1,482 research outputs found
Bars and spirals in tidal interactions with an ensemble of galaxy mass models
We present simulations of the gaseous and stellar material in several
different galaxy mass models under the influence of different tidal fly-bys to
assess the changes in their bar and spiral morphology. Five different mass
models are chosen to represent the variety of rotation curves seen in nature.
We find a multitude of different spiral and bar structures can be created, with
their properties dependent on the strength of the interaction. We calculate
pattern speeds, spiral wind-up rates, bar lengths, and angular momentum
exchange to quantify the changes in disc morphology in each scenario. The
wind-up rates of the tidal spirals follow the 2:1 resonance very closely for
the flat and dark matter dominated rotation curves, whereas the more baryon
dominated curves tend to wind-up faster, influenced by their inner bars. Clear
spurs are seen in most of the tidal spirals, most noticeable in the flat
rotation curve models. Bars formed both in isolation and interactions agree
well with those seen in real galaxies, with a mixture of "fast" and "slow"
rotators. We find no strong correlation between bar length or pattern speed and
the interaction strength. Bar formation is, however, accelerated/induced in
four out of five of our models. We close by briefly comparing the morphology of
our models to real galaxies, easily finding analogues for nearly all
simulations presenter here, showing passages of small companions can easily
reproduce an ensemble of observed morphologies.Comment: 30 pages, 29 colour figures, accepted for publication in MNRAS.
Videos of simulations can be found at
http://www.youtube.com/playlist?list=PLQKy--XcWrIVBc1sS2RNc-ekyfeBsGtD
Gas and stellar spiral structures in tidally perturbed disc galaxies
Tidal interactions between disc galaxies and low mass companions are an
established method for generating galactic spiral features. In this work we
present a study of the structure and dynamics of spiral arms driven in
interactions between disc galaxies and perturbing companions in 3-D
N-body/smoothed hydrodynamical numerical simulations. Our specific aims are to
characterize any differences between structures formed in the gas and stars
from a purely hydrodynamical and gravitational perspective, and to find a
limiting case for spiral structure generation. Through analysis of a number of
different interacting cases, we find that there is very little difference
between arm morphology, pitch angles and pattern speeds between the two media.
The main differences are a minor offset between gas and stellar arms, clear
spurring features in gaseous arms, and different radial migration of material
in the stronger interacting cases. We investigate the minimum mass of a
companion required to drive spiral structure in a galactic disc, finding the
limiting spiral generation cases with companion masses of the order
, equivalent to only 4% of the stellar disc mass, or 0.5%
of the total galactic mass of a Milky Way analogue.Comment: 20 pages, 23 figures, accepted for publication by MNRA
The Morphology of the Milky Way
This thesis presents an investigation into the morphological features of the Milky Way, the exact structure of which is somewhat of an unknown. We begin with a discussion of the problem at hand, and a review of the literature and methodology associated with determining Galactic structure (Chapter 1).
The methodology of the investigation is to use numerical simulations to reproduce the structure of the interstellar medium (ISM) gas under the effect of gravitational forces that represent possible morphologies of the Milky Way, such as spiral arms and inner bars. The ISM is simulated using smoothed particle hydrodynamics (SPH), which has been tailored to ISM scales by the inclusion of cooling, heating and a simple chemical network, discussed in Chapter 2.
The Milky Way is first assumed to be grand design in nature, with analytic potentials representing the various arm and bar components. Simulations are then compared to longitude velocity CO emission observations to assess the quality of the reproduction of Galactic morphology. These results are shown in Chapter 3, where best fitting models have a bar pattern speed within 50-60km/s/kpc, an arm pattern speed of approximately 20km/s/kpc, a bar orientation of approximately 45 degrees,and arm pitch angle between 10-15 degrees. While nearly all observed emission features are reproducible, there is no model that reproduces all simultaneously. Using both bar and arm components together we find a better match to the data, but still no perfect reproduction. Models with two arms lack many of the observed features, but models with four arms produce too much local emission in the inner quadrants. Chapter 4 shows more sophisticated synthetic observations, created using a radiative transfer code. Resulting emission features are broadly in keeping with those seen in observations, the strength of which appears a strong function of gas surface density.
