1,814 research outputs found
Seeding high-redshift QSOs by collisional runaway in primordial star clusters
We study how runaway stellar collisions in high-redshift, metal-poor star
clusters form very massive stars (VMSs) that can directly collapse to
intermediate-mass black holes (IMBHs). We follow the evolution of a pair of
neighbouring high-redshift mini-haloes with high-resolution, cosmological
hydrodynamical zoom-in simulations using the adaptive mesh refinement code
RAMSES combined with the non-equilibrium chemistry package KROME. The first
collapsing mini-halo is assumed to enrich the central nuclear star cluster
(NSC) of the other to a critical metallicity, sufficient for Population II
(Pop. II) star formation at redshift . Using the spatial
configuration of the flattened, asymmetrical gas cloud forming in the core of
the metal enriched halo, we set the initial conditions for simulations of an
initially non-spherical star cluster with the direct summation code NBODY6
which are compared to about 2000 NBODY6 simulations of spherical star clusters
for a wide range of star cluster parameters. The final mass of the VMS that
forms depends strongly on the initial mass and initial central density of the
NSC. For the initial central densities suggested by our RAMSES simulations,
VMSs with mass > 400 M can form in clusters with stellar masses of
M, and this can increase to well over 1000 M
for more massive and denser clusters. The high probability we find for forming
a VMS in these mini-haloes at such an early cosmic time makes collisional
runaway of Pop. II star clusters a promising channel for producing large
numbers of high-redshift IMBHs that may act as the seeds of supermassive black
holes
Approaches to analysis with infinitesimals following Robinson, Nelson, and others
This is a survey of several approaches to the framework for working with infinitesimals and infinite numbers, originally developed by Abraham Robinson in the 1960s, and their constructive engagement with the Cantor-Dedekind postulate and the Intended Interpretation hypothesis. We highlight some applications including (1) Loeb's approach to the Lebesgue measure, (2) a radically elementary approach to the vibrating string, (3) true infinitesimal differential geometry. We explore the relation of Robinson's and related frameworks to the multiverse view as developed by Hamkins. Keywords: axiomatisations, infinitesimal, nonstandard analysis, ultraproducts, superstructure, set-theoretic foundations, multiverse, naive integers, intuitionism, soritical properties, ideal elements, protozoa
From Nonstandard Analysis to various flavours of Computability Theory
As suggested by the title, it has recently become clear that theorems of
Nonstandard Analysis (NSA) give rise to theorems in computability theory (no
longer involving NSA). Now, the aforementioned discipline divides into
classical and higher-order computability theory, where the former (resp. the
latter) sub-discipline deals with objects of type zero and one (resp. of all
types). The aforementioned results regarding NSA deal exclusively with the
higher-order case; we show in this paper that theorems of NSA also give rise to
theorems in classical computability theory by considering so-called textbook
proofs.Comment: To appear in the proceedings of TAMC2017 (http://tamc2017.unibe.ch/
New Methods for Identifying Lyman Continuum Leakers and Reionization-Epoch Analogues
Identifying low-redshift galaxies that emit Lyman continuum radiation (LyC leakers) is one of the primary, indirect methods of studying galaxy formation in the epoch of reionization. However, not only has it proved challenging to identify such systems, it also remains uncertain whether the low-redshift LyC leakers are truly ‘analogues’ of the sources that reionized the Universe. Here, we use high-resolution cosmological radiation hydrodynamics simulations to examine whether simulated galaxies in the epoch of reionization share similar emission line properties to observed LyC leakers at z ∼ 3 and z ∼ 0. We find that the simulated galaxies with high LyC escape fractions (fesc) often exhibit high O32 and populate the same regions of the R23–O32 plane as z ∼ 3 LyC leakers. However, we show that viewing angle, metallicity, and ionization parameter can all impact where a galaxy resides on the O32–fesc plane. Based on emission line diagnostics and how they correlate with fesc, lower metallicity LyC leakers at z ∼ 3 appear to be good analogues of reionization-era galaxies. In contrast, local [S II]-deficient galaxies do not overlap with the simulated high-redshift LyC leakers on the S II Baldwin–Phillips–Terlevich (BPT) diagram; however, this diagnostic may still be useful for identifying leakers. We use our simulated galaxies to develop multiple new diagnostics to identify LyC leakers using infrared and nebular emission lines. We show that our model using only [C II]158 μm and [O III]88 μm can identify potential leakers from non-leakers from the local Dwarf Galaxy Survey. Finally, we apply this diagnostic to known high-redshift galaxies and find that MACS 1149_JD1 at z = 9.1 is the most likely galaxy to be actively contributing to the reionization of the Universe
Probing cosmic dawn with emission lines: predicting infrared and nebular line emission for ALMA and JWST
Infrared and nebular lines provide some of our best probes of the physics
regulating the properties of the interstellar medium (ISM) at high-redshift.
