arXiv subjects

Paul Alexander

Publications and source records attributed to Paul Alexander.

At least 55 records · Page 3Linked to original sources

arXiv2014

Radio Source Evolution on Galactic Scales

There is mounting evidence that mechanical radio source feedback is important in galaxy evolution and in order to quantify this feedback, detailed models of radio source evolution are required. We present an extension to current analytic models that encompasses young radio sources with physical sizes on sub-kiloparsec scales. This work builds on an existing young source dynamical model to include radiative losses in a flat environment, and as such, is the best physically-motivated Compact Symmetric Object model to date. Results predict that young radio sources experience significant radiative loss on length scales and spectral scales consistent with observed Compact Steep-Spectrum sources. We include full expressions for the transition to self-similar expansion and present this complete model of radio source evolution from first cocoon formation to end of source lifetime around 10^8 years within the context of a simplified King profile external atmosphere.

astro-ph.GA↗

arXiv2013

How to take the interstellar weather with you in pulsar timing analysis

Here we present a Bayesian method of including discrete measurements of dispersion measure due to the interstellar medium in the direction of a pulsar as prior information in the analysis of that pulsar. We use a simple simulation to show the efficacy of this method, where the inclusion of the additional measurements results in both a significant increase in the precision with which the timing model parameters can be obtained, and an improved upper limit on the amplitude of any red noise in the dataset. We show that this method can be applied where no multi-frequency data exists across much of the dataset, and where there is no simultaneous multi-frequency data for any given observing epoch. Including such information in the analysis of upcoming International Pulsar Timing Array (IPTA) and European Pulsar Timing Array (EPTA) data releases could therefore prove invaluable in obtaining the most constraining limits on gravitational wave signals within those datasets.

astro-ph.IM↗

arXiv2013

TempoNest: A Bayesian approach to pulsar timing analysis

A new Bayesian software package for the analysis of pulsar timing data is presented in the form of TempoNest which allows for the robust determination of the non-linear pulsar timing solution simultaneously with a range of additional stochastic parameters. This includes both red spin noise and dispersion measure variations using either power law descriptions of the noise, or through a model-independent method that parameterises the power at individual frequencies in the signal. We use TempoNest to show that at noise levels representative of current datasets in the European Pulsar Timing Array (EPTA) and International Pulsar Timing Array (IPTA) the linear timing model can underestimate the uncertainties of the timing solution by up to an order of magnitude. We also show how to perform Bayesian model selection between different sets of timing model and stochastic parameters, for example, by demonstrating that in the pulsar B1937+21 both the dispersion measure variations and spin noise in the data are optimally modelled by simple power laws. Finally we show that not including the stochastic parameters simultaneously with the timing model can lead to unpredictable variation in the estimated uncertainties, compromising the robustness of the scientific results extracted from such analysis.

astro-ph.IM↗

arXiv2013

Hyper-efficient model-independent Bayesian method for the analysis of pulsar timing data

A new model independent method is presented for the analysis of pulsar timing data and the estimation of the spectral properties of an isotropic gravitational wave background (GWB). We show that by rephrasing the likelihood we are able to eliminate the most costly aspects of computation normally associated with this type of data analysis. When applied to the International Pulsar Timing Array Mock Data Challenge data sets this results in speedups of approximately 2 to 3 orders of magnitude compared to established methods. We present three applications of the new likelihood. In the low signal to noise regime we sample directly from the power spectrum coefficients of the GWB signal realization. In the high signal to noise regime, where the data can support a large number of coefficients, we sample from the joint probability density of the power spectrum coefficients for the individual pulsars and the GWB signal realization. Critically in both these cases we need make no assumptions about the form of the power spectrum of the GWB, or the individual pulsars. Finally we present a method for characterizing the spatial correlation between pulsars on the sky, making no assumptions about the form of that correlation, and therefore providing the only truly general Bayesian method of confirming a GWB detection from pulsar timing data.