The analytic potentials are then replaced by a set of discretised mass components that represent the stellar system, which is the subject of Chapter 5. Using a live N-body disc then allows for the dynamic creation of bar and arm features, from which further synthetic observations are produced. Transient arm and bar features are relatively easy to produce, though not necessarily simultaneously. Arm patterns showing two to five arms and some with an effectively flocculent structure are created, with pitch angles around 20 degrees. The pattern speed of which tends to decrease with radius, highlighting that the arms are material rather than wave-like in nature. Best fitting synthetic observations show that a four-armed spiral pattern provides good agreement with observations, more so than that of the fixed potentials, with clear reproduction of nearly all arm features. However, an inner bar appears necessary to remove excess emission seen towards the Galactic centre, which was not present in these models
The morphology of the Milky Way - II. Reconstructing CO maps from disc galaxies with live stellar distributions
The arm structure of the Milky Way remains somewhat of an unknown, with
observational studies hindered by our location within the Galactic disc. In the
work presented here we use smoothed particle hydrodynamics (SPH) and radiative
transfer to create synthetic longitude-velocity observations. Our aim is to
reverse-engineer a top down map of the Galaxy by comparing synthetic
longitude-velocity maps to those observed. We set up a system of N-body
particles to represent the disc and bulge, allowing for dynamic creation of
spiral features. Interstellar gas, and the molecular content, is evolved
alongside the stellar system. A 3D-radiative transfer code is then used to
compare the models to observational data. The resulting models display arm
features that are a good reproduction of many of the observed emission
structures of the Milky Way. These arms however are dynamic and transient,
allowing for a wide range of morphologies not possible with standard density
wave theory. The best fitting models are a much better match than previous work
using fixed potentials. They favour a 4-armed model with a pitch angle of
approximately 20 degrees, though with a pattern speed that decreases with
increasing Galactic radius. Inner bars are lacking however, which appear
required to fully reproduce the central molecular zone.Comment: 16 pages, 15 figures, accepted by MNRA
The Impact of Galactic Disc Environment on Star-Forming Clouds
We explore the effect of different galactic disc environments on the
properties of star-forming clouds through variations in the background
potential in a set of isolated galaxy simulations. Rising, falling and flat
rotation curves expected in halo dominated, disc dominated and Milky Way-like
galaxies were considered, with and without an additional two-arm spiral
potential. The evolution of each disc displayed notable variations that are
attributed to different regimes of stability, determined by shear and
gravitational collapse. The properties of a typical cloud were largely
unaffected by the changes in rotation curve, but the production of small and
large cloud associations was strongly dependent on this environment. This
suggests that while differing rotation curves can influence where clouds are
initially formed, the average bulk properties are effectively independent of
the global environment. The addition of a spiral perturbation made the greatest
difference to cloud properties, successfully sweeping the gas into larger,
seemingly unbound, extended structures and creating large arm-interarm
contrasts.Comment: Accepted to MNRAS on 3rd December, 201
Acoustic environmental accuracy requirements for response determination
A general purpose computer program was developed for the prediction of vehicle interior noise. This program, named VIN, has both modal and statistical energy analysis capabilities for structural/acoustic interaction analysis. The analytic models and their computer implementation were verified through simple test cases with well-defined experimental results. The model was also applied in a space shuttle payload bay launch acoustics prediction study. The computer program processes large and small problems with equal efficiency because all arrays are dynamically sized by program input variables at run time. A data base is built and easily accessed for design studies. The data base significantly reduces the computational costs of such studies by allowing the reuse of the still-valid calculated parameters of previous iterations
