124 research outputs found
Cloud and Star Formation in Spiral Arms
We present the results from simulations of GMC formation in spiral galaxies.
First we discuss cloud formation by cloud-cloud collisions, and gravitational
instabilities, arguing that the former is prevalent at lower galactic surface
densities and the latter at higher. Cloud masses are also limited by stellar
feedback, which can be effective before clouds reach their maximum mass. We
show other properties of clouds in simulations with different levels of
feedback. With a moderate level of feedback, properties such as cloud rotations
and virial parameters agree with observations. Without feedback, an unrealistic
population of overly bound clouds develops. Spiral arms are not found to
trigger star formation, they merely gather gas into more massive GMCs. We
discuss in more detail interactions of clouds in the ISM, and argue that these
are more complex than early ideas of cloud-cloud collisions. Finally we show
ongoing work to determine whether the Milky Way is a flocculent or grand design
spiral.Comment: 10 pages, 5 figures, to be published in Seychelles conference
"Lessons from the Local Group", ed. K. C. Freeman, B. G. Elmegreen, D. L.
Block, and M. Woolway (Dordrecht: Springer), 201
Variational bayes for estimating the parameters of a hidden Potts model
Hidden Markov random field models provide an appealing representation of images and other spatial problems. The drawback is that inference is not straightforward for these models as the normalisation constant for the likelihood is generally intractable except for very small observation sets. Variational methods are an emerging tool for Bayesian inference and they have already been successfully applied in other contexts. Focusing on the particular case of a hidden Potts model with Gaussian noise, we show how variational Bayesian methods can be applied to hidden Markov random field inference. To tackle the obstacle of the intractable normalising constant for the likelihood, we explore alternative estimation approaches for incorporation into the variational Bayes algorithm. We consider a pseudo-likelihood approach as well as the more recent reduced dependence approximation of the normalisation constant. To illustrate the effectiveness of these approaches we present empirical results from the analysis of simulated datasets. We also analyse a real dataset and compare results with those of previous analyses as well as those obtained from the recently developed auxiliary variable MCMC method and the recursive MCMC method. Our results show that the variational Bayesian analyses can be carried out much faster than the MCMC analyses and produce good estimates of model parameters. We also found that the reduced dependence approximation of the normalisation constant outperformed the pseudo-likelihood approximation in our analysis of real and synthetic datasets
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
Reading Materials: Pots, Plaster, Poetry, and the Press, 1766-1861
This is the proposal for 'Reading Materials: Pots, Plaster, Poetry, and the Press, 1766-1861', a (putative) panel for MLA 2024. Individual paper abstracts and other materials will follow
How do different spiral arm models impact the ISM and GMC population?
The nature of galactic spiral arms in disc galaxies remains elusive.
Regardless of the spiral model, arms are expected to play a role in sculpting
the star-forming interstellar medium. As such, different arm models may result
in differences in the structure of the interstellar medium and molecular cloud
properties. In this study we present simulations of galactic discs subject to
spiral arm perturbations of different natures. We find very little difference
in how the cloud population or gas kinematics vary between the different
grand-design spirals, indicting that the interstellar medium on cloud scales
cares little about where spiral arms come from. We do, however, see a
difference in the interarm/arm mass spectra, {and minor differences in tails of
the distributions of cloud properties} (as well as radial variations in the
stellar/gaseous velocity dispersions). These features can be attributed to
differences in the radial dependence of the pattern speeds between the
different spiral models, and could act as a metric of the nature of spiral
structure in observational studies.Comment: 18 pages, 17 figures. Accepted for publication in MNRA
The variational Bayesian approach to fitting mixture models to circular wave direction data
The emerging variational Bayesian (VB) technique for approximate Bayesian statistical inference is a nonsimulation- based and time-efficient approach. It provides a useful, practical alternative to other Bayesian statistical approaches such as Markov chain Monte Carlo–based techniques, particularly for applications involving large datasets. This article reviews the increasingly popular VB statistical approach and illustrates how it can be used to fit Gaussian mixture models to circular wave direction data. This is done by taking the straightforward approach of padding the data; this method involves adding a repeat of a complete cycle of the data to the existing dataset to obtain a dataset on the real line. The padded dataset can then be analyzed using the standard VB technique. This results in a practical, efficient approach that is also appropriate for modeling other types of circular, or directional, data such as wind direction
Effects of antiplatelet therapy on stroke risk by brain imaging features of intracerebral haemorrhage and cerebral small vessel diseases: subgroup analyses of the RESTART randomised, open-label trial
Background
Findings from the RESTART trial suggest that starting antiplatelet therapy might reduce the risk of recurrent symptomatic intracerebral haemorrhage compared with avoiding antiplatelet therapy. Brain imaging features of intracerebral haemorrhage and cerebral small vessel diseases (such as cerebral microbleeds) are associated with greater risks of recurrent intracerebral haemorrhage. We did subgroup analyses of the RESTART trial to explore whether these brain imaging features modify the effects of antiplatelet therapy
Targeting DNA Damage Response and Replication Stress in Pancreatic Cancer
Background and aims:
Continuing recalcitrance to therapy cements pancreatic cancer (PC) as the most lethal malignancy, which is set to become the second leading cause of cancer death in our society. The study aim was to investigate the association between DNA damage response (DDR), replication stress and novel therapeutic response in PC to develop a biomarker driven therapeutic strategy targeting DDR and replication stress in PC.
