17 research outputs found

    The influence of baryons on dark matter halos: A cosmic tale of stripping, destruction, and statistics

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    Small scale tests of the nature of dark matter require simulations which incorporate baryonic physics. In this thesis we study how the inclusion of baryonic physics affects the abundance and properties of dark matter halos and their substructures. We introduce a new high-resolution hydrodynamical zoom simulation of a 10^13 Msun galaxy group, which we use to study the properties of halos and subhalos relevant to strong lensing tests of the cold dark matter model. We also compare two hydrodynamical simulations of Milky Way-mass halos, Apostle and Auriga. We find that the number of subhalos, as well as the slope of the subhalo mass function and the subhalo velocity distribution, is altered significantly depending on the implementation of baryonic physics

    Energy equipartition between stellar and dark matter particles in cosmological simulations results in spurious growth of galaxy sizes

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    The impact of 2-body scattering on the innermost density profiles of dark matter haloes is well established. We use a suite of cosmological simulations and idealized numerical experiments to show that 2-body scattering is exacerbated in situations where there are two species of unequal mass. This is a consequence of mass segregation and reflects a flow of kinetic energy from the more to less massive particles. This has important implications for the interpretation of galaxy sizes in cosmological hydrodynamic simulations, which nearly always model stars with less massive particles than are used for the dark matter. We compare idealized models as well as simulations from the eagle project that differ only in the mass resolution of the dark matter component, but keep subgrid physics, baryonic mass resolution, and gravitational force softening fixed. If the dark matter particle mass exceeds the mass of stellar particles, then galaxy sizes – quantified by their projected half-mass radii, R50 – increase systematically with time until R50 exceeds a small fraction of the redshift-dependent mean interparticle separation, l (⁠ R 50 ≳0.05×l R50≳0.05×l ⁠). Our conclusions should also apply to simulations that adopt different hydrodynamic solvers, subgrid physics, or adaptive softening, but in that case may need quantitative revision. Any simulation employing a stellar-to-dark matter particle mass ratio greater than unity will escalate spurious energy transfer from dark matter to baryons on small scales

    Unravelling the photoprotection properties of mycosporine amino acid motifs

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    Photoprotection from harmful ultraviolet (UV) radiation exposure is a key problem in modern society. Mycosporine like amino acids found in fungi, cyanobacteria, macroalgae, phytoplankton and humans, are already presenting a promising form of natural photoprotection in sunscreen formulations. Using time-resolved transient electronic absorption spectroscopy and guided by complementary ab initio calculations, we help to unravel how the core structures of these molecules perform under UV irradiation. Through such detailed insight into the relaxation mechanisms of these ubiquitous molecules, we hope to inspire new thinking in developing next generation sun protecting molecules

    Experimental and computational analysis of para-hydroxy methylcinnamate following photoexcitation

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    Para-hydroxy methylcinnamate is part of the cinnamate family of molecules. Experimental and computational studies have suggested conflicting non-radiative decay routes after photoexcitation to its S1(ππ*) state. One non-radiative decay route involves intersystem crossing mediated by an optically dark singlet state, whilst the other involves direct intersystem crossing to a triplet state. Furthermore, irrespective of the decay mechanism, the lifetime of the initially populated S1(ππ*) state is yet to be accurately measured. In this study, we use time-resolved ion-yield and photoelectron spectroscopies to precisely determine the S1(ππ*) lifetime for the s-cis conformer of para-hydroxy methylcinnamate, combined with time-dependent density functional theory to determine the major non-radiative decay route. We find the S1(ππ*) state lifetime of s-cis para-hydroxy methylcinnamate to be ∼2.5 picoseconds, and the major non-radiative decay route to follow the [1ππ*→1nπ*→3ππ*→S0] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as para-coumaric acid and methylcinnamate

    Subhalo destruction in the Apostle and Auriga simulations

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    N-body simulations make unambiguous predictions for the abundance of substructures within dark matter halos. However, the inclusion of baryons in the simulations changes the picture because processes associated with the presence of a large galaxy in the halo can destroy subhalos and substantially alter the mass function and velocity distribution of subhalos. We compare the effect of galaxy formation on subhalo populations in two state-of-the-art sets of hydrodynamical ΛCDM simulations of Milky Way mass halos, APOSTLE and AURIGA. We introduce a new method for tracking the orbits of subhalos between simulation snapshots that gives accurate results down to a few kiloparsecs from the centre of the halo. Relative to a dark matter-only simulation, the abundance of subhalos in APOSTLE is reduced by 50% near the centre and by 10% within r200. In AURIGA the corresponding numbers are 80% and 40%. The velocity distributions of subhalos are also affected by the presence of the galaxy, much more so in AURIGA than in APOSTLE. The differences on subhalo properties in the two simulations can be traced back to the mass of the central galaxies, which in AURIGA are typically twice as massive as those in APOSTLE. We show that some of the results from previous studies are inaccurate due to systematic errors in the modelling of subhalo orbits near the centre of halos

