Fuelling Active Galactic Nuclei

We suggest that most nearby active galactic nuclei are fed by a series of small--scale, randomly--oriented accretion events. Outside a certain radius these events promote rapid star formation, while within it they fuel the supermassive black hole. We show that the events have a characteristic time evolution. This picture agrees with several observational facts. The expected luminosity function is broadly in agreement with that observed for moderate--mass black holes. The spin of the black hole is low, and aligns with the inner disc in each individual feeding event. This implies radio jets aligned with the axis of the obscuring torus, and uncorrelated with the large--scale structure of the host galaxy. The ring of young stars observed about the Galactic Centre are close to where our picture predicts that star formation should occur.Comment: MNRAS, in pres

Radio and optical orientations of galaxies

We investigate the correlations between optical and radio isophotal position angles for 14302 SDSS galaxies with $r$ magnitudes brighter than 18 and which have been associated with extended FIRST radio sources. We identify two separate populations of galaxies using the colour, concentration and their principal components. Surprisingly strong statistical alignments are found: late-type galaxies are overwhelmingly biased towards a position angle differences of $0^{\circ}$ and early-type galaxies to $90^{\circ}$. The late-type alignment can be easily understood in terms of the standard picture in which the radio emission is intimately related to areas of recent star-formation. In early-type galaxies the radio emission is expected to be driven by accretion on to a nuclear black hole. We argue that the observed correlation of the radio axis with the minor axis of the large-scale stellar distribution gives a fundamental insight into the structure of elliptical galaxies, for example, whether or not the nuclear kinematics are decoupled form the rest of the galaxy. Our results imply that the galaxies are oblate spheroids with their radio emission aligned with the minor axis. Remarkably the strength of the correlation of the radio major axis with the optical minor axis depends on radio loudness. Those objects with a low ratio of FIRST radio flux density to total stellar light show a strong minor axis correlation while the stronger radio sources do not. This may reflect different formation histories for the different objects and we suggest we may be seeing the different behaviour of rationally supported and non-rotationally supported ellipticals.Comment: Version to appear in MNRA

Simultaneous rocket and MST radar observation of an internal gravity wave breaking in the mesosphere

In June, 1983, the Structure and Atmospheric Turbulence Environment (STATE) rocket and Poker Flat Mesophere-Stratosphere-Troposphere radar campaign was conducted to measure the interaction between turbulence, electron density and electron density gradient that has produced unusually strong MST radar echoes from the summer mesosphere over Poker Flat, Alaska. Analysis or radar wind measurements and a concurrent wind and temperature profile obtained from a rocket probe carrying a three-axis accelerometer are given. The two data sets provide a fairly complete (and in some cases, redundant) picture of the breaking (or more correctly, the saturation) of a large-amplitude, low-frequency, long-wavelength internal gravity wave. The data show that small-scale turbulence and small-scale wave intensity is greatest at those altitudes where the large-scale wave-induced temperature lapse rate is most negative or most nearly unstable, but the wind shear due to the large-scale wave is a minimum. A brief review of linear gravity-wave theory is presented as an aid to the identification of the gravity-wave signature in the radar and rocket data. Analysis of the time and height cross sections of wind speed and turbulence intensity observed by the Poker Flat MST radar follows. Then, the vertical profile of temperature and winds measured by a rocket probe examined. Finally, the use of the independent data sets provided by the rocket and the radar are discussed and implications for theories of wave saturation are presented

Magnetic and Gravitational Disk-Star Interactions: An Interdependence of PMS Stellar Rotation Rates and Spin-Orbit Misalignments

The presence of giant gaseous planets that reside in close proximity to their host stars may be a consequence of large-scale radial migration through the proto-planetary nebulae. Within the context of this picture, significant orbital obliquities characteristic of a substantial fraction of such planets can be attributed to external torques that perturb the disks out of alignment with the spin axes of their host stars. Therefore, the acquisition of orbital obliquity exhibits sensitive dependence on the physics of disk-star interactions. Here, we analyze the primordial excitation of spin-orbit misalignment of Sun-like stars, in light of disk-star angular momentum transfer. We begin by calculating the stellar pre-main sequence rotational evolution, accounting for spin-up due to gravitational contraction and accretion as well as spin-down due to magnetic star-disk coupling. We devote particular attention to angular momentum transfer by accretion, and show that while generally subdominant to gravitational contraction, this process is largely controlled by the morphology of the stellar magnetic field (i.e. specific angular momentum accreted by stars with octupole-dominated surface fields is smaller than that accreted by dipole-dominated stars by an order of magnitude). Subsequently, we examine the secular spin-axis dynamics of disk-bearing stars, accounting for the time-evolution of stellar and disk properties and demonstrate that misalignments are preferentially excited in systems where stellar rotation is not overwhelmingly rapid. Moreover, we show that the excitation of spin-orbit misalignment occurs impulsively, through an encounter with a resonance between the stellar precession frequency and the disk-torquing frequency. Cumulatively, the model developed herein opens up a previously unexplored avenue towards understanding star-disk evolution and its consequences in a unified manner.Comment: 18 pages, 7 figures, accepted to Ap

