V. R. Eke

Durham University, Durham, England, United Kingdom

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Publications (92)284.32 Total impact

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    ABSTRACT: The variation of remotely sensed neutron count rates is measured as a function of cratercentric distance using data from the Lunar Prospector Neutron Spectrometer. The count rate, stacked over many craters, peaks over the crater centre, has a minimum near the crater rim and at larger distances it increases to a mean value that is up to 1% lower than the mean count rate observed over the crater. A simple model is presented, based upon an analytical topographical profile for the stacked craters fitted to data from the Lunar Orbiter Laser Altimeter (LOLA). The effect of topography coupled with neutron beaming from the surface largely reproduces the observed count rate profiles. However, a model that better fits the observations can be found by including the additional freedom to increase the neutron emissivity of the crater area by ~0.35% relative to the unperturbed surface. It is unclear what might give rise to this effect, but it may relate to additional surface roughness in the vicinities of craters. The amplitude of the crater-related signal in the neutron count rate is small, but not too small to demand consideration when inferring water-equivalent hydrogen (WEH) weight percentages in polar permanently shaded regions (PSRs). If the crater-wide count rate excess is levered into a much smaller PSR, then it can lead to a significantly biased inferred WEH weight percentage. For instance, it may increase the inferred WEH for Cabeus crater at the Moon's South Pole from ~1% to ~4%.
    Preview · Article · May 2015 · Icarus
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    ABSTRACT: We use the ‘Evolution and Assembly of GaLaxies and their Environments’ (eagle) suite of hydrodynamical cosmological simulations to measure offsets between the centres of stellar and dark matter components of galaxies. We find that the vast majority (>95 per cent) of the simulated galaxies display an offset smaller than the gravitational softening length of the simulations (Plummer-equivalent ϵ = 700 pc), both for field galaxies and satellites in clusters and groups. We also find no systematic trailing or leading of the dark matter along a galaxy's direction of motion. The offsets are consistent with being randomly drawn from a Maxwellian distribution with σ ≤ 196 pc. Since astrophysical effects produce no feasible analogues for the $1.62^{+0.47}_{-0.49}$ kpc offset recently observed in Abell 3827, the observational result is in tension with the collisionless cold dark matter model assumed in our simulations.
    Full-text · Article · May 2015 · Monthly Notices of the Royal Astronomical Society Letters
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    ABSTRACT: We estimate the rate of dark matter scattering in collapsed structures throughout the history of the Universe. If the scattering cross-section is velocity-independent, then the canonical picture is correct that scatterings occur mainly at late times. The scattering rate peaks slightly at redshift z~6, and remains significant today. Half the scatterings occur after z~1, in structures more massive than 10^12 M_sun. Within a factor of two, these numbers are robust to changes in the assumed astrophysics, and the scatterings would be captured in cosmological simulations. However, for particle physics models with a velocity-dependent cross-section (as for Yukawa potential interactions via a massive mediator), the scattering rate peaks before z~20, in objects with mass less than 10^4 M_sun. These precise values are sensitive to the redshift-dependent mass-concentration relation and the small-scale cutoff in the matter power spectrum. In extreme cases, the qualitative effect of early interactions may be reminiscent of warm dark matter and strongly affect the subsequent growth of structure. However, these scatterings are being missed in existing cosmological simulations with limited mass resolution.
    Full-text · Article · May 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Here, we report on the findings obtained from our initial suite of 35 SPH simulations of the impact origin of the obliquity of Uranus.
    No preview · Conference Paper · Mar 2015
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    ABSTRACT: We assess local-time variations of epithermal-neutron count rates measured by the Lunar Prospector Neutron Spectrometer. We investigate the nature of these variations and find no evidence to support the idea that such variations are caused by diurnal varia-tions of hydrogen concentration across the lunar surface. Rather we find an anticorrelation between instrumental temperature and epithermal-neutron count rate. We have also found that the measured counts are dependent on the temperatures of the top decime-ters of the lunar subsurface as constrained by the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment temperature measurements. Finally, we have made the first measurement of the effective leakage depth for epithermal-neutrons of ∼20 cm.
