Research Articles (Physics)

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Promoting physics and development in Africa
(American Institute of Physics, 2004) Zingu, Edmund
To excel in physics research in Africa is to conquer Mount Everest without the aid of additional oxygen. In a continent that lacks the infrastructure of research laboratories, technical support, and so forth, relatively few physicists have managed to perform at levels competitive with the best in the world. Are the challenges of physicists in Africa any different from those facing physicists elsewhere? Physicists everywhere have to convince their governments, businesses, and the public that investment in physics is beneficial and will lead to economic development and an enhanced quality of life.
Figure rotation of dark haloes in cold dark matter simulations
(Oxford University Press, 2007) Bryan, Scott E.; Cress, Christine M
We investigate the figure rotation of dark matter haloes identified in cold dark matter (CDM) simulations.We find that when strict criteria are used to select suitable haloes for study, five of the 222 haloes identified in our z = 0 simulation output undergo coherent figure rotation over a 5 h−1 Gyr period.We discuss the effects of varying the selection criteria and find that pattern speeds for a much larger fraction of the haloes can be measured when the criteria are relaxed. Pattern speeds measured over a 1 h−1 Gyr period follow a lognormal distribution, centred at p = 0.2 h rad Gyr−1 with a maximum value of 0.94 h rad Gyr−1. Over a 5 h−1 Gyr period, the average pattern speed of a halo is about 0.1 h rad Gyr−1 and the largest pattern speed found is 0.24 h rad Gyr−1. Less than half of the selected haloes showed alignment between their figure rotation axis and minor axis, the exact fraction being somewhat dependent on how one defines a halo. While the pattern speeds observed are lower than those generally thought capable of causing spiral structure, we note that coherent figure rotation is found over very long periods and argue that further simulations would be required before strong conclusions about spiral structure in all galaxies could be drawn.
A CHANDRA STUDY OF TEMPERATURE SUBSTRUCTURES IN INTERMEDIATE-REDSHIFT GALAXY CLUSTERS
(2009) Gu, Liyi; Wang, Yu; Zhang, Zhongli
By analyzing the gas temperature maps created from the Chandra archive data, we reveal the prevailing existence of temperature substructures on ∼ 100 h −1 70 kpc scales in the central regions of nine intermediate-redshift (z ≈ 0.1) galaxy clusters, which resemble those found in the Virgo and Coma Clusters. Each substructure contains a clump of hot plasma whose temperature is about 2 − 3 keV higher than the environment, corresponding to an excess thermal energy of ∼ 1058−60 erg per clump. Since if there were no significant non-gravitational heating sources, these substructures would have perished in 108−9 yrs due to thermal conduction and turbulent flows, whose velocity is found to range from about 200 to 400 km s−1 , we conclude that the substructures cannot be created and sustained by inhomogeneous radiative cooling. We also eliminate the possibilities that the temperature substructures are caused by supernova explosions, or by the non-thermal X-ray emission due to the inverse-Comptonization of the CMB photons. By calculating the rising time of AGN-induced buoyant bubbles, we speculate that the intermittent AGN outbursts (≥ 1060 erg per burst) may have played a crucial role in the forming of the high temperature substructures. Our results are supported by recent study of McNamara & Nulsen (2007), posing a tight observational constraint on future theoretical and numerical studies
Relativistic mean field formulation of clustering in heavy nuclei
(IOP, 2010) S.M. Wyngaardt; H. W. Groenewald; T.T Ibrahim; J. Ndayishimye; S.M. Perez
Very little is known about clustering in heavy nuclei and in particular the interaction between the correlated cluster nucleons and remaining core nucleons. Currently the phenomenological Saxon-Woods plus cubic Saxon-Woods core-cluster potential successfully predicts the alpha decay half-life and energy band spectra of a number of heavy nuclei. This model, however, lacks a microscopic understanding of clustering phenomenon in these heavy nuclear systems. A fully relativistic microscopic formalism is presented, which generates the core-cluster potential by means of the McNeil, Ray and Wallace based double folding procedure. The core and cluster baryon densities are calculated by using a relativistic mean field approach. The Lorentz covariant IA1 representation of the nucleon-nucleon interaction is folded with the core and cluster densities. Theoretical predictions of the ground-state decay half-life and positive parity energy band of 212Po are obtained with the relativistic mean field formalism and which are compared to the results from the phenomenological Saxon-Woods plus cubic Saxon-Wood core-cluster potential and microscopic M3Y interaction.
