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  • Articles  (117)
  • Oxford University Press  (117)
  • Monthly Notices of the Royal Astronomical Society  (25)
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  • Articles  (117)
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  • 1
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    Testing light-traces-mass in Hubble Frontier Fields Cluster MACS-J0416.1-2403 (2016)
    Oxford University Press
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    Publication Date: 2016-07-13
    Description: We reconstruct the projected mass distribution of a massive merging Hubble Frontier Fields cluster MACSJ0416 using the genetic algorithm based free-form technique called Grale. The reconstructions are constrained by 149 lensed images identified by Jauzac et al. using HFF data. No information about cluster galaxies or light is used, which makes our reconstruction unique in this regard. Using visual inspection of the maps, as well as galaxy-mass correlation functions we conclude that overall light does follow mass. Furthermore, the fact that brighter galaxies are more strongly clustered with mass is an important confirmation of the standard biasing scenario in galaxy clusters. On the smallest scales, approximately less than a few arcseconds, the resolution afforded by 149 images is still not sufficient to confirm or rule out galaxy-mass offsets of the kind observed in ACO 3827. We also compare the mass maps of MACSJ0416 obtained by three different groups: Grale, and two parametric Lenstool reconstructions from the CATS and Sharon/Johnson teams. Overall, the three agree well; one interesting discrepancy between Grale and Lenstool galaxy-mass correlation functions occurs on scales of tens of kpc and may suggest that cluster galaxies are more biased tracers of mass than parametric methods generally assume.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    Implications of the pulsar wind nebula scenario for a TeV gamma-ray source VER J2016+371 (2016)
    Oxford University Press
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    Publication Date: 2016-06-24
    Description: We present multiwavelength studies of a TeV gamma-ray source VER J2016+371 suggested to be associated with a supernova remnant CTB 87 (G74.9+1.2) and based on X-ray and radio morphologies, CTB 87 is identified as an evolved pulsar wind nebula. A source in the vicinity of VER J2016+371 is also detected at GeV energies by Fermi Gamma Ray Space Telescope suggesting a likely counterpart at GeV energies. We find that a broken power-law (BPL) distribution of electrons can explain the observed data at radio, X-ray and TeV energies, however, is not sufficient to explain the data at MeV–GeV energies. A Maxwellian distribution of electrons along with the BPL distribution of electrons in low magnetic fields can explain the observed multiwavelength data spanned from radio to TeV energies suggesting this as the most likely scenario for this source. We also find that although the hadronic model can explain the observed GeV–TeV data for the ambient matter density of ~ 20 cm – 3 , no observational support for such high ambient density makes this hadronic scenario unlikely for this source.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Light versus dark in strong-lens galaxies: dark matter haloes that are rounder than their stars (2015)
    Oxford University Press
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    Publication Date: 2015-12-20
    Description: We measure the projected density profile, shape and alignment of the stellar and dark matter mass distribution in 11 strong-lens galaxies. We find that the projected dark matter density profile – under the assumption of a Chabrier stellar initial mass function – shows significant variation from galaxy to galaxy. Those with an outermost image beyond ~10 kpc are very well fit by a projected Navarro–Frenk–White (NFW) profile; those with images within 10 kpc appear to be more concentrated than NFW, as expected if their dark haloes contract due to baryonic cooling. We find that over several half-light radii, the dark matter haloes of these lenses are rounder than their stellar mass distributions. While the haloes are never more elliptical than e dm = 0.2, their stars can extend to e * 〉 0.2. Galaxies with high dark matter ellipticity and weak external shear show strong alignment between light and dark; those with strong shear ( 0.1) can be highly misaligned. This is reassuring since isolated misaligned galaxies are expected to be unstable. Our results provide a new constraint on galaxy formation models. For a given cosmology, these must explain the origin of both very round dark matter haloes and misaligned strong-lens systems.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 4
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    The behaviour of dark matter associated with four bright cluster galaxies in the 10 kpc core of Abell 3827 (2015)
    Oxford University Press
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    Publication Date: 2015-04-16
    Description: Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10 kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and Very Large Telescope/Multi-Unit Spectroscopic Explorer integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is $1.62^{+0.47}_{-0.49}$  kpc, where the 68 per cent confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments – but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section DM / m  ~ (1.7 ± 0.7) 10 –4  cm 2  g –1  ( t infall /10 9 yr) –2 , where t infall is the infall duration.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 5
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    The evolution of disc galaxies with and without classical bulges since z ~ 1 (2015)
    Oxford University Press
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    Publication Date: 2015-05-24
    Description: Establishing relative role of internally and externally driven mechanisms responsible for disc and bulge growth is essential to understand the evolution of disc galaxies. In this context, we have studied the physical properties of disc galaxies without classical bulges in comparison to those with classical bulges since z  ~ 0.9. Using images from the Hubble Space Telescope and Sloan Digital Sky Survey, we have computed both parametric and non-parametric measures, and examined the evolution in size, concentration, stellar mass, effective stellar mass density and asymmetry. We find that both disc galaxies with and without classical bulges have gained more than 50 per cent of their present stellar mass over the last ~8 Gyr. Also, the increase in disc size is found to be peripheral. While the average total (Petrosian) radius almost doubles from z  ~ 0.9 to z  ~ 0, the average effective radius undergoes a marginal increase in comparison. Additionally, increase in the density of the inner region is evident through the evolution of both concentration and effective stellar mass density. We find that the asymmetry index falls from higher to lower redshifts, but this is more pronounced for the bulgeless disc sample. Also, asymmetry correlates with the global effective radius, and concentration correlates with the global Sérsic index, but better so for higher redshifts only. The substantial increase in mass and size indicates that accretion of external material has been a dominant mode of galaxy growth, where the circumgalactic environment plays a significant role.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 6