    Full-text · Article · Jan 2015 · Earth and Planetary Science Letters
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    ABSTRACT: We reconstruct the abundance of thorium near the Compton-Belkovich Volcanic Complex on the Moon, using data from the Lunar Prospector Gamma Ray Spectrometer. We enhance the resolution via a pixon image reconstruction technique, and find that the thorium is distributed over a larger (40 km × 75 km) area than the (25 km × 35 km) high albedo region normally associated with Compton-Belkovich. Our reconstructions show that inside this region, the thorium concentration is 14–26 ppm. We also find additional thorium, spread up to 300 km eastward of the complex at ~2 ppm. The thorium must have been deposited during the formation of the volcanic complex, because subsequent lateral transport mechanisms, such as small impacts, are unable to move sufficient material. The morphology of the feature is consistent with pyroclastic dispersal and we conclude that the present distribution of thorium was likely created by the explosive eruption of silicic magma.
    Full-text · Article · Jan 2015 · Journal of Geophysical Research Atmospheres
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    ABSTRACT: Circular Polarisation Ratio (CPR) mosaics from Mini-SAR on Chandrayaan-1 and Mini-RF on LRO are used to study craters near to the lunar north pole. The look direction of the detectors strongly affects the appearance of the crater CPR maps. Rectifying the mosaics to account for parallax also significantly changes the CPR maps of the crater interiors. It is shown that the CPRs of crater interiors in unrectified maps are biased to larger values than crater exteriors, because of a combination of the effects of parallax and incidence angle. Using the LOLA Digital Elevation Map (DEM), the variation of CPR with angle of incidence has been studied. For fresh craters, CPR~0.7 with only a weak dependence on angle of incidence or position interior or just exterior to the crater, consistent with dihedral scattering from blocky surface roughness. For anomalous craters, the CPR interior to the crater increases with both incidence angle and distance from the crater centre. Central crater CPRs are similar to those in the crater exteriors. CPR does not appear to correlate with temperature within craters. Furthermore, the anomalous polar craters have diameter-to-depth ratios that are lower than those of typical polar craters. These results strongly suggest that the high CPR values in anomalous polar craters are not providing evidence of significant volumes of water ice. Rather, anomalous craters are of intermediate age, and maintain sufficiently steep walls that deep regolith does not cover all rough surfaces.
    Full-text · Article · Oct 2014 · Icarus
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    ABSTRACT: We present a maximum-likelihood weak-lensing analysis of the mass distribution in optically selected spectroscopic Galaxy Groups (G3Cv5) in the Galaxy And Mass Assembly (GAMA) survey, using background Sloan Digital Sky Survey (SDSS) photometric galaxies. The scaling of halo mass, Mh, with various group observables is investigated. Our main results are as follows. (1) The measured relations of halo mass with group luminosity, virial volume and central galaxy stellar mass, M*, agree very well with predictions from mock group catalogues constructed from a galform semi-analytical galaxy formation model implemented in the Millennium ΛCDM N-body simulation. (2) The measured relations of halo mass with velocity dispersion and projected half-abundance radius show weak tension with mock predictions, hinting at problems in the mock galaxy dynamics and their small-scale distribution. (3) The median Mh|M* measured from weak lensing depends more sensitively on the lognormal dispersion in M* at fixed Mh than it does on the median M*|Mh. Our measurements suggest an intrinsic dispersion of σlog(M*)∼ 0.15. (4) Comparing our mass estimates with those in the catalogue, we find that the G3Cv5 mass can give biased results when used to select subsets of the group sample. Of the various new halo-mass estimators that we calibrate using our weak-lensing measurements, group luminosity is the best single-proxy estimator of group mass.
