3,600 research outputs found
Astrophysical Effects of Scalar Dark Matter Miniclusters
We model the formation, evolution and astrophysical effects of dark compact
Scalar Miniclusters (``ScaMs''). These objects arise when a scalar field, with
an axion-like or Higgs-like potential, undergoes a second order phase
transition below the QCD scale. Such a scalar field may couple too weakly to
the standard model to be detectable directly through particle interactions, but
may still be detectable by gravitational effects, such as lensing and baryon
accretion by large, gravitationally bound miniclusters. The masses of these
objects are shown to be constrained by the Ly power spectrum to be less
than , but they may be as light as classical axion
miniclusters, of the order of . We simulate the formation and
nonlinear gravitational collapse of these objects around matter-radiation
equality using an N-body code, estimate their gravitational lensing properties,
and assess the feasibility of studying them using current and future lensing
experiments. Future MACHO-type variability surveys of many background sources
can reveal either high-amplification, strong lensing events, or measure density
profiles directly via weak-lensing variability, depending on ScaM parameters
and survey depth. However, ScaMs, due to their low internal densities, are
unlikely to be responsible for apparent MACHO events already detected in the
Galactic halo. A simple estimate is made of parameters that would give rise to
early structure formation; in principle, early stellar collapse could be
triggered by ScaMs as early as recombination, and significantly affect cosmic
reionization.Comment: 13 pages, 12 figures. Replaced to reflect published versio
Ceramic composition at Chalcolithic Shiqmim, northern Negev desert, Israel: investigating technology and provenance using thin section petrography, instrumental geochemistry and calcareous nannofossils
Technological innovations in ceramic production and other crafts are hallmarks of the Chalcolithic period (4500â3600 BCE) in the southern Levant, but details of manufacturing traditions have not been fully investigated using the range of analytical methods currently available. This paper presents results of a compositional study of 51 sherds of ceramic churns and other pottery types from the Chalcolithic site of Shiqmim in the northern Negev desert. By applying complementary thin section petrography, instrumental geochemistry and calcareous nannofossil analyses, connections between the raw materials, clay paste recipes and vessel forms of the selected ceramic samples are explored and documented. The study indicates that steps in ceramic manufacturing can be related to both technological choices and local geology. Detailed reporting of the resulting data facilitates future comparative ceramic compositional research that is needed as a basis for testable regional syntheses and to better resolve networks of trade/exchange and social group movement
Production of single-domain magnetite throughout life by sockeye salmon, Oncorhynchus nerka
Although single-domain particles of biogenic magnetite have been found in different species of pelagic fishes, nothing is known about when it is synthesized, or about whether the time during life when it is produced is correlated with the
development of responses to magnetic field stimuli. We have investigated production of biogenic magnetite suitable for use in magnetoreception in different life stages of the sockeye salmon, Oncorhynchus nerka (Walbaum). Sockeye
salmon were chosen because responses in orientation arenas to magnetic field stimuli have been demonstrated in both fry and smolt stages of this species.
We found significant quantities of single-domain magnetite in connective tissue from the ethmoid region of the skull of adult (4-year-old) sockeye salmon. The ontogenetic study revealed an orderly increase in the amount of magnetic material in the same region of the skull but not in other tissues of sockeye salmon fry, yearlings and smolts. The physical properties of this material closely matched
those of magnetite particles extracted from the ethmoid tissue of the adult fish. We suggest that single-domain magnetite particles suitable for use in magnetoreception
are produced throughout life in the ethmoid region of the skull in sockeye salmon. Based on theoretical calculations, we conclude that there are enough particles present in the skulls of the fry to mediate their responses to magnetic field direction. By the smolt stage, the amount of magnetite present in the front of the skull is sufficient to provide the fish with a magnetoreceptor capable of detecting small changes in the intensity of the geomagnetic field.
Other tissues of the salmon, such as the eye and skin, often contained ferromagnetic material, although the magnetizations of these tissues were usually more variable than in the ethmoid tissue. These deposits of unidentified magnetic material, some of which may be magnetite, appear almost exclusively in adults and so would not be useful in magnetoreception by young fish. We suggest that tissue from within the ethmoid region of the skull in pelagic fishes is the only site yet identified where magnetite suitable for use in magnetoreception is concentrated
Neural Machine Translation
Neural Machine Translation is the primary algorithm used in industry to perform machine translation. This state-of-the-art algorithm is an application of deep learning in which massive datasets of translated sentences are used to train a model capable of translating between any two languages. The architecture behind neural machine translation is composed of two recurrent neural networks used together in tandem to create an Encoder Decoder structure. Attention mechanisms have recently been developed to further increase the accuracy of these models. In this senior thesis, the various parts of Neural Machine Translation are explored towards the eventual creation of a tutorial on the topic. In the first half of this paper, each of the aspects that go into creating a NMT model are explained in depth. With an understanding of the mechanics of NMT, the second portion of this paper briefly outlines enhancements that were made to the PyTorch tutorial on NMT to create an updated and more effective tutorial on the topic
A comparison of black hole growth in galaxy mergers with Gasoline and Ramses
Supermassive black hole dynamics during galaxy mergers is crucial in
determining the rate of black hole mergers and cosmic black hole growth. As
simulations achieve higher resolution, it becomes important to assess whether
the black hole dynamics is influenced by the treatment of the interstellar
medium in different simulation codes. We here compare simulations of black hole
growth in galaxy mergers with two codes: the Smoothed Particle Hydrodynamics
code Gasoline, and the Adaptive Mesh Refinement code Ramses. We seek to
identify predictions of these models that are robust despite differences in
hydrodynamic methods and implementations of sub-grid physics. We find that the
general behavior is consistent between codes. Black hole accretion is minimal
while the galaxies are well-separated (and even as they "fly-by" within 10 kpc
at first pericenter). At late stages, when the galaxies pass within a few kpc,
tidal torques drive nuclear gas inflow that triggers bursts of black hole
accretion accompanied by star formation. We also note quantitative
discrepancies that are model-dependent: our Ramses simulations show less star
formation and black hole growth, and a smoother gas distribution with larger
clumps and filaments, than our Gasoline simulations. We attribute these
differences primarily to the sub-grid models for black hole fueling and
feedback and gas thermodynamics. The main conclusion is that differences exist
quantitatively between codes, and this should be kept in mind when making
comparisons with observations. However, reassuringly, both codes capture the
same dynamical behaviors in terms of triggering of black hole accretion, star
formation, and black hole dynamics.Comment: 11 pages, 7 figures. Submitted to A&A. Comments welcom
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