The orbital periods of aa dor and ny vir
(Oxford University Press, 2011) Kilkenny, Dave
New timings of eclipses made between 2000 and 2010 are presented for two binary systems with hot subdwarf primary stars. In the case of AA Dor, an sdOB star with a very cool secondary, the period is found to be constant at a level of about 10−14 d per orbit. In the case of NY Vir, a rapidly pulsating sdBVr with a cool companion, the period is discovered to be decreasing at a rate of −11.2 × 10−13 d per orbit. Close binary stars are particularly useful for the determination of fundamental stellar parameters. Double-lined spectroscopic binaries enable the mass ratio of the binary components to be determined and, if the inclination of the binary orbit can be measured or reasonably constrained (as in the case of an eclipsing system), then the absolute masses can be found. In addition, the light curve of an eclipsing system allows relative stellar radii to be found and even the absolute radii if the system is a double-lined binary.
Clustering of HI galaxies in the HI parkes all sky survey and arecibo legacy fast alfa survey
(Oxford University Press, 2011) Passmoor, S. S.; Cress, Catherine; Faltenbacher, A
We investigate the clustering of H I-selected galaxies in the Arecibo Legacy Fast ALFA Survey (ALFALFA) and compare results with those obtained for the H I Parkes All Sky Survey (HIPASS). Measurements of the angular correlation function and the inferred 3D clustering are compared with results from direct spatial-correlation measurements. We are able to measure clustering on smaller angular scales and for galaxies with lower H I masses than was previously possible. We calculate the expected clustering of dark matter using the redshift distributions of the HIPASS and ALFALFA, and show that the ALFALFA sample is somewhat more antibiased with respect to dark matter than the HIPASS sample.
Reconstruction of dark energy and expansion dynamics using Gaussian processes
(IOP Publishing, 2012) Seikel, Marina; Clarkson, Chris
An important issue in cosmology is reconstructing the effective dark energy equation of state directly from observations. With few physically motivated models, future dark energy studies cannot only be based on constraining a dark energy parameter space, as the errors found depend strongly on the parametrisation considered. We present a new non-parametric approach to reconstructing the history of the expansion rate and dark energy using Gaussian Processes, which is a fully Bayesian approach for smoothing data. We present a pedagogical introduction to Gaussian Processes, and discuss how it can be used to robustly differentiate data in a suitable way. Using this method we show that the Dark Energy Survey - Supernova Survey (DES) can accurately recover a slowly evolving equation of state to w = ±0.05 (95% CL) at z = 0 and ±0.25 at z = 0.7, with a minimum error of ±0.025 at the sweet-spot at z 0.16, provided the other parameters of the model are known. Errors on the expansion history are an order of magnitude smaller, yet make no assumptions about dark energy whatsoever.
Disentangling non-gaussianity, bias, and general relativistic effects in the galaxy distribution
(American Physical Society, 2012) Bruni, Marco; Crittenden, Robert; Maartens, Roy
Local non-Gaussianity, parametrized by f NL , introduces a scale-dependent bias that is strongest at large scales, precisely where general relativistic (GR) effects also become significant. With future data, it should be possible to constrain f NL = O ( 1 ) with high redshift surveys. GR corrections to the power spectrum and ambiguities in the gauge used to define bias introduce effects similar to f NL = O ( 1 ) , so it is essential to disentangle these effects. For the first time in studies of primordial non-Gaussianity, we include the consistent GR calculation of galaxy power spectra, highlighting the importance of a proper definition of bias. We present observable power spectra with and without GR corrections, showing that an incorrect definition of bias can mimic non-Gaussianity. However, these effects can be distinguished by their different redshift and scale dependence, so as to extract the true primordial non-Gaussianity.
Comparison of Hi and optical redshifts of galaxies - The impact of redshift uncertainties on spectral line stacking
(Oxford University Press, 2013) Maddox, Natasha; Hess, Kelley M.; Blyth, S.-L.; Jarvis, Matt
Accurate optical redshifts will be critical for spectral co-adding techniques used to extract detections from below the noise level in ongoing and upcoming surveys for Hi, which will extend our current understanding of gas reservoirs in galaxies to lower column densities and higher redshifts. We have used existing, high quality optical and radio data from the SDSS and ALFALFA surveys to investigate the relationship be- tween redshifts derived from optical spectroscopy and neutral hydrogen (Hi) spectral line observations.We find that the two redshift measurements agree well, with a negli- gible systematic offset and a small distribution width. Employing simple simulations, we determine how the width of an ideal stacked Hi profile depends on these redshift offsets, as well as larger redshift errors more appropriate for high redshift galaxy sur- veys. The width of the stacked profile is dominated by the width distribution of the input individual profiles when the redshift errors are less than the median width of the input profiles, and only when the redshift errors become large, 150 kms−1, do they significantly affect the width of the stacked profile. This redshift accuracy can be achieved with moderate resolution optical spectra. We provide guidelines for the number of spectra required for stacking to reach a specified mass sensitivity, given tele- scope and survey parameters, which will be useful for planning optical spectroscopy observing campaigns to supplement the radio data.