    Full-text · Article · Apr 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: A detailed comparison is made of results from the Lunar Prospector Neutron Spectrometer (LPNS) and the Lunar Exploration Neutron Detector Collimated Sensors for EpiThermal Neutrons (LEND CSETN). Using the autocorrelation function and power spectrum of the polar count rate maps produced by these experiments, it is shown that the LEND CSETN has a footprint that is at least as big as would be expected for an omni-directional detector at an orbital altitude of 50 km. The collimated flux into the field of view of the collimator is negligible. Arguments put forward asserting otherwise are considered and found wanting for various reasons. The maps of lunar polar hydrogen with the highest contrast, i.e. spatial resolution, are those resulting from pixon image reconstructions of the LPNS data. These typically provide weight percentages of water equivalent hydrogen that are accurate to 30% within the polar craters.
    Full-text · Article · Mar 2014 · Journal of Geophysical Research: Space Physics
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    ABSTRACT: Well-motivated elementary particle candidates for the dark matter, such as the sterile neutrino, behave as warm dark matter (WDM).For particle masses of order a keV, free streaming produces a cutoff in the linear fluctuation power spectrum at a scale corresponding to dwarf galaxies. We investigate the abundance and structure of WDM haloes and subhaloes on these scales using high resolution cosmological N-body simulations of galactic haloes of mass similar to the Milky Way's. On scales larger than the free-streaming cutoff, the initial conditions have the same power spectrum and phases as one of the cold dark matter (CDM) haloes previously simulated by Springel et al as part of the Virgo consortium Aquarius project. We have simulated four haloes with WDM particle masses in the range 1.4-2.3keV and, for one case, we have carried out further simulations at varying resolution. N-body simulations in which the power spectrum cutoff is resolved are known to undergo artificial fragmentation in filaments producing spurious clumps which, for small masses (<10^7Msun in our case) outnumber genuine haloes. We have developed a robust algorithm to identify these spurious objects and remove them from our halo catalogues. We find that the WDM subhalo mass function is suppressed by well over an order magnitude relative to the CDM case for masses <10^9Msun. Requiring that there should be at least as many subhaloes as there are observed satellites in the Milky Way leads to a conservative lower limit to the (thermal equivalent) WDM particle mass of ~1.5\rmn{keV}. WDM haloes and subhaloes have cuspy density distributions that are well described by NFW or Einasto profiles. Their central densities are lower for lower WDM particle masses and none of the models we have considered suffer from the "too big to fail" problem recently highlighted by Boylan-Kolchin et al.
    Full-text · Article · Aug 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The flux of epithermal neutrons leaking from the lu-nar surface provides information about the abundance of hydrogen in the top ∼ 70 cm of the regolith. Hydro-gen nuclei efficiently moderate neutrons knocked from their nuclei by incoming cosmic rays, and decrease the epithermal neutron leakage flux. This technique was first used for the Lunar Prospector Neutron Spectrom-eter (LPNS) to indicate hydrogen concentrations near to the lunar poles [1]. Having an accurate map of the lunar near-surface hydrogen is crucial for the study of volatile delivery and retention in the inner solar system. It is also a key ingredient in planning future lunar missions to investigate permanently shaded regions near the lunar poles. The Lunar Exploration Neutron Detector (LEND) Collimated Sensors for EpiThermal Neutrons (CSETN) on LRO aimed to map the lunar hydrogen with improved spatial resolution using neutron spectroscopy [2]. Se-rious questions have been raised concerning the effec-tiveness of the LEND CSETN for actually returning a sharper map of the lunar epithermal neutron flux [3, 4, 5]. A number of recent papers by the LEND team have re-asserted that the collimated detector does have a 10 km footprint [6, 7, 8, 9, 10]. This work will use data from both the LPNS and the LEND CSETN to create a uni-fied, consistent picture of the available data. The LEND CSETN commissioning phase data will also be used to help discriminate between the two points of view. Global maps of the neutron count rate The LEND uncollimated Sensor for EpiThermal Neu-trons (SETN), which is strapped to the outside of the col-limator, gives an obvious first check of the data from the CSETN. The purpose of a collimated neutron detector is to produce a sharper version of the blurred map that an omnidirectional detector would measure. Whole-Moon maps, determined by [7], are shown in figures 1 and 2, from which it can immediately be seen that the CSETN is measuring not a sharper map, but a map of something else. The lunar variation is being driven by neutrons with different energies in the SETN and CSETN cases. In fact, the CSETN map bears some resemblance to maps of higher energy neutrons, of the sort that would make it through the collimator walls [7]. If the lunar compo-nent of the CSETN count rate is dominated by neutrons Figure 1: A Robinson projection of the smoothed (FWHM=60 km), 'background subtracted' LEND SETN count rate map from figure 16 of [7].