astro-ph.IM↗

arXiv2012

Future Science Prospects for AMI

The Arcminute Microkelvin Imager (AMI) is a telescope specifically designed for high sensitivity measurements of low-surface-brightness features at cm-wavelength and has unique, important capabilities. It consists of two interferometer arrays operating over 13.5-18 GHz that image structures on scales of 0.5-10 arcmin with very low systematics. The Small Array (AMI-SA; ten 3.7-m antennas) couples very well to Sunyaev-Zel'dovich features from galaxy clusters and to many Galactic features. The Large Array (AMI-LA; eight 13-m antennas) has a collecting area ten times that of the AMI-SA and longer baselines, crucially allowing the removal of the effects of confusing radio point sources from regions of low surface-brightness, extended emission. Moreover AMI provides fast, deep object surveying and allows monitoring of large numbers of objects. In this White Paper we review the new science - both Galactic and extragalactic - already achieved with AMI and outline the prospects for much more.

astro-ph.IM↗

arXiv2012

A new connection between the opening angle and the large-scale morphology of extragalactic radio sources

In the case of an initially conical jet, we study the relation between jet collimation by the external pressure and large-scale morphology. We first consider the important length-scales in the problem, and then carry out axisymmetric hydrodynamic simulations that include, for certain parameters, all these length-scales. We find three important scales related to the collimation region: (i) where the sideways ram-pressure equals the external pressure, (ii) where the jet density equals the ambient density, and (iii) where the forward ram-pressure falls below the ambient pressure. These scales are set by the external Mach-number and opening angle of the jet. We demonstrate that the relative magnitudes of these scales determine the collimation, Mach-number, density and morphology of the large scale jet. Based on analysis of the shock structure, we reproduce successfully the morphology of Fanaroff-Riley (FR) class I and II radio sources. Within the framework of the model, an FR I radio source must have a large intrinsic opening angle. Entrainment of ambient gas might also be important. We also show that all FR I sources with radio lobes or similar features must have had an earlier FR II phase.

astro-ph.CO↗

arXiv2011

Interaction of Fanaroff-Riley class II jets with a magnetised intra-cluster medium

We present 3-D MHD and synthetic numerical simulations to follow the evolution of randomly magnetized intra-cluster medium plasma under the effects of powerful, light, hypersonic and bipolar jets. We prescribe the cluster magnetic field (CMF) as a Gaussian random field with power law energy spectrum tuned to the expectation for Kolmogorov turbulence. We investigate the power of jets and the viewing angle used for the synthetic Rotation Measure (RM) observations. We find the model radio sources introduce and amplify fluctuations on the RM statistical properties; the average RM and the RM standard deviation are increased by the action of the jets. This may lead to overestimations of the CMFs' strength up to 70%. The effect correlates with the jet power. Jets distort and amplify CMFs especially near the edges of the lobes and the jets' heads. Thus the RM structure functions are flattened at scales comparable to the source size. Jet-produced RM enhancements depend on the orientation of the jet axis to the line of sight and are thus more apparent in quasars than in radio galaxies.

astro-ph.CO↗

arXiv2011

Interaction of Fanaroff-Riley class II radio jets with a randomly magnetised intra-cluster medium

A combination of three-dimensional (3D) magnetohydrodynamics (MHD) and synthetic numerical simulations are presented to follow the evolution of a randomly magnetised plasma that models the intra-cluster medium (ICM), under the isolated effects of powerful, light, hypersonic and bipolar Fanaroff-Riley class II (FR II) jets. We prescribe the cluster magnetic field (CMF) as a Gaussian random field with a Kolmogorov-like energy spectrum. Both the power of the jets and the viewing angle that is used for the synthetic Rotation Measure (RM) observations are investigated. We find the model radio sources introduce and amplify fluctuations on the RM statistical properties which we analyse as a function of time as well as the viewing angle. The average RM and the RM standard deviation are increased by the action of the jets. Energetics, RM statistics and magnetic power spectral analysis consistently show that the effects also correlate with the jets' power, and that the lightest, fastest jets produce the strongest changes in their environment. We see jets distort and amplify the CMFs especially near the edges of the lobes and the jets' heads. This process leads to a flattening of the RM structure functions at scales comparable to the source size. The edge features we find are similar to ones observed in Hydra A. The results show that jet-produced RM enhancements are more apparent in quasars than in radio galaxies. Globally, jets tend to enhance the RM standard deviation which may lead to overestimations of the CMFs' strength by about 70%. This study means to serve as a pathfinder for the SKA, EVLA and LOFAR to follow the evolution of cosmic magnetic fields.

