merger is larger than 10%, and it takes about 4 Gyr to get relaxed, which
is substantially longer than typically assumed in the turbulent re-acceleration
models, invoked to explain the statistics of observed radio halos. Striking
similarities in the morphology and other physical parameters between our
simulations and the "symmetrical radio relics" found at the periphery of the
merging cluster A3376 are finally discussed. In particular, the interaction
between the merger shock and the filaments surrounding the cluster could
explain the presence of "notch-like" features at the edges of the double
relics.Comment: 16 pages, 19 figures, Published in Astrophysical Journal (online) and
printed version will be published on 1st January, 201
BOSS-LDG: A Novel Computational Framework that Brings Together Blue Waters, Open Science Grid, Shifter and the LIGO Data Grid to Accelerate Gravitational Wave Discovery
We present a novel computational framework that connects Blue Waters, the
NSF-supported, leadership-class supercomputer operated by NCSA, to the Laser
Interferometer Gravitational-Wave Observatory (LIGO) Data Grid via Open Science
Grid technology. To enable this computational infrastructure, we configured,
for the first time, a LIGO Data Grid Tier-1 Center that can submit
heterogeneous LIGO workflows using Open Science Grid facilities. In order to
enable a seamless connection between the LIGO Data Grid and Blue Waters via
Open Science Grid, we utilize Shifter to containerize LIGO's workflow software.
This work represents the first time Open Science Grid, Shifter, and Blue Waters
are unified to tackle a scientific problem and, in particular, it is the first
time a framework of this nature is used in the context of large scale
gravitational wave data analysis. This new framework has been used in the last
several weeks of LIGO's second discovery campaign to run the most
computationally demanding gravitational wave search workflows on Blue Waters,
and accelerate discovery in the emergent field of gravitational wave
astrophysics. We discuss the implications of this novel framework for a wider
ecosystem of Higher Performance Computing users.Comment: 10 pages, 10 figures. Accepted as a Full Research Paper to the 13th
IEEE International Conference on eScienc
Properties of hierarchically forming star clusters
We undertake a systematic analysis of the early (< 0.5 Myr) evolution of
clustering and the stellar initial mass function in turbulent fragmentation
simulations. These large scale simulations for the first time offer the
opportunity for a statistical analysis of IMF variations and correlations
between stellar properties and cluster richness. The typical evolutionary
scenario involves star formation in small-n clusters which then progressively
merge; the first stars to form are seeds of massive stars and achieve a
headstart in mass acquisition. These massive seeds end up in the cores of
clusters and a large fraction of new stars of lower mass is formed in the outer
parts of the clusters. The resulting clusters are therefore mass segregated at
an age of 0.5 Myr, although the signature of mass segregation is weakened
during mergers. We find that the resulting IMF has a smaller exponent
(alpha=1.8-2.2) than the Salpeter value (alpha=2.35). The IMFs in subclusters
are truncated at masses only somewhat larger than the most massive stars (which
depends on the richness of the cluster) and an universal upper mass limit of
150 Msun is ruled out. We also find that the simulations show signs of the
IGIMF effect proposed by Weidner & Kroupa, where the frequency of massive stars
is suppressed in the integrated IMF compared to the IMF in individual clusters.
We identify clusters through the use of a minimum spanning tree algorithm which
allows easy comparison between observational survey data and the predictions of
turbulent fragmentation models. In particular we present quantitative
predictions regarding properties such as cluster morphology, degree of mass
segregation, upper slope of the IMF and the relation between cluster richness
and maximum stellar mass. [abridged]Comment: 21 Pages, 25 Figure
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