    Full-text · Conference Paper · Apr 2013
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    ABSTRACT: We will apply a PIXON reconstruction technique to the MONS epithermal data over the full surface of Mars to produce an accurate H distribution map.
    No preview · Article · Mar 2013
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    Quan Guo · Shaun Cole · Vincent Eke · Carlos Frenk · John Helly
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    ABSTRACT: We investigate the luminosity functions (LFs) and projected number density profiles of galactic satellites around isolated primaries of different luminosities. We measure these quantities for model satellites placed into the Millennium and Millennium II dark matter simulations by the GALFORM semi-analytic galaxy formation model for different bins of primary galaxy magnitude and we investigate their dependence on satellite luminosity. We compare our model predictions to the data of Guo et al. from the Sloan Digital Sky Survey Data Release 8 (SDSS DR8). First, we use a mock light-cone catalogue to verify that the method we used to count satellites in the SDSS DR8 is unbiased. We find that the radial distributions of model satellites are similar to those around comparable primary galaxies in the SDSS DR8, with only slight differences at low luminosities and small projected radii. However, when splitting the satellites by colour, the model and SDSS satellite systems no longer resemble one another, with many red model satellites, in contrast to the dominant blue fraction at similar luminosity in SDSS. The few model blue satellites are also significantly less centrally concentrated in the halo of their stacked primary than their SDSS counterparts. The implications of this result for the GALFORM model are discussed.
    Full-text · Article · Jan 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Cold dark matter models predict the existence of a large number of substructures within dark matter halos. If the cold dark matter consists of weakly interacting massive particles, their annihilation within these substructures could lead to diffuse GeV emission that would dominate over the annihilation signal of the host halo. In this work we search for GeV emission from three nearby galaxy clusters: Coma, Virgo and Fornax. We first remove known extragalactic and galactic diffuse gamma-ray backgrounds and point sources from the Fermi 2-year catalog and find a significant residual diffuse emission in all three clusters. We then investigate whether this emission is due to (i) unresolved point sources; (ii) dark matter annihilation; or (iii) cosmic rays (CR). Using 45 months of Fermi-LAT data we detect several new point sources (not present in the Fermi 2-year point source catalogue) which contaminate the signal previously analyzed by Han et al.(arxiv:1201.1003). Including these and accounting for the effects of undetected point sources, we find no significant detection of extended emission from the three clusters studied. Instead, we determine upper limits on emission due to dark matter annihilation and cosmic rays. For Fornax and Virgo the limits on CR emission are consistent with theoretical models, but for Coma the upper limit is a factor of 2 below the theoretical expectation. Allowing for systematic uncertainties associated with the treatment of CR, the upper limits on the cross section for dark matter annihilation from our clusters are more stringent than those from analyses of dwarf galaxies in the Milky Way. We rule out the thermal cross section for supersymmetric dark matter particles for masses as large as 100 GeV (depending on the annihilation channel).