astro-ph.CO↗

arXiv2011

Three-dimensional magnetohydrodynamic simulations of the evolution of magnetic fields in Fanaroff-Riley class II radio sources

Radio observations of Fanaroff-Riley class II sources often show correlations between the synchrotron emission and the linear-polarimetric distributions. Magnetic position vectors seem to align with the projected emission of both the radio jets and the sources' edges. Using statistics we study such relation as well as its unknown time evolution via synthetic polarisation maps of model FR II sources formed in 3D-MHD numerical simulations of bipolar, hypersonic and weakly magnetised jets. The magnetic field is initially random with a Kolmogorov power spectrum, everywhere. We investigate the structure and evolution of magnetic fields in the sources as a function of the power of jets and the observational viewing angle. Our synthetic polarisation maps agree with observations, showing B-field vectors which are predominantly aligned with the jet axis, and show that magnetic fields inside sources are shaped by the jets' backflow. Polarimetry is found to correlate with time, the viewing angle and the jet-to-ambient density contrast. The magnetic structure inside thin elongated sources is more uniform than inside more spherical ones. We see jets increase the magnetic energy in cocoons in proportion to the jet velocity and the cocoon width. Filaments in the synthetic emission maps suggest turbulence develops in evolved sources.

astro-ph.CO↗

arXiv2010

3D-MHD simulations of the evolution of magnetic fields in FR II radio sources

3D-MHD numerical simulations of bipolar, hypersonic, weakly magnetized jets and synthetic synchrotron observations are presented to study the structure and evolution of magnetic fields in FR II radio sources. The magnetic field setup in the jet is initially random. The power of the jets as well as the observational viewing angle are investigated. We find that synthetic polarization maps agree with observations and show that magnetic fields inside the sources are shaped by the jets' backflow. Polarimetry statistics correlates with time, the viewing angle and the jet-to-ambient density contrast. The magnetic structure inside thin elongated sources is more uniform than for ones with fatter cocoons. Jets increase the magnetic energy in cocoons, in proportion to the jet velocity. Both, filaments in synthetic emission maps and 3D magnetic power spectra suggest that turbulence develops in evolved sources.

astro-ph.CO↗

arXiv2010

Multipath Parameter Estimation from OFDM Signals in Mobile Channels

We study multipath parameter estimation from orthogonal frequency division multiplex signals transmitted over doubly dispersive mobile radio channels. We are interested in cases where the transmission is long enough to suffer time selectivity, but short enough such that the time variation can be accurately modeled as depending only on per-tap linear phase variations due to Doppler effects. We therefore concentrate on the estimation of the complex gain, delay and Doppler offset of each tap of the multipath channel impulse response. We show that the frequency domain channel coefficients for an entire packet can be expressed as the superimposition of two-dimensional complex sinusoids. The maximum likelihood estimate requires solution of a multidimensional non-linear least squares problem, which is computationally infeasible in practice. We therefore propose a low complexity suboptimal solution based on iterative successive and parallel cancellation. First, initial delay/Doppler estimates are obtained via successive cancellation. These estimates are then refined using an iterative parallel cancellation procedure. We demonstrate via Monte Carlo simulations that the root mean squared error statistics of our estimator are very close to the Cramer-Rao lower bound of a single two-dimensional sinusoid in Gaussian noise.

