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.
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.
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.
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.
Radio-loud active calactic nucleus: is there a link between luminosity and cluster environment?
(American Physical Society, 2013) Ineson, J.; Croston, J. H.; Jarvis, Matt; Hardcastle, M.J.; Evans, D. A.; Kraft, R. P.
We present here the first results from the Chandra ERA (Environments of Radio-loud AGN) Large Project, characterizing the cluster environments of a sample of 26 radio-loud active galactic nuclei (AGNs) at z ∼ 0.5 that covers three decades of radio luminosity. This is the first systematic X-ray environmental study at a single epoch, and has allowed us to examine the relationship between radio luminosity and cluster environment without the problems ofMalmquist bias.We have found a weak correlation between radio luminosity and host cluster X-ray luminosity, as well as tentative evidence that this correlation is driven by the subpopulation of low-excitation radio galaxies, with high-excitation radio galaxies showing no significant correlation. The considerable scatter in the environments may be indicative of complex relationships not currently included in feedback models.
The neutral hydrogen content of galaxies in cosmological hydrodynamic simulations
(Oxford University Press, 2013) Dave, Romeel; Katz, Neal; Oppenheimer, Benjamin D.; Kollmeier, Juna A.; Weinberg, David H.
We examine the global HI properties of galaxies in quarter billion particle cosmological simulations using GADGET-2, focusing on howgalactic outflows impactHI content.We consider four outflow models, including a new one (ezw) motivated by recent interstellar medium simulations in which the wind speed and mass loading factor scale as expected for momentumdriven outflows for larger galaxies and energy-driven outflows for dwarfs (σ <75 km s−1). To obtain predicted HI masses, we employ a simple but effective local correction for particle selfshielding and an observationally constrained transition from neutral to molecular hydrogen. Our ezw simulation produces an HI mass function whose faint-end slope of −1.3 agrees well with observations from the Arecibo Fast Legacy ALFA survey; other models agree less well. Satellite galaxies have a bimodal distribution in HI fraction versus halo mass, with smaller satellites and/or those in larger haloes more often being HI deficient. At a given stellar mass, HI content correlates with the star formation rate and inversely correlates with metallicity, as expected if driven by stochasticity in the accretion rate. To higher redshifts, massive HI galaxies disappear and the mass function steepens. The global cosmic HI density conspires to remain fairly constant from z ∼ 5→0, but the relative contribution from smaller galaxies increases with redshift.
The slow flow model of dust eflux in local star-forming galaxies
(Oxford University Press, 2013) Zahid, H.; Torrey, Paul; Dave, Romeel; Geller, Margaret; Kudritzki, Rolf; Kewley, Lisa J.
We develop a dust efflux model of radiation pressure acting on dust grains which successfully reproduces the relation between stellar mass, dust opacity and star forma- tion rate observed in local star-forming galaxies. The dust content of local star-forming galaxies is set by the competition between the physical processes of dust production and dust loss in our model. The dust loss rate is proportional to the dust opacity and star formation rate. Observations of the relation between stellar mass and star formation rate at several epochs imply that the majority of local star-forming galax- ies are best characterized as having continuous star formation histories. Dust loss is a consequence of sustained interaction of dust with the radiation field generated by continuous star formation. Dust efflux driven by radiation pressure rather than dust destruction offers a more consistent physical interpretation of the dust loss mechanism. By comparing our model results with the observed relation between stellar mass, dust extinction and star formation rate in local star-forming galaxies we are able to con- strain the timescale and magnitude of dust loss. The timescale of dust loss is long and therefore dust is effluxed in a “Slow Flow". Dust loss is modest in low mass galaxies but massive galaxies may lose up to 70 80% of their dust over their lifetime. Our Slow Flow model shows that mass loss driven by dust opacity and star formation may be an important physical process for understanding normal star-forming galaxy evolution.
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.