    Full-text · Article · Jul 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The construction of a catalogue of galaxy groups from the Two-degree Field Galaxy Redshift Survey (2dFGRS) is described. Groups are identified by means of a friends-of-friends percolation algorithm which has been thoroughly tested on mock versions of the 2dFGRS generated from cosmological N-body simulations. The tests suggest that the algorithm groups all galaxies that it should be grouping, with an additional 40 per cent of interlopers. About 55 per cent of the ~190000 galaxies considered are placed into groups containing at least two members of which ~29000 are found. Of these, ~7000 contain at least four galaxies, and these groups have a median redshift of 0.11 and a median velocity dispersion of 260km/s. This 2dFGRS Percolation-Inferred Galaxy Group (2PIGG) catalogue represents the largest available homogeneous sample of galaxy groups. (8 data files).
    No preview · Article · Apr 2012
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    ABSTRACT: Much of our current knowledge about Mars' climate and atmospheric global circulation stems from measurements taken by landers and orbiters. Thus for many years the details of the atmospheric circulation were studied using numerical global circulation models (GCMs) that have been successful in reproducing most of the available observations [1]. More than ever, GCMs will play a central role in analyzing the existing data and in planning and execution of upcoming missions. The Mars Odyssey Neutron Spectrometer (MONS) has enabled a comprehensive study of the overall distribution of hydrogen in the surface of Mars [2]. Deposits ranging between 20% and 100% Water-Equivalent Hydrogen (WEH) by mass are found pole-ward of 55 deg. latitude, while less H-rich deposits are found at lower latitudes. These results assume that the H distribution is uniform in the top meter of the martian soil. The Mars Reconnaissance Orbiter-Compact Reconnaissance Imaging Spectrometer for Mars (MRO-CRISM) has identified numerous locations on Mars where hydrous minerals occur [3]. The information collected by MRO-CRISM samples the top few mm's to cm's of the surface. This independent information can impose additional constrains on the 3-D H distribution inferred from the MONS data. For instance, the absence of a correlation between WEH wt% drawn from the MONS and CRISM data at a location where the neutron data indicate high WEH implies the presence of a 3-D structure that is characterized by a top layer with a low abundance of water, either ice or hydrated minerals, and some buried layers where the concentration of H is higher than that expected in a uniformly mixed layer. However, the spatial resolution of MONS and MRO-CRISM are ~550 km and ~20-200m, respectively. Hence, one must assure the MRO-CRISM and MONS data are on the same scales. The MRO-CRISM data can be re-binned to lower resolution, but additionally the MONS instrumental smearing must be properly understood and removed. Usually, in the presence of noise, this is an ill posed problem that requires the use of a statistical approach [4,5]. Here we present the most recent results of applying such an approach to the MONS epithermal neutron data coupled with independent information regarding the distribution of water and hydroxyls, including hydrous mineralogy. An exciting prospect is that this approach can provide estimates of the real extent or the original volume of surface water ice. Such estimates can then be used to constrain the Mars GCM. The Ames Mars GCM depends on several important parameters associated with the atmosphere and surface properties. In particular ice content is directly related to thermal conductivity and thermal inertia, and spatial variations of these govern the input and release of energy (and water vapor) seasonally [6] . Deviations from a uniform ice distribution poleward of 80 deg. N may thus influence local circulation and precipitation. Replicating the Viking and later missions atmospheric pressure histories requires taking into account near-surface water ice content and spatial distribution at high latitudes. To the extent that these can be constrained by MONS measurements, the results of the GCM can be tied to physical parameters that characterize the near-surface materials at high latitudes. Perhaps more important is what the derived distribution of polar ground ice reveals about recent climatic trends. [1] Forget et al, 1999, JGR, 104, 24155; [2] Feldman et al, 2004, JGR, 109, 9006; [3] Brown et al, 2010, In LPSC, report 1278; [4] Pina et al, 1992, PASP, 104, 1096, [5] Eke, 2001, MNRAS, 324, 108; [6] Haberle et al, 2008, PSS, 56, 251
    No preview · Article · Apr 2012
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    ABSTRACT: A comprehensive analysis of data from the Lunar Exploration Neutron Detector Collimated Sensors for Epithermal Neutrons is performed, with significant implications for the lunar hydrogen distribution.