cs.IT↗

arXiv2010

Magnetic fields in galaxies: I. Radio disks in local late-type galaxies

We develop an analytical model to follow the cosmological evolution of magnetic fields in disk galaxies. Our assumption is that fields are amplified from a small seed field via magnetohydrodynamical (MHD) turbulence. We further assume that this process is fast compared to other relevant timescales, and occurs principally in the cold disk gas. We follow the turbulent energy density using the Shabala & Alexander (2009) galaxy formation and evolution model. Three processes are important to the turbulent energy budget: infall of cool gas onto the disk and supernova feedback increase the turbulence; while star formation removes gas and hence turbulent energy from the cold gas. Finally, we assume that field energy is continuously transferred from the incoherent random field into an ordered field by differential galactic rotation. Model predictions are compared with observations of local late type galaxies by Fitt & Alexander (1993) and Shabala et al. (2008). The model reproduces observed magnetic field strengths and luminosities in low and intermediate-mass galaxies. These quantities are overpredicted in the most massive hosts, suggesting that inclusion of gas ejection by powerful AGNs is necessary in order to quench gas cooling and reconcile the predicted and observed magnetic field strengths.

astro-ph.CO↗

arXiv2009

Estimation of Magnetic Field Strength in the Turbulent Warm Ionized Medium

We studied Faraday rotation measure (RM) in turbulent media with the rms Mach number of unity, using isothermal, magnetohydrodynamic turbulence simulations. Four cases with different values of initial plasma beta were considered. Our main findings are as follows. (1) There is no strong correlation between the fluctuations of magnetic field strength and gas density. So the magnetic field strength estimated with RM/DM (DM is the dispersion measure) correctly represents the true mean strength of the magnetic field along the line of sight. (2) The frequency distribution of RMs is well fitted to the Gaussian. In addition, there is a good relation between the width of the distribution of RM/$\bar{\rm RM}$ ($\bar{\rm RM}$ is the average value of RMs) and the strength of the regular field along the line of sight; the width is narrower, if the field strength is stronger. We discussed the implications of our findings in the warm ionized medium where the Mach number of turbulent motions is around unity.

astro-ph.GA↗

arXiv2009

Measurements of the Cosmological Evolution of Magnetic Fields with the Square Kilometre Array

We investigate the potential of the Square Kilometre Array (SKA) for measuring the magnetic fields in clusters of galaxies via Faraday rotation of background polarised sources. [...] We find that about 10 per cent of the sky is covered by a significant extragalactic Faraday screen. Most of it has rotation measures between 10 and 100 rad/m/m. We argue that the cluster centres should have up to about 5000 rad/m/m. We show that the proposed mid frequency aperture array of the SKA as well as the lowest band of the SKA dish array are well suited to make measurements for most of these rotation measure values, typically requiring a signal-to-noise of ten. We calculate the spacing of sources forming a grid for the purpose of measuring foreground rotation measures: it reaches a spacing of 36 arcsec for a 100 hour SKA observation per field. We also calculate the statistics for background RM measurements in clusters of galaxies. We find that a first phase of the SKA would allow us to take stacking experiments out to high redshifts (>1), and provide improved magnetic field structure measurements for individual nearby clusters. The full SKA aperture array would be able to make very detailed magnetic field structure measurements of clusters with more than 100 background sources per cluster up to a redshift of 0.5 and more than 1000 background sources per cluster for nearby clusters, and could for reasonable assumptions about future measurements of electron densities in high redshift clusters constrain the power law index for the magnetic field evolution to better than dm=0.4, if the magnetic field in clusters should follow B ~ (1+z)^m.