    The effects of integrated care: a systematic review of UK and international evidence

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    BACKGROUND: Healthcare systems around the world have been responding to the demand for better integrated models of service delivery. However, there is a need for further clarity regarding the effects of these new models of integration, and exploration regarding whether models introduced in other care systems may achieve similar outcomes in a UK national health service context. METHODS: The study aimed to carry out a systematic review of the effects of integration or co-ordination between healthcare services, or between health and social care on service delivery outcomes including effectiveness, efficiency and quality of care. Electronic databases including MEDLINE; Embase; PsycINFO; CINAHL; Science and Social Science Citation Indices; and the Cochrane Library were searched for relevant literature published between 2006 to March 2017. Online sources were searched for UK grey literature, and citation searching, and manual reference list screening were also carried out. Quantitative primary studies and systematic reviews, reporting actual or perceived effects on service delivery following the introduction of models of integration or co-ordination, in healthcare or health and social care settings in developed countries were eligible for inclusion. Strength of evidence for each outcome reported was analysed and synthesised using a four point comparative rating system of stronger, weaker, inconsistent or limited evidence. RESULTS: One hundred sixty seven studies were eligible for inclusion. Analysis indicated evidence of perceived improved quality of care, evidence of increased patient satisfaction, and evidence of improved access to care. Evidence was rated as either inconsistent or limited regarding all other outcomes reported, including system-wide impacts on primary care, secondary care, and health care costs. There were limited differences between outcomes reported by UK and international studies, and overall the literature had a limited consideration of effects on service users. CONCLUSIONS: Models of integrated care may enhance patient satisfaction, increase perceived quality of care, and enable access to services, although the evidence for other outcomes including service costs remains unclear. Indications of improved access may have important implications for services struggling to cope with increasing demand. TRIAL REGISTRATION: Prospero registration number: 42016037725

    A high-resolution cosmological simulation of a strong gravitational lens

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    We present a cosmological hydrodynamical simulation of a 1013 M⊙ galaxy group and its environment (out to 10 times the virial radius) carried out using the EAGLE model of galaxy formation. Exploiting a novel technique to increase the resolution of the dark matter calculation independently of that of the gas, the simulation resolves dark matter haloes and subhaloes of mass 5 × 106 M⊙. It is therefore useful for studying the abundance and properties of the haloes and subhaloes targeted in strong lensing tests of the cold dark matter model. We estimate the halo and subhalo mass functions and discuss how they are affected both by the inclusion of baryons in the simulation and by the environment. We find that the halo and subhalo mass functions have lower amplitude in the hydrodynamical simulation than in its dark-matter-only counterpart. This reflects the reduced growth of haloes in the hydrodynamical simulation due to the early loss of gas by reionization and galactic winds and, additionally, in the case of subhaloes, disruption by enhanced tidal effects within the host halo due to the presence of a massive central galaxy. The distribution of haloes is highly anisotropic reflecting the filamentary character of mass accretion on to the cluster. As a result, there is significant variation in the number of structures with viewing direction. The median number of structures near the centre of the halo, when viewed in projection, is reduced by a factor of 2 when baryons are included

    Investigating bounds on decoherence in quantum mechanics via B and D-mixing

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    We investigate bounds on decoherence in quantum mechanics by studying B and D-mixing observables, making use of many precise new measurements, particularly from the LHC and B factories. In that respect we show that the stringent bounds obtained by a different group in 2013 rely on unjustified assumptions. Finally, we point out which experimental measurements could improve the decoherence bounds considerably

    Ultrafast Spectroelectrochemistry of the Catechol/o‐Quinone Redox Couple in Aqueous Buffer Solution

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    Eumelanin is a natural pigment found in many organisms that provides photoprotection from harmful UV radiation. As a redox‐active biopolymer, the structure of eumelanin is thought to contain different redox states of quinone, including catechol subunits. To further explore the excited state properties of eumelanin, we have investigated the catechol/o‐quinone redox couple by spectroelectrochemical means, in a pH 7.4 aqueous buffered solution, and using a boron doped diamond mesh electrode. At pH 7.4, the two proton, two electron oxidation of catechol is promoted, which facilitates continuous formation of the unstable o‐quinone product in solution. Ultrafast transient absorption (femtosecond to nanosecond) measurements of o‐quinone species involve initial formation of an excited singlet state followed by triplet state formation within 24 ps. In contrast, catechol in aqueous buffer leads to formation of the semiquinone radical Δt>500 ps. Our results demonstrate the rich photochemistry of the catechol/o‐quinone redox couple and provides further insight into the excited state processes of these key building blocks of eumelanin
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