    No preview · Article · Mar 2012
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    ABSTRACT: Data from the Lunar Exploration Neutron Detector (LEND) Collimated Sensors for Epithermal Neutrons (CSETN) are used in conjunction with a model based on results from the Lunar Prospector mission to quantify the extent of the background in the LEND CSETN. A simple likelihood analysis implies that at least 90% of the lunar component of the LEND CSETN flux results from high energy epithermal neutrons passing through the walls of the collimator. Thus, the effective full-width at half-maximum of the LEND CSETN is comparable with that of the omni-directional Lunar Prospector Neutron Spectrometer. The resulting map of high energy epithermal neutrons offers the opportunity to probe the hydrogen abundance at low latitudes, and provide constraints on the distribution of lunar water.
    Full-text · Article · Mar 2012 · The Astrophysical Journal
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    Quan Guo · Shaun Cole · Vincent Eke · Carlos Frenk
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    ABSTRACT: We study the spatial distribution of satellite galaxies around isolated primaries using the Sloan Digital Sky Survey (SDSS) spectroscopic and photometric galaxy catalogues. We select isolated primaries from the spectroscopic sample and search for potential satellites in the much deeper photometric sample. For specific luminosity primaries we obtain robust statistical results by stacking as many as ∼50 000 galaxy systems. We find no evidence for any anistropy in the satellite galaxy distribution relative to the major axes of the primaries. We derive accurate projected number density profiles of satellites down to 4 mag fainter than their primaries. We find that the normalized satellite profiles generally have a universal form and can be well fitted by projected NFW profiles. The NFW concentration parameter increases with decreasing satellite luminosity while being independent of the luminosity of the primary except for very bright primaries. The profiles of the faintest satellites show deviations from the NFW form with an excess at small galactocentric projected distances. In addition, we quantify how the radial distribution of satellites depends on the colour of the satellites and on the colour and concentration of their primaries.
    Full-text · Article · Jan 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We report evidence for extended gamma-ray emission from the Virgo, Fornax and Coma clusters based on a maximum-likelihood analysis of the 3-year Fermi-LAT data. For all three clusters, excess emission is observed within three degrees of the center, peaking at the GeV scale. This emission cannot be accounted for by known Fermi sources or by the galactic and extragalactic backgrounds. If interpreted as annihilation emission from supersymmetric dark matter (DM) particles, the data prefer models with a particle mass in the range 20-60 GeV annihilating into the b-bbar channel, or 2-10 GeV and >1 TeV annihilating into mu-mu final states. Our results are consistent with those obtained by Hooper and Linden from a recent analysis of Fermi-LAT data in the region of the Galactic Centre. An extended DM annihilation profile dominated by emission from substructures is preferred over a simple point source model. The significance of DM detection is 4.4 sigma in Virgo and lower in the other two clusters. We also consider the possibility that the excess emission arises from cosmic ray (CR) induced gamma-rays, and infer a CR level within a factor of three of that expected from analytical models. However, the significance of a CR component is lower than the significance of a DM component, and there is no need for such a CR component in the presence of a DM component in the preferred DM mass range. We also set flux and cross-section upper limits for DM annihilation into the b-bbar and mu-mu channels in all three clusters.
    Full-text · Article · Jan 2012

Publication Stats

5k Citations
284.32 Total Impact Points

Institutions

  • 1996-2015
    • Durham University
      • • Department of Physics
      • • Institute for Computational Cosmology "ICC"
      Durham, England, United Kingdom
  • 1998-2001
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
    • University of Oxford
      • Department of Physics
      Oxford, England, United Kingdom
  • 2000
    • The University of Arizona
      Tucson, Arizona, United States