VLT/XSHOOTER & Subaru/MOIRCS spectroscopy of HUDF-YD3: No evidence for Lyman-alpha emission at z=8.55
(Oxford University Press, 2013) Bunker, Andrew J.; Caruana, Joseph; Wilkins, Stephen M.; Jarvis, Matt
We present spectroscopic observations with VLT/XSHOOTER and Subaru/MOIRCS of a relatively bright Y -band drop-out galaxy in the Hubble Ultra Deep Field, first selected by Bunker et al. (2010), McLure et al. (2010) and Bouwens et al. (2010) to be a likely z ≈ 8 − 9 galaxy on the basis of its colours in the HST ACS and WFC 3 images. This galaxy, HUDF.YD3 (also known as UDFy-38135539) has been targetted for VLT/SINFONI integral field spectroscopy by Lehnert et al. (2010), who published a candidate Lyman-α emission line at z = 8.55 from this source. In our independent spectroscopy using two different infrared spectrographs (5 hours with VLT/XSHOOTER and 11 hours with Subaru/MOIRCS) we are unable to reproduce this line. We do not detect any emission line at the spectral and spatial location reported in Lehnert et al. (2010), despite the expected signal in our combined MOIRCS & XSHOOTER data being 5 σ. The line emission also seems to be ruled out by the faintness of this object in recently extremely deep F105W (Y -band) HST/WFC 3 imaging from HUDF12; the line would fall within this filter and such a galaxy should have been detected at YAB = 28.6 mag ( ∼ 20σ) rather than the marginal YAB ≈ 30 mag observed in the Y -band image, > 3 times fainter than would be expected if the emission lie was real. Hence it appears highly unlikely that the reported Lyman-α line emission at z > 8 is real, meaning that the highest-redshift sources for which Lyman-α emission has been seen are at z = 6.9 − 7.2. It is conceivable that Lyman-α does not escape galaxies at higher redshifts, where the Gunn-Peterson absorption renders the Universe optically thick to this line. However, deeper spectroscopy on a larger sample of candidate z > 7 galaxies will be needed to test this.
Cosmology with photometrically classified type IA supernovae from the SDSS-II supernova survey
(The American Astronomical Society, 2013) Campbell, Heather; D’Andrea, Chris B.; Nichol, Robert C.; Sako, Masao; Smith, Mathew
We present the cosmological analysis of 752 photometrically–classified Type Ia Supernovae (SNe Ia) obtained from the full Sloan Digital Sky Survey II (SDSS-II) Supernova (SN) Survey, supplemented with host–galaxy spectroscopy from the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). Our photometric–classificationmethod is based on the SN typing technique of Sako et al. (2011), aided by host galaxy redshifts (0.05 < z < 0.55). SNANA simulations of our methodology estimate that we have a SN Ia typing efficiency of 70.8%, with only 3.9% contamination from core-collapse (non-Ia) SNe. We demonstrate that this level of contamination has no effect on our cosmological constraints. We quantify and correct for our selection effects (e.g., Malmquist bias) using simulations. When fitting to a flat _CDM cosmological model, we find that our photometric sample alone gives Ωm = 0.24+0.07/−0.05 (statistical errors only). If we relax the constraint on flatness, then our sample provides competitive joint statistical constraints on Ωm and Ω∆, comparable to those derived from the spectroscopically- confirmed three-year Supernova Legacy Survey (SNLS3). Using only our data, the statistics–only result favors an accelerating universe at 99.96% confidence. Assuming a constant wCDM cosmological model, and combining with H0, CMB and LRG data, we obtain w = −0.96+0.10/−0.10, Ωm = 0.29+0.02/−0.02 and Ωk = 0.00+0.03/−0.02 (statistical errors only), which is competitive with similar spectroscopically confirmed SNe Ia analyses. Overall this comparison is reassuring, considering the lower redshift leverage of the SDSS-II SN sample (z < 0.55) and the lack of spectroscopic confirmation used herein. These results demonstrate the potential of photometrically–classified SNe Ia samples in improving cosmological constraints.
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.
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.
Black hole - galaxy correlations without self-regulation
(American Astronomical Society, 2013) Angles-Alcazar, Daniel; Ozel, Feryal; Dave, Romeel
Recent models of black hole growth in a cosmological context have forwarded a paradigm in which the growth is self-regulated by feedback from the black hole itself. Here we use cosmological zoom simulations of galaxy formation down to z =2 to show that such strong self-regulation is required in the popular spherical Bondi accretion model, but that a plausible alternative model in which black hole growth is limited by galaxy-scale torques does not require self-regulation. Instead, this torque-limited accretion model yields black holes and galaxies evolving on average along the observed scaling relations by relying only on a fixed, 5% mass retention rate onto the black hole from the radius at which the accretion flow is fed. Feedback from the black hole may (and likely does) occur, but does not need to couple to galaxy-scale gas in order to regulate black hole growth. We show that this result is insensitive to variations in the initial black hole mass, stellar feedback, or other implementation details. The torque-limited model allows for high accretion rates at very early epochs (unlike the Bondi case), which if viable can help explain the rapid early growth of black holes, while by z ∼ 2 it yields Eddington factors of ∼1%–10%. This model also yields a less direct correspondence between major merger events and rapid phases of black hole growth. Instead, growth is more closely tied to cosmological disk feeding, which may help explain observational studies showing that, at least at z >~ 1, active galaxies do not preferentially show merger signatures.
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.

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