Maternal Glucose at 28 Weeks of Gestation Is Not Associated With Obesity in 2-Year-Old Offspring: The Belfast Hyperglycemia and Adverse Pregnancy Outcome (HAPO) Family Study
Distinct phospholipase C-regulated signalling pathways in Swiss 3T3 fibroblasts induce the rapid generation of the same polyunsaturated diacylglycerols
AbstractProstaglandin F2α, platelet-derived growth factor (PDGF) and calcium ionophore A23187 stimulated the rapid (within 25 s) generation of polyunsaturated 1,2-diacylglycerol (DAG) species, in particular 18:0/20:3n-9, 18:0/20:4n-6 and 18:0/20:5n-3, in Swiss 3T3 fibroblasts. This was followed by a second sustained phase characterised by saturated, monounsaturated and diunsaturated DAG species derived, at least partially, from a phospholipase D/phosphatidate phosphohydrolase-linked pathway. This could be directly activated by phorbol ester. Assay of rat brain protein kinase C (PKC) in lipid vesicles showed that first phase, polyunsaturated-enriched DAG isolated from Swiss 3T3 cells was a more potent activator of kinase activity compared to that achieved with DAG from control or 5 min stimulated cells. Thus activation of distinct members of the phospholipase C family leads to the rapid and almost identical generation of polyunsaturated DAG species which are capable of preferentially activating protein kinase C (PKC)
Star formation and ISM morphology in tidally induced spiral structures
Tidal encounters are believed to be one of the key drivers of galactic spiral
structure in the Universe. Such spirals are expected to produce different
morphological and kinematic features compared to density wave and dynamic
spiral arms. In this work we present high resolution simulations of a tidal
encounter of a small mass companion with a disc galaxy. Included are the
effects of gas cooling and heating, star formation and stellar feedback. The
structure of the perturbed disc differs greatly from the isolated galaxy,
showing clear spiral features that act as sites of new star formation, and
displaying interarm spurs. The two arms of the galaxy, the bridge and tail,
appear to behave differently; with different star formation histories and
structure. Specific attention is focused on offsets between gas and stellar
spiral features which can be directly compared to observations. We find some
offsets do exist between different media, with gaseous arms appearing mostly on
the convex side of the stellar arms, though the exact locations appear highly
time dependent. These results further highlight the differences between tidal
spirals and other theories of arm structure.Comment: 17 pages, 19 colour figures, accepted for publication in MNRA
Expression of functional sphingosine-1 phosphate receptor-1 is reduced by B cell receptor signaling and increased by inhibition of PI3 kinase δ but not SYK or BTK in chronic lymphocytic leukaemia cells
BCR signaling pathway inhibitors such as ibrutinib, idelalisib, and fostamatinib (respective inhibitors of Bruton’s tyrosine kinase,
PI3Kδ, and spleen tyrosine kinase) represent a significant therapeutic advance in B cell malignancies, including chronic lymphocytic
leukemia (CLL). These drugs are distinctive in increasing blood lymphocytes while simultaneously shrinking enlarged lymph
nodes, suggesting anatomical redistribution of CLL cells from lymph nodes into the blood. However, the mechanisms underlying
this phenomenon are incompletely understood. In this study, we showed that the egress receptor, sphingosine-1-phosphate (S1P)
receptor 1 (S1PR1), was expressed at low levels in normal germinal centers and CLL lymph nodes in vivo but became upregulated
on normal B cells and, to a variable and lesser extent, CLL cells following in vitro incubation in S1P-free medium. Spontaneous
recovery of S1PR1 expression on normal B and CLL cells was prevented by BCR cross-linking, whereas treatment of CLL cells
with idelalisib increased S1PR1 expression and migration toward S1P, the greatest increase occurring in cases with unmutated
IgH V region genes. Intriguingly, ibrutinib and fostamatinib had no effect on S1PR1 expression or function. Conversely,
chemokine-induced migration, which requires integrin activation and is essential for the entry of lymphocytes into lymph nodes as
well as their retention, was blocked by ibrutinib and fostamatinib, but not idelalisib. In summary, our results suggest that different
BCR signaling inhibitors redistribute CLL cells from lymph nodes into the blood through distinct mechanisms: idelalisib actively
promotes egress by upregulating S1PR1, whereas fostamatinib and ibrutinib may reduce CLL cell entry and retention by
suppressing chemokine-induced integrin activation
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