Evolution of faint radio sources in the VIDEO-XMM3 field
(OUP, 2013) McAlpine, K.; Jarvis, Matt; Bonfield, David G.
It has been speculated that low luminosity radio-loud AGN have the potential to serve as an important source of AGN feedback, and may be responsible for suppressing star-formation activity in massive elliptical galaxies at late times. As such the cosmic evolution of these sources is vitally important to understanding the significance of such AGN feedback processes and their influence on the global star-formation history of the universe. In this paper we present a new investigation of the evolution of faint radio sources out to z~2.5. We combine a 1 square degree VLA radio survey, complete to a depth of 100 μJy, with accurate 10 band photometric redshifts from the VIDEO and CFHTLS surveys. The results indicate that the radio population experiences mild positive evo- lution out to z~1.2 increasing their space density by a factor of ~3, consistent with results of several previous studies. Beyond z=1.2 there is evidence of a slowing down of this evolution. Star-forming galaxies drive the more rapid evolution at low redshifts, z<1.2, while more slowly evolving AGN populations dominate at higher redshifts resulting in a decline in the evolution of the radio luminosity function at z>1.2. The evolution is best fit by pure luminosity evolution with star-forming galaxies evolving as (1 + z)2.47±0.12 and AGN as (1 + z)1.18±0.21.
Anti-lensing: the bright side of voids
(American Physical Society, 2013) Bolejko, Krzysztof; Clarkson, Chris; Maartens, Roy; Bacon, David; Meures, Nikolai; Beynon, Emma
More than half of the volume of our Universe is occupied by cosmic voids. The lensing magni ca- tion e ect from those under-dense regions is generally thought to give a small dimming contribution: objects on the far side of a void are supposed to be observed as slightly smaller than if the void were not there, which together with conservation of surface brightness implies net reduction in photons received. This is predicted by the usual weak lensing integral of the density contrast along the line of sight. We show that this standard e ect is swamped at low redshifts by a relativistic Doppler term that is typically neglected. Contrary to the usual expectation, objects on the far side of a void are brighter than they would be otherwise. Thus the local dynamics of matter in and near the void is crucial and is only captured by the full relativistic lensing convergence. There are also signi cant nonlinear corrections to the relativistic linear theory, which we show actually under-predicts the e ect. We use exact solutions to estimate that these can be more than 20% for deep voids. This remains an important source of systematic errors for weak lensing density reconstruction in galaxy surveys and for supernovae observations, and may be the cause of the reported extra scatter of eld supernovae located on the edge of voids compared to those in clusters.
Periodic variations in the O-C diagrams of five pulsation frequencies of the DB white dwarf EC 20058-5234
(American Physical Society, 2013) Dalessio, J.; Sullivan, D. J.; Kilkenny, David; Provencal, J. L.; Shipman, H. L.; Sullivan, T.; Fraga, L.; Sefako, R.
Variations in the pulsation arrival time of five independent pulsation frequencies of the DB white dwarf EC 20058−5234 individually imitate the effects of reflex motion induced by a planet or companion but are inconsistent when considered in unison. The pulsation frequencies vary periodically in a 12.9 year cycle and undergo secular changes that are inconsistent with simple neutrino plus photon-cooling models. The magnitude of the periodic and secular variations increases with the period of the pulsations, possibly hinting that the corresponding physical mechanism is located near the surface of the star. The phase of the periodic variations appears coupled to the sign of the secular variations. The standards for pulsation-timing-based detection of planetary companions around pulsating white dwarfs, and possibly other variables such as subdwarf B stars, should be re-evaluated. The physical mechanism responsible for this surprising result may involve a redistribution of angular momentum or a magnetic cycle. Additionally, variations in a supposed combination frequency are shown to match the sum of the variations of the parent frequencies to remarkable precision, an expected but unprecedented confirmation of theoretical predictions.