Phenomenology with a recoil-free jet axis: TMD fragmentation and the jet shape

We study the phenomenology of recoil-free jet axes using analytic calculations and Monte Carlo simulations. Our focus is on the average energy as function of the angle with the jet axis (the jet shape), and the energy and transverse momenta of hadrons in a jet (TMD fragmentation). We find that the dependence on the angle (or transverse momentum) is governed by a power law, in contrast to the double-logarithmic dependence for the standard jet axis. The effects of the jet radius, jet algorithm, angular resolution and grooming are investigated. TMD fragmentation is important for constraining the structure of the proton through semi-inclusive deep-inelastic scattering. These observables are also of interest to the LHC, for example to constrain $\alpha_s$ from precision jet measurements, or probe the quark-gluon plasma in heavy-ion collisions.Comment: 26 pages, 16 figures, version 2: matches Journal versio

Predicting beef carcass fatness using an image analysis system

The amount and distribution of subcutaneous fat is an important factor affecting beef carcass quality. The degree of fatness is determined by visual assessments scored on a scale of five fatness levels (the SEUROP system). New technologies such as the image analysis method have been developed and applied in an effort to enhance the accuracy and objectivity of this classification system. In this study, 50 young bulls were slaughtered (570 ± 52.5 kg) and after slaughter the carcasses were weighed (360 ± 33.1 kg) and a SEUROP system fatness score assigned. A digital picture of the outer surface of the left side of the carcass was taken and the area of fat cover (fat area) was measured using an image analysis system. Commercial cutting of the carcasses was performed 24 h post-mortem. The fat trimmed away on cutting (cutting fat) was weighed. A regression analysis was carried out for the carcass cutting fat (y-axis) on the carcass fat area (x-axis) to establish the accuracy of the image analysis system. A greater accuracy was obtained by the image analysis (R2 = 0.72; p 0.001). These results show the image analysis to be more accurate than the visual assessment system for predicting beef carcass fatness

Pulsar scintillations from corrugated reconnection sheets in the ISM

We show that surface waves along interstellar current sheets closely aligned with the line of sight lead to pulsar scintillation properties consistent with those observed. This mechanism naturally produces the length and density scales of the ISM scattering lenses that are required to explain the magnitude and dynamical spectrum of the scintillations. In this picture, the parts of warm ionized interstellar medium that are responsible for the scintillations are relatively quiescent, with scintillation and scattering resulting from weak waves propagating along magnetic domain boundary current sheets, which are both expected from helicity conservation and have been observed in numerical simulations. The model statistically predicts the spacing and amplitudes of inverted parabolic arcs seen in Fourier-transformed dynamical spectra of strongly scintillating pulsars with only 3 parameters. Multi-frequency, multi-epoch low frequency VLBI observations can quantitatively test this picture. If successful, in addition to mapping the ISM, this may open the door to precise nanoarcsecond pulsar astrometry, distance measurements, and emission studies using these 10AU interferometers in the sky.Comment: 9 pages, 10 figures, accepted in MNRA

How Filaments are Woven into the Cosmic Web

Observations indicate galaxies are distributed in a filament-dominated web-like structure. Numerical experiments at high and low redshift of viable structure formation theories also show filament-dominance. We present a simple quantitative explanation of why this is so, showing that the final-state web is actually present in embryonic form in the overdensity pattern of the initial fluctuations, with nonlinear dynamics just sharpening the image. The web is largely defined by the position and primordial tidal fields of rare events in the medium, with the strongest filaments between nearby clusters whose tidal tensors are nearly aligned. Applications of the cosmic web theory to observations include probing cluster-cluster bridges by weak gravitational lensing, X-rays, and the Sunyaev-Zeldovich effect and probing high redshift galaxy-galaxy bridges by low column density Lyman alpha absorption lines.Comment: 9 pages, gzipped uuencoded postscript file, 4 figures in separate files. The text + figures are also available from anonymous ftp site: ftp://ftp.cita.utoronto.ca/ftp/cita/bond/bkp_natur

Gribov's Equation for the Green Function of Light Quarks

Gribov's scenario of supercritical charges in QCD is investigated. We perform a numerical study of the corresponding equation for the Green function of light quarks. This is done in an approximation which neglects all pion contributions. Different types of solutions in the Euclidean region are discussed and the mass function of the quark is calculated. The solutions of the equation are shown to have a qualitatively different behaviour if the strong coupling constant alpha_s exceeds a critical value alpha_c = 0.43 in the infrared region. Chiral symmetry breaking is found to occur at supercritical coupling. The analytic structure of the solutions is investigated. Earlier results obtained by Gribov are confirmed and extended.Comment: 30 pages, 18 figure