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  • Articles  (526)
  • 2015-2019  (526)
  • 2016  (526)
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  • Articles  (526)
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  • 2015-2019  (526)
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  • 1
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    Peanut-shaped metallicity distributions in bulges of edge-on galaxies: the case of NGC 4710 (2016)
    Oxford University Press
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    Publication Date: 2016-12-31
    Description: Bulges of edge-on galaxies are often boxy/peanut-shaped (B/PS), and unsharp masks reveal the presence of an X shape. Simulations show that these shapes can be produced by dynamical processes driven by a bar which vertically thickens the centre. In the Milky Way, which contains such a B/PS bulge, the X-shaped structure is traced by the metal-rich stars but not by the metal-poor ones. Recently, Debattista et al. interpreted this property as a result of the varying effect of the bar on stellar populations with different starting kinematics. This kinematic fractionation model predicts that cooler populations at the time of bar formation go on to trace the X shape, whereas hotter populations are more uniformly distributed. As this prediction is not specific to the Milky Way, we test it with Multi Unit Spectroscopic Explorer (MUSE) observations of the B/PS bulge in the nearby galaxy NGC 4710. We show that the metallicity map is more peanut-shaped than the density distribution itself, in good agreement with the prediction. This result indicates that the X-shaped structure in B/PS bulges is formed of relatively metal-rich stars that have been vertically redistributed by the bar, whereas the metal-poor stars have a more uniform, box-shaped distribution.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    A strongly truncated inner accretion disc in the Rapid Burster (2016)
    Oxford University Press
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    Publication Date: 2016-12-31
    Description: The neutron star (NS) low-mass X-ray binary (LMXB) the Rapid Burster (RB; MXB 1730-335) uniquely shows both Type I and Type II X-ray bursts. The origin of the latter is ill-understood but has been linked to magnetospheric gating of the accretion flow. We present a spectral analysis of simultaneous Swift , NuSTAR and XMM–Newton observations of the RB during its 2015 outburst. Although a broad Fe K line has been observed before, the high quality of our observations allows us to model this line using relativistic reflection models for the first time. We find that the disc is strongly truncated at $41.8^{+6.7}_{-5.3}$ gravitational radii (~87 km), which supports magnetospheric Type II burst models and strongly disfavours models involving instabilities at the innermost stable circular orbit. Assuming that the RB magnetic field indeed truncates the disc, we find B  = (6.2 ± 1.5)  x  10 8  G, larger than typically inferred for NS LMXBs. In addition, we find a low inclination ( $i = 29{\degr } \pm 2{\degr }$ ). Finally, we comment on the origin of the Comptonized and thermal components in the RB spectrum.
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    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Non-linear shrinkage estimation of large-scale structure covariance (2016)
    Oxford University Press
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    Publication Date: 2016-12-30
    Description: In many astrophysical settings, covariance matrices of large data sets have to be determined empirically from a finite number of mock realizations. The resulting noise degrades inference and precludes it completely if there are fewer realizations than data points. This work applies a recently proposed non-linear shrinkage estimator of covariance to a realistic example from large-scale structure cosmology. After optimizing its performance for the usage in likelihood expressions, the shrinkage estimator yields subdominant bias and variance comparable to that of the standard estimator with a factor of ~50 less realizations. This is achieved without any prior information on the properties of the data or the structure of the covariance matrix, at a negligible computational cost.
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    Topics: Physics
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  • 4
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    AGN feedback and the origin of the {alpha} enhancement in early-type galaxies - insights from the GAEA model (2016)
    Oxford University Press
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    Publication Date: 2016-12-30
    Description: We take advantage of our recently published model for GAlaxy Evolution and Assembly (GAEA) to study the origin of the observed correlation between [α/Fe] and galaxy stellar mass. In particular, we analyse the role of radio-mode active galactic nuclei (AGN) feedback, which recent work has identified as a crucial ingredient to reproduce observations. In GAEA, this process introduces the observed trend of star formation histories extending over shorter time-scales for more massive galaxies, but does not provide a sufficient condition to reproduce the observed α enhancements of massive galaxies. In the framework of our model, this is possible only by assuming that any residual star formation is truncated for galaxies more massive than 10 10.5 M . This results, however, in even shorter star formation time-scales for the most massive galaxies, which translate in total stellar metallicities significantly lower than observed. Our results demonstrate that (i) trends of [α/Fe] ratios cannot be simply converted into relative time-scale indicators; and (ii) AGN feedback cannot explain alone the positive correlation between [α/Fe] and galaxy mass/velocity dispersion. Reproducing simultaneously the mass–metallicity relation and the α enhancements observed pose a challenge for hierarchical models, unless more exotic solutions are adopted such as metal-rich winds or a variable initial mass function.
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    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Polarized emission from an off-centred dipole (2016)
    Oxford University Press
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    Publication Date: 2016-12-28
    Description: Radio polarization measurements of pulsed emission from pulsars offer a valuable insight into the basic geometry of the neutron star: inclination angle between the magnetic and rotation axis and inclination of the line of sight. So far, all studies about radio polarization focused on the standard rotating vector model with the underlying assumption of a centred dipole. In this Letter, we extend this model to the most general off-centred dipole configuration and give an exact closed analytic expression for the phase-resolved polarization angle. It is shown that contrary to the rotating vector model, for an off-centred dipole, the polarization angle also depends on the emission altitude. Although the fitting parameter space increases from two to six (position of the dipole, altitude and shift of the zero phase), statistical analysis should remain tractable. Observations revealing an evolution of the polarization angle with frequency would undeniably furnish a strong hint for the presence of a decentred magnetic dipole in neutron stars.