However, interpreting the physical conditions of high-redshift galaxies
directly from emission lines remains complicated due to inhomogeneities in
temperature, density, metallicity, ionisation parameter, and spectral hardness.
We present a new suite of cosmological, radiation-hydrodynamics simulations,
each centred on a massive Lyman-break galaxy that resolves such properties in
an inhomogeneous ISM. Many of the simulated systems exhibit transient but well
defined gaseous disks that appear as velocity gradients in [CII]~158.6m
emission. Spatial and spectral offsets between [CII]~158.6m and
[OIII]~88.33m are common, but not ubiquitous, as each line probes a
different phase of the ISM. These systems fall on the local [CII]-SFR relation,
consistent with newer observations that question previously observed
[CII]~158.6m deficits. Our galaxies are consistent with the nebular line
properties of observed galaxies and reproduce offsets on the BPT and
mass-excitation diagrams compared to local galaxies due to higher star
formation rate (SFR), excitation, and specific-SFR, as well as harder spectra
from young, metal-poor binaries. We predict that local calibrations between
H and [OII]~3727 luminosity and galaxy SFR apply up to , as
do the local relations between certain strong line diagnostics (R23 and
[OIII]~5007/H) and galaxy metallicity. Our new simulations are well
suited to interpret the observations of line emission from current (ALMA and
HST) and upcoming facilities (JWST and ngVLA)
Cycle-centrality in complex networks
Networks are versatile representations of the interactions between entities
in complex systems. Cycles on such networks represent feedback processes which
play a central role in system dynamics. In this work, we introduce a measure of
the importance of any individual cycle, as the fraction of the total
information flow of the network passing through the cycle. This measure is
computationally cheap, numerically well-conditioned, induces a centrality
measure on arbitrary subgraphs and reduces to the eigenvector centrality on
vertices. We demonstrate that this measure accurately reflects the impact of
events on strategic ensembles of economic sectors, notably in the US economy.
As a second example, we show that in the protein-interaction network of the
plant Arabidopsis thaliana, a model based on cycle-centrality better accounts
for pathogen activity than the state-of-art one. This translates into
pathogen-targeted-proteins being concentrated in a small number of triads with
high cycle-centrality. Algorithms for computing the centrality of cycles and
subgraphs are available for download
New methods for identifying Lyman continuum leakers and reionization-epoch analogues
Identifying low-redshift galaxies that emit Lyman Continuum radiation (LyC
leakers) is one of the primary, indirect methods of studying galaxy formation
in the epoch of reionization. However, not only has it proved challenging to
identify such systems, it also remains uncertain whether the low-redshift LyC
leakers are truly "analogues" of the sources that reionized the Universe. Here,
we use high-resolution cosmological radiation hydrodynamics simulations to
examine whether simulated galaxies in the epoch of reionization share similar
emission line properties to observed LyC leakers at and . We
find that the simulated galaxies with high LyC escape fractions ()
often exhibit high O32 and populate the same regions of the R23-O32 plane as
LyC leakers. However, we show that viewing angle, metallicity, and
ionisation parameter can all impact where a galaxy resides on the O32- plane. Based on emission line diagnostics and how they correlate with
, lower-metallicity LyC leakers at appear to be good
analogues of reionization-era galaxies. In contrast, local [SII]-deficient
galaxies do not overlap with the simulated high-redshift LyC leakers on the
SII-BPT diagram; however, this diagnostic may still be useful for identifying
leakers. We use our simulated galaxies to develop multiple new diagnostics to
identify LyC leakers using IR and nebular emission lines. We show that our
model using only [CII] and [OIII] can identify
potential leakers from non-leakers from the local Dwarf Galaxy Survey. Finally,
we apply this diagnostic to known high-redshift galaxies and find that
MACS1149_JD1 at is the most likely galaxy to be actively contributing
to the reionization of the Universe
Tracing the sources of reionization in cosmological radiation hydrodynamics simulations
We use the photon flux and absorption tracer algorithm presented in Katz et
al.~2018, to characterise the contribution of haloes of different mass and