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    Photometric evidence of an intermediate-age stellar population in the inner bulge of M31 (2015)
    Oxford University Press
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    Publication Date: 2015-07-02
    Description: We explore the assembly history of the M31 bulge within a projected major-axis radius of 180 arcsec (~680 pc) by studying its stellar populations in Hubble Space Telescope Wide Field Camera 3 and Advanced Camera for Surveys observations. Colours formed by comparing near-ultraviolet versus optical bands are found to become bluer with increasing major-axis radius, which is opposite to that predicted if the sole sources of near-ultraviolet light were old extreme horizontal branch stars with a negative radial gradient in metallicity. Spectral energy distribution fits require a metal-rich intermediate-age stellar population (300 Myr to 1 Gyr old, ~Z ) in addition to the dominant old population. The radial gradients in age and metallicity of the old stellar population are consistent with those in previous works. For the intermediate-age population, we find an increase in age with radius and a mass fraction that increases up to 2 per cent at 680 pc away from the centre. We exclude contamination from the M31 disc and/or halo as the main origin for this population. Our results thus suggest that intermediate-age stars exist beyond the central 5 arcsec (19 pc) of M31 and contribute ~1 per cent of the total stellar mass in the bulge. These stars could be related to the secular growth of the M31 bulge.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 7
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    Intensity interferometry with more than two detectors? (2013)
    Oxford University Press
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    Publication Date: 2013-12-06
    Description: The original intensity interferometers were instruments built in the 1950s and 1960s by Hanbury Brown and collaborators, achieving milliarcsec resolutions in visible light without optical-quality mirrors. They exploited a then-novel physical effect, nowadays known as HBT correlation after the experiments of Hanbury Brown and Twiss, and considered fundamental in quantum optics. Now a new generation of intensity interferometers is being designed, raising the possibility of measuring intensity correlations with three or more detectors. Quantum optics predicts two interesting features in many-detector HBT: (i) the signal contains spatial information about the source (such as the bispectrum or closure phase) not present in standard HBT and (ii) correlation increases combinatorially with the number of detectors. The signal-to-noise ratio (SNR) depends crucially on the number of photons – in practice always 〈〈1 – detected per coherence time. A simple SNR formula is derived for thermal sources, indicating that three-detector HBT is feasible for bright stars. The many-detector enhancement of HBT would be much more difficult to measure, but seems plausible for bright masers.
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    Topics: Physics
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  • 8
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    Mass-galaxy offsets in Abell 3827, 2218 and 1689: intrinsic properties or line-of-sight substructures? (2014)
    Oxford University Press
    Details
    Publication Date: 2014-03-21
    Description: We have made mass maps of three strong-lensing clusters, Abell 3827, Abell 2218 and Abell 1689, in order to test for mass–light offsets. The technique used is GRALE, which enables lens reconstruction with minimal assumptions, and specifically with no information about the cluster light being given. In the first two of these clusters, we find local mass peaks in the central regions that are displaced from the nearby galaxies by a few to several kpc. These offsets could be due to line-of-sight structure unrelated to the clusters, but that is very unlikely, given the typical levels of chance line-of-sight coincidences in cold dark matter simulations – for Abell 3827 and Abell 2218 the offsets appear to be intrinsic. In the case of Abell 1689, we see no significant offsets in the central region, but we do detect a possible line-of-sight structure: it appears only when sources at z 3 are used for reconstructing the mass. We discuss possible origins of the mass–galaxy offsets in Abell 3827 and Abell 2218: these include pure gravitational effects like dynamical friction, but also non-standard mechanisms like self-interacting dark matter.
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    Topics: Physics
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  • 9
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    Clocks around Sgr A (2014)
    Oxford University Press
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    Publication Date: 2014-09-19
    Description: The S stars near the Galactic Centre and any pulsars that may be on similar orbits can be modelled in a unified way as clocks orbiting a black hole, and hence are potential probes of relativistic effects, including black hole spin. The high eccentricities of many S stars mean that relativistic effects peak strongly around pericentre; for example, orbit precession is not a smooth effect but almost a kick at pericentre. We argue that concentration around pericentre will be an advantage when analysing redshift or pulse-arrival data to measure relativistic effects, because cumulative precession will be drowned out by Newtonian perturbations from other mass in the Galactic Centre region. Wavelet decomposition may be a way to disentangle relativistic effects from Newton perturbations. Assuming a plausible model for Newtonian perturbations on S2, relativity appears to be strongest in a two-year interval around pericentre, in wavelet modes of time-scale 6 months.
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    Topics: Physics
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  • 10
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    Feasibility of observing Hanbury Brown and Twiss phase (2014)
    Oxford University Press
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    Publication Date: 2014-11-27
    Description: The interferometers of Hanbury Brown and collaborators in the 1950s and 1960s, and their modern descendants now being developed (intensity interferometers) measure the spatial power spectrum of the source from intensity correlations at two points. The quantum optical theory of the Hanbury Brown and Twiss effect shows that more is possible, in particular the phase information can be recovered by correlating intensities at three points (bispectrum). In this paper we argue that such three-point measurements are possible for bright stars such as Sirius and Betelgeuse using off the shelf single photon counters with collecting areas of the order of 100 m 2 . It seems possible to map individual features on the stellar surface. Simple diameter measurements would be possible with amateur class telescopes.
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    Topics: Physics
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