Annealing effect of hybrid solar cells based on poly (3-hexylthiophene) and zinc-oxide nanostructures
(Elsevier, 2013) Motaung, David; Malgas, Gerald; Ray, Suprakas S.; Arendse, Christopher
The structural growth and optical and photovoltaic properties of the organic–inorganic hybrid structures of zinc oxide (ZnO)-nanorods/poly-3-hexylthiophene (P3HT) and two variations of organic polymer blends of ZnO/ P3HT:C60 fullerene and ZnO/P3HT:6,6]-phenyl C61 butyric acid methyl ester were studied in detail during thermal annealing. The ordering of the P3HT nanocrystals increased during annealing, which also improved hole transport in the hybrid structures. The optical constants of the ZnO/P3HT:[6,6]-phenyl C61 butyric acid methyl ester (PCBM) ﬁlms elevated with annealing temperature due to the improved crystallisation induced by the formation of P3HT crystalline domains. As a result, a maximum power conversion efﬁciency of approximately 1.03% was achieved for the annealed ZnO/P3HT:PCBM device at 140 °C. These ﬁndings indicate that ZnO-nanorods/P3HT:PCBM ﬁlms are stable at temperatures up to 160 °C.
Herschel-ATLAS/GAMA: What determines the far-infrared properties of radio galaxies?
(OUP, 2013) Virdee, J.S.; Hardcastle, M.J.; Jarvis, Matt
We perform a stacking analysis of Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) data in order to obtain isothermal dust temperatures and rest-frame luminosities at 250 µm (L250), for a well-defined sample of 1599 radio sources over the H-ATLAS Phase 1/Galaxy and Mass Assembly (GAMA) area. The radio sample is generated using a combination of NRAO VLA Sky Survey data and K-band United Kingdom Infrared Telescope Deep Sky Survey–Large Area Survey data, over the redshift range 0.01 30 kpc) counterparts. The higher dust temperature suggests that this may be attributed to enhanced SFRs in compact radio galaxies, but whether this is directly or indirectly due to radio activity (e.g. jet-induced or merger-driven star formation) is as yet unknown. For matched samples in LK and g –r , sub-1.5L∗ K and super-1.5L∗ K radio-detected galaxies have 0.89±0.18 and 0.49±0.12 times the 250μm luminosity of their non-radio-detected counterparts. Thus, while no difference in L250 is observed in sub-1.5L∗ K radio-detected galaxies, a strong deficit is observed in super-1.5L∗ K radio-detected galaxies. We explain these results in terms of the hotter, denser and richer halo environments massive radio galaxies maintain and are embedded in. These environments are expected to quench the cold gas and dust supply needed for further star formation and therefore dust production. Our results indicate that all massive radio galaxies (>1.5L∗ K) may have systematically lower FIR luminosities (∼25 per cent) than their colour-matched non-radio-detected counterparts. Finally, no relation between radio spectral index and L250 is found for the subset of 1.4-GHz radio sources with detections at 330 MHz.
Sample variance, source clustering and their influence on the counts of faint radio sources
(Oxford University Press, 2013) Heywood, Ian; Jarvis, Matt; Condon, James J.
The shape of the curves defined by the counts of radio sources per unit area as a function of their flux density was one of the earliest cosmological probes. Radio source counts continue to be an area of astrophysical interest as they can be used to study the relative populations of galaxy types in the Universe (as well as investigate any cosmological evolution in their respective luminosity functions). They are also a vital consideration for determining howsource confusion may limit the depth of a radio interferometer observation, and are essential for characterizing the extragalactic foregrounds in cosmicmicrowave background experiments. There is currently no consensus as to the relative populations of the faintest (sub-mJy) source types, where the counts show a turn-up. Most of the source count data in this regime are gathered from multiple observations that each use a deep, single pointing with an interferometric radio telescope. These independent count measurements exhibit large amounts of scatter (factors of the order of a few) that significantly exceeds their respective stated uncertainties. In this paper, we use a simulation of the extragalactic radio continuum emission to assess the level at which sample variance may be the cause of the scatter. We find that the scatter induced by sample variance in the simulated counts decreases towards lower flux density bins as the raw source counts increase. The field-to-field variations make significant contributions to the scatter in the measurements of counts derived from deep observations that consist of a single pointing, and could even be the sole cause at >100 μJy. We present a method for evaluating the flux density limit that a radio survey must reach in order to reduce the count uncertainty induced by sample variance to a specific value. We also derive a method for correcting Poisson errors on source counts from existing and future deep radio surveys in order to include the uncertainties due to the cosmological clustering of sources. A conclusive empirical constraint on the effect of sample variance at these low luminosities is unlikely to arise until the completion of future large-scale radio surveys with next-generation radio telescopes.
Gama/H-Atlas: The dust opacity-stellar mass surface density relation for spiral galaxies
(IOP Science, 2013) Grootes, M.W.; Tuffs, R.J.; Jarvis, Matt
We report the discovery of a well-defined correlation between B-band face-on central optical depth due to dust and the stellar mass surface density of nearby spiral galaxies. This relation was derived from a sample of spiral galaxies taken from the Galaxy and Mass Assembly (GAMA) survey, which were detected in the FIR/submm in the Herschel -ATLAS science demonstration phase field. Using a quantitative analysis of the NUV attenuation-inclination relation for complete samples of GAMA spirals categorized according to stellar mass surface density we demonstrate that this correlation can be used to statistically correct for dust attenuation purely on the basis of optical photometry and S´ersic-profile morphological fits.