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    Topics: Physics
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  • 6
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    On similarity of binary black hole gravitational-wave skymaps: to observe or to wait? (2016)
    Oxford University Press
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    Publication Date: 2016-12-28
    Description: Localization estimates for GW150914, the first binary black hole detected by the LIGO instruments, were shared with partner facilities for electromagnetic follow-up. While the source was a compact binary coalescence (CBC), it was first identified by algorithms that search for unmodelled signals, which produced the skymaps that directed electromagnetic observations. Later on, CBC-specific algorithms produced refined versions, which showed significant differences. In this Letter, we show that those differences were not accidental and that CBC and unmodelled skymaps for binary black holes will frequently be different; we thus provide a way to determine whether to observe electromagnetically as promptly as possible (following a gravitational-wave detection), or to wait until CBC skymaps become available, should they not be available in low latency. We also show that, unsurprisingly, CBC algorithms can yield much smaller searched areas.
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    Topics: Physics
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  • 7
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    The origin of single low-mass WDs: another problem that consequential angular momentum loss in CVs might solve (2016)
    Oxford University Press
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    Publication Date: 2016-12-26
    Description: Low-mass helium-core white dwarfs (WDs) with masses below $0.5\,{\mathrm{M_{{\odot}}}}$ are known to be formed in binary star systems but unexpectedly, a significant fraction of them seems to be single. On the other hand, in cataclysmic variables (CVs), a large number of low-mass WD primary stars is predicted but not observed. We recently showed that the latter problem can be solved if consequential angular momentum loss causes especially CVs with low-mass WDs to merge and form single stars. Here we simulate the population of single WDs resulting from single-star evolution and from binary star mergers taking into account these new merging CVs. We show that according to the revised model of CV evolution, merging CVs might be the dominant channel leading to the formation of low-mass single WDs and that the predicted relative numbers are consistent with observations. This can be interpreted as further evidence for the revised model of CV evolution we recently suggested. This model includes consequential angular momentum loss that increases with decreasing WD mass and might not only explain the absence of low-mass WD primaries in CVs but also the existence of single low-mass WDs.
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    Topics: Physics
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  • 8
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    Cosmological N-body simulations including radiation perturbations (2016)
    Oxford University Press
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    Publication Date: 2016-12-26
    Description: Cosmological N -body simulations are the standard tools to study the emergence of the observed large-scale structure of the Universe. Such simulations usually solve for the gravitational dynamics of matter within the Newtonian approximation, thus discarding general relativistic effects such as the coupling between matter and radiation ( photons and neutrinos). In this Letter, we investigate novel hybrid simulations that incorporate interactions between radiation and matter to the leading order in General Relativity, whilst evolving the matter dynamics in full non-linearity according to Newtonian theory. Our hybrid simulations come with a relativistic space–time and make it possible to investigate structure formation in a unified framework. In this work, we focus on simulations initialized at z = 99 and show that the extracted matter power spectrum receives up to 3 per cent corrections on very large scales through radiation. Our numerical findings compare favourably with linear analytical results from Fidler et al., from which we deduce that there cannot be any significant non-linear mode-coupling induced through linear radiation corrections.
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  • 9
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    Variable protostellar mass accretion rates in cloud cores (2016)
    Oxford University Press
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    Publication Date: 2016-12-22
    Description: Spherical hydrodynamic models with a polytropic equation of state (EoS) for forming protostars are revisited in order to investigate the so-called luminosity conundrum highlighted by observations. For a molecular cloud (MC) core with such an EoS with polytropic index 〉 1, the central mass accretion rate (MAR) decreases with increasing time as a protostar emerges, offering a sensible solution to this luminosity problem. As the MAR decreases, the protostellar luminosity also decreases, meaning that it is invalid to infer the star formation time from the currently observed luminosity using an isothermal model. Furthermore, observations of radial density profiles and the radio continua of numerous MC cores evolving towards protostars also suggest that polytropic dynamic spheres of 〉 1 should be used in physical models.
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    Topics: Physics
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  • 10
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    A ghostly damped Ly {alpha} system revealed by metal absorption lines (2016)
    Oxford University Press
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    Publication Date: 2016-12-22
    Description: We report the discovery of the first ‘ghostly’ damped Ly α absorption system (DLA), which is identified by the presence of absorption from strong low-ion species at z abs = 1.704 65 along the line of sight to the quasar SDSS J113341.29–005740.0 with z em = 1.704 41. No Ly α absorption trough is seen associated with these absorptions because the DLA trough is filled with the leaked emission from the broad emission-line region of the quasar. By modelling the quasar spectrum and analysing the metal lines, we derive log  N (H i )(cm –2 ) ~21.0 ± 0.3. The DLA cloud is small (≤0.32 pc), thus not covering entirely the broad-line region and is located at ≥39 pc from the central active galactic nucleus (AGN). Although the DLA is slightly redshifted relative to the quasar, its metallicity ([S/H] = –0.41 ± 0.30) is intermediate between what is expected from infalling and outflowing gas. It could be possible that the DLA is part of some infalling material accreting on to the quasar host galaxy through filaments, and that its metallicity is raised by mixing with the enriched outflowing gas emanating from the central AGN. Current DLA surveys miss these ‘ghostly’ DLAs, and it would be important to quantify the statistics of this population by searching the Sloan Digital Sky Survey (SDSS) data base using metal absorption templates.
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    Topics: Physics
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