Methods:
We interrogated the transcriptome, genome, proteome and functional characteristics of 61 novel PC patient-derived cell lines to define novel therapeutic strategies targeting DDR and replication stress. Validation was done in patient derived xenografts and human PC organoids.
Results:
Patient-derived cell lines faithfully recapitulate the epithelial component of pancreatic tumors including previously described molecular subtypes. Biomarkers of DDR deficiency, including a novel signature of homologous recombination deficiency, co-segregates with response to platinum (P < 0.001) and PARP inhibitor therapy (P < 0.001) in vitro and in vivo. We generated a novel signature of replication stress with which predicts response to ATR (P < 0.018) and WEE1 inhibitor (P < 0.029) treatment in both cell lines and human PC organoids. Replication stress was enriched in the squamous subtype of PC (P < 0.001) but not associated with DDR deficiency.
Conclusions:
Replication stress and DDR deficiency are independent of each other, creating opportunities for therapy in DDR proficient PC, and post-platinum therapy
Effects of antiplatelet therapy after stroke due to intracerebral haemorrhage (RESTART): a randomised, open-label trial
Background:
Antiplatelet therapy reduces the risk of major vascular events for people with occlusive vascular disease, although it might increase the risk of intracranial haemorrhage. Patients surviving the commonest subtype of intracranial haemorrhage, intracerebral haemorrhage, are at risk of both haemorrhagic and occlusive vascular events, but whether antiplatelet therapy can be used safely is unclear. We aimed to estimate the relative and absolute effects of antiplatelet therapy on recurrent intracerebral haemorrhage and whether this risk might exceed any reduction of occlusive vascular events.
Methods:
The REstart or STop Antithrombotics Randomised Trial (RESTART) was a prospective, randomised, open-label, blinded endpoint, parallel-group trial at 122 hospitals in the UK. We recruited adults (≥18 years) who were taking antithrombotic (antiplatelet or anticoagulant) therapy for the prevention of occlusive vascular disease when they developed intracerebral haemorrhage, discontinued antithrombotic therapy, and survived for 24 h. Computerised randomisation incorporating minimisation allocated participants (1:1) to start or avoid antiplatelet therapy. We followed participants for the primary outcome (recurrent symptomatic intracerebral haemorrhage) for up to 5 years. We analysed data from all randomised participants using Cox proportional hazards regression, adjusted for minimisation covariates. This trial is registered with ISRCTN (number ISRCTN71907627).
Findings:
Between May 22, 2013, and May 31, 2018, 537 participants were recruited a median of 76 days (IQR 29–146) after intracerebral haemorrhage onset: 268 were assigned to start and 269 (one withdrew) to avoid antiplatelet therapy. Participants were followed for a median of 2·0 years (IQR [1·0– 3·0]; completeness 99·3%). 12 (4%) of 268 participants allocated to antiplatelet therapy had recurrence of intracerebral haemorrhage compared with 23 (9%) of 268 participants allocated to avoid antiplatelet therapy (adjusted hazard ratio 0·51 [95% CI 0·25–1·03]; p=0·060). 18 (7%) participants allocated to antiplatelet therapy experienced major haemorrhagic events compared with 25 (9%) participants allocated to avoid antiplatelet therapy (0·71 [0·39–1·30]; p=0·27), and 39 [15%] participants allocated to antiplatelet therapy had major occlusive vascular events compared with 38 [14%] allocated to avoid antiplatelet therapy (1·02 [0·65–1·60]; p=0·92).
Interpretation:
These results exclude all but a very modest increase in the risk of recurrent intracerebral haemorrhage with antiplatelet therapy for patients on antithrombotic therapy for the prevention of occlusive vascular disease when they developed intracerebral haemorrhage. The risk of recurrent intracerebral haemorrhage is probably too small to exceed the established benefits of antiplatelet therapy for secondary prevention
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