stars of different age and metallicity to the reionization of the Universe. We
employ a suite of cosmological multifrequency radiation hydrodynamics AMR
simulations that are carefully calibrated to reproduce a realistic reionization
history and galaxy properties at . In our simulations, haloes with
mass , stars with
metallicity , and stars with age
dominate reionization by both mass and
volume. We show that the sources that reionize most of the volume of the
Universe by are not necessarily the same sources that dominate the
meta-galactic UV background at the same redshift. We further show that in our
simulations, the contribution of each type of source to reionization is not
uniform across different gas phases. The IGM, CGM, filaments, ISM, and rarefied
supernova heated gas have all been photoionized by different classes of
sources. Collisional ionisation contributes at both the lowest and highest
densities. In the early stages of the formation of individual HII bubbles,
reionization proceeds with the formation of concentric shells of gas ionised by
different classes of sources, leading to large temperature variations as a
function of galacto-centric radius. The temperature structure of individual HII
bubbles may thus give insight into the star formation history of the galaxies
acting as the first ionising sources. Our explorative simulations highlight how
the complex nature of reionization can be better understood by using our photon
tracer algorithm
Higher order QCD corrections to charged-lepton deep-inelastic scattering and global fits of parton distributions
We study the perturbative QCD corrections to heavy-quark structure functions
of charged-lepton deep-inelastic scattering and their impact on global fits of
parton distributions. We include the logarithmically enhanced terms near
threshold due to soft gluon resummation in the QCD corrections at
next-to-next-to-leading order. We demonstrate that this approximation is
sufficient to describe the available HERA data in most parts of the kinematic
region. The threshold-enhanced next-to-next-to-leading order corrections
improve the agreement between predictions based on global fits of the parton
distribution functions and the HERA collider data even in the small-x region.Comment: 11 pages, 6 figures, latex, extended journal versio
The nature of high [O III]88 μ m/[C II]158 μm galaxies in the epoch of reionization: Low carbon abundance and a top-heavy IMF?
ALMA observations of z > 6 galaxies have revealed abnormally high [O III]88 μm/[C II]158 μm ratios and [C II]158 μm deficits compared to local galaxies. The origin of this behaviour is unknown. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties, including ionization parameter, gas density, or photodissociation region (PDR) covering fraction. In order to elucidate the underlying physics that drives this high-redshift phenomenon, we employ SPHINX20, a state-of-the-art, cosmological radiation–hydrodynamics simulation, that resolves detailed ISM properties of thousands of galaxies in the epoch of reionization which has been post-processed with CLOUDY to predict emission lines. We find that the observed z > 6 [O III]88 μm–SFR and [C II]158 μm–SFR relations can only be reproduced when the C/O abundance ratio is ∼8 × lower than Solar and the total metal production is ∼4 × higher than that of a Kroupa IMF. This implies that high-redshift galaxies are potentially primarily enriched by low-metallicity core–collapse supernovae with a more top-heavy IMF. As AGB stars and type-Ia supernova begin to contribute to the galaxy metallicity, both the [C II]158 μm–SFR and [C II]158 μm luminosity functions are predicted to converge to observed values at z ∼ 4.5. While we demonstrate that ionization parameter, LyC escape fraction, ISM gas density, and CMB attenuation all drive galaxies towards higher [O III]88 μm/[C II]158 μm, observed values at z > 6 can only be reproduced with substantially lower C/O abundances compared to Solar. The combination of [C II]158 μm and [O III]88 μm can be used to predict the values of ionization parameter, ISM gas density, and LyC escape fraction and we provide estimates of these quantities for nine observed z > 6 galaxies. Finally, we demonstrate that [O I]63 μm can be used as a replacement for [C II]158 μ m in high-redshift galaxies where [C II]158 μ m is unobserved and argue that more observation time should be used to target [O I]63 μm at z > 6. Future simulations will be needed to self-consistently address the numerous uncertainties surrounding a varying IMF at high redshift and the associated metal returns
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