astro-ph.CO↗

arXiv2009

Radio source feedback in galaxy evolution

We present a galaxy evolution model which incorporates a physically motivated implementation of AGN feedback. Intermittent jets inflate cocoons of radio plasma which then expand supersonically, shock heating the ambient gas. The model reproduces observed star formation histories to the highest redshifts for which reliable data exists, as well as the observed galaxy colour bimodality. Intermittent radio source feedback also naturally provides a way of keeping the black hole and spheroid growth in step. We find possible evidence for a top-heavy Initial Mass Function (IMF) for $z>2$, consistent with observations of element abundances, and sub-mm and Lyman break galaxy counts.

astro-ph.CO↗

arXiv2009

The relationship between star formation rate and radio synchrotron luminosity at 0 < z < 2

We probe the relationship between star formation rate (SFR) and radio synchrotron luminosity in galaxies at 0 < z < 2 within the northern Spitzer Wide-area Infrared Extragalactic survey (SWIRE) fields, in order to investigate some of the assumptions that go into calculating the star formation history of the Universe from deep radio observations. We present new 610-MHz Giant Metrewave Radio Telescope (GMRT) observations of the European Large-Area ISO Survey (ELAIS)-N2 field, and using this data, along with previous GMRT surveys carried out in the ELAIS-N1 and Lockman Hole regions, we construct a sample of galaxies which have redshift and SFR information available from the SWIRE survey. We test whether the local relationship between SFR and radio luminosity is applicable to z = 2 galaxies, and look for evolution in this relationship with both redshift and SFR in order to examine whether the physical processes which lead to synchrotron radiation have remained the same since the peak of star formation in the Universe. We find that the local calibration between radio luminosity and star formation can be successfully applied to radio-selected high-redshift, high-SFR galaxies, although we identify a small number of sources where this may not be the case; these sources show evidence for inaccurate estimations of their SFR, but there may also be some contribution from physical effects such as the recent onset of starburst activity, or suppression of the radio luminosity within these galaxies.

astro-ph.GA↗

arXiv2008

Radio source stacking and the infrared / radio correlation at microJy flux densities

We investigate the infrared / radio correlation using the technique of source stacking, in order to probe the average properties of radio sources that are too faint to be detected individually. We compare the two methods used in the literature to stack sources, and demonstrate that the creation of stacked images leads to a loss of information. We stack infrared sources in the Spitzer extragalactic First Look Survey (xFLS) field, and the three northern Spitzer Wide-area Infrared Extragalactic survey (SWIRE) fields, using radio surveys created at 610 MHz and 1.4 GHz, and find a variation in the absolute strength of the correlation between the xFLS and SWIRE regions, but no evidence for significant evolution in the correlation over the 24-um flux density range 150 uJy - 2 mJy. We carry out the first radio source stacking experiment using 70-um-selected galaxies, and find no evidence for significant evolution over the 70-um flux density range 10 mJy - 100 mJy.

astro-ph↗

arXiv2008

Dynamical and chemical effects of FR II radio sources on the intra-cluster medium

We investigate the effects of intermittent strong jets from an Active Galactic Nuclei (AGN) of a massive galaxy in the core of a cool core galaxy cluster, on the dynamics and metal distribution of the intra-cluster medium (ICM). We use a simple model for the metal distribution within the host galaxy which includes metal injection via star formation. We carry out 2D axisymmetric hydrodynamic simulations of these systems. After having established a cooling flow, two light jets are injected in opposite directions with a range of (intermittent) active phases. We follow the time evolution of the system from the jets' active phases up to 3 Gyr. The general metallicity evolution for all our simulations is very similar on large-scales. The convective flows driven by the jets advect gas and metals from the central galaxy to distances beyond 1.5 Mpc within the cluster. Intermittent jets are able to distribute metals to grater radii. The metal injection has effects on the general metal abundances present in the ICM, the faster the metal replenishment time the higher the metal abundance. Although metallicity gradients in the very central regions of cool core clusters are likely to be shaped by less-energetic phenomena, we find evidence in our simulations for metallicity gradients similar to those observed out to ~400 kpc radius. The metal distribution details of the central galaxy have no effects on the dynamical evolution of the ICM metals.

astro-ph↗