The host haloes of O I absorbers in the reionization epoch
(Oxford University Press, 2013) Finlator, Kristian; Muñoz, Joseph A.; Dave, Romeel; Oppenheimer, B. D.; Peng Oh, S.; Özel, Feryal
We use a radiation hydrodynamic simulation of the hydrogen reionization epoch to study OI absorbers at z ∼ 6. The intergalactic medium (IGM) is reionized before it is enriched; hence, OI absorption originates within dark matter haloes. The predicted abundance of OI absorbers is in reasonable agreement with observations. At z = 10, ≈70 per cent of sightlines through atomically cooled haloes encounter a visible (NOI > 1014cm−2) column. Reionization ionizes and removes gas from haloes less massive than 108.4M , but 20 per cent of sightlines through more massive haloes encounter visible columns even at z = 5. The mass scale of absorber host haloes is 10–100 times smaller than the haloes of Lyman-break galaxies and Lyman α emitters, hence absorption probes the dominant ionizing sources more directly. OI absorbers have neutral hydrogen columns of 1019–1021 cm−2, suggesting a close resemblance between objects selected in OI and HI absorption. Finally, the absorption in the foreground of the z = 7.085 quasar ULAS J1120+0641 cannot originate in a dark matter halo because halo gas at the observed HI column density is enriched enough to violate the upper limits on the OI column. By contrast, gas at less than one-third the cosmic mean density satisfies the constraints. Hence, the foreground absorption likely originates in the IGM.
Existence domains of dust-acoustic solitons and supersolitons
(AIP Publishing, 2013) Maharaj, S.K.; Bharuthram, Ramashwar; Singh, S. V.; Lakhina, G. S.
Using the Sagdeev potential method, the existence of large amplitude dust-acoustic solitons and supersolitons is investigated in a plasma comprising cold negative dust, adiabatic positive dust, Boltzmann electrons, and non-thermal ions. This model supports the existence of positive potential supersolitons in a certain region in parameter space in addition to regular solitons having negative and positive potentials. The lower Mach number limit for supersolitons coincides with the occurrence of double layers whereas the upper limit is imposed by the constraint that the adiabatic positive dust number density must remain real valued. The upper Mach number limits for negative potential (positive potential) solitons coincide with limiting values of the negative (positive) potential for which the negative (positive) dust number density is real valued. Alternatively, the existence of positive potential solitons can terminate when positive potential double layers occur.
Octupole correlations in the structure of O2 bands in the N=88 nuclei150Sm Gd
(American Physical Society, 2013) Bvumbi, S.P.; Sharpey-Schafer, J.F.; Jones, P.M.; Mullins, S.M.
Knowledge of the exact microscopic structure of the 01 + ground state and first excited 02 + state in 150Sm is required to understand the branching of double β decay to these states from 150Nd. The detailed spectroscopy of 150Sm and 152Gd has been studied using (α,xn) reactions and the γ -ray arrays AFRODITE and JUROGAM II. Consistently strong E1 transitions are observed between the excited Kπ = 02 + bands and the lowest negative parity bands in both nuclei. These results are discussed in terms of the possible permanent octupole deformation in the first excited Kπ = 02 + band and also in terms of the “tidal wave” model of Frauendorf.
Galaxy correlations and the BAO in a void universe: structure formation as a test of the Copernican Principle
(IOP Science, 2013) February, Sean; Clarkson, Chris; Maartens, Roy
A suggested solution to the dark energy problem is the void model, where accelerated expansion is replaced by Hubble-scale inhomogeneity. In these models, density perturbations grow on a radially inhomogeneous background. This large scale inhomogeneity distorts the spherical Baryon Acoustic Oscillation feature into an ellipsoid which implies that the bump in the galaxy correlation function occurs at different scales in the radial and transverse correlation functions. We compute these for the first time, under the approximation that curvature gradients do not couple the scalar modes to vector and tensor modes. The radial and transverse correlation functions are very different from those of the concordance model, even when the models have the same average BAO scale. This implies that if void models are fine-tuned to satisfy average BAO data, there is enough extra information in the correlation functions to distinguish a void model from the concordance model. We expect these new features to remain when the full perturbation equations are solved, which means that the radial and transverse galaxy correlation functions can be used as a powerful test of the Copernican Principle.

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