Radio Loudness of AGNs: Host Galaxy Morphology and the Spin Paradigm

We investigate how the total radio luminosity of AGN-powered radio sources depends on their accretion luminosity and the central black hole mass. We find that AGNs form two distinct and well separated sequences on the radio-loudness - Eddington-ratio plane. We argue that these sequences mark the real upper bounds of radio-loudness of two distinct populations of AGNs: those hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radio-loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that another parameter in addition to the accretion rate must play a role in determining the jet production efficiency in active galactic nuclei, and that this parameter is related to properties of the host galaxy. The revealed host-related radio dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars hosted by elliptical galaxies is radio quiet. We argue that the huge difference between the radio-loudness reachable by AGNs in disc and elliptical galaxies can be explained by the scenario according to which the spin of a black hole determines the outflow's power, and central black holes can reach large spins only in early type galaxies (following major mergers), and not (in a statistical sense) in spiral galaxies.Comment: 7 pages, 4 figures included. Proceedings of the Workshop `Extragalactic Jets: Theory and Observation from Radio to Gamma Ray', Girdwood, May 200

Sound propagation around underwater seamounts

Submitted in partial fulfillment of the requirements for Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2005This thesis develops and utilizes a method for analyzing data from the North Pacific Acoustic Laboratory's (NPAL) Basin Acoustic Seamount Scattering Experiment (BASSEX). BASSEX was designed to provide data to support the development of analytical techniques and methods which improve the understanding of sound propagation around underwater seamounts. The depth-dependent sound velocity profile of typical ocean waveguides force sound to travel in convergence zones about a minimum sound speed depth. This ducted nature of the ocean makes modeling the acoustic field around seamounts particularly challenging, compared to an isovelocity medium. The conical shape of seamounts also adds to the complexity of the scatter field. It is important to the U.S. Navy to understand how sound is diffracted around this type of topographic feature. Underwater seamounts can be used to conceal submarines by absorbing and scattering the sound they emit. BASSEX measurements have characterized the size and shape of the forward scatter field around the Kermit-Roosevelt Seamount in the Pacific Ocean. Kermit- Roosevelt is a large, conical seamount which shoals close to the minimum sound speed depth, making it ideal for study. Acoustic sources, including M-sequence and linear frequency-modulated sources, were stationed around the seamount at megameter ranges. A hydrophone array was towed around the seamount to locations which allowed measurement of the perturbation zone. Results from the method developed in this thesis show that the size and shape of the perturbation zone measured coincides with theoretical and experimental results derived in previous work

QPTAS and Subexponential Algorithm for Maximum Clique on Disk Graphs

A (unit) disk graph is the intersection graph of closed (unit) disks in the plane. Almost three decades ago, an elegant polynomial-time algorithm was found for Maximum Clique on unit disk graphs [Clark, Colbourn, Johnson; Discrete Mathematics '90]. Since then, it has been an intriguing open question whether or not tractability can be extended to general disk graphs. We show the rather surprising structural result that a disjoint union of cycles is the complement of a disk graph if and only if at most one of those cycles is of odd length. From that, we derive the first QPTAS and subexponential algorithm running in time 2^{O~(n^{2/3})} for Maximum Clique on disk graphs. In stark contrast, Maximum Clique on intersection graphs of filled ellipses or filled triangles is unlikely to have such algorithms, even when the ellipses are close to unit disks. Indeed, we show that there is a constant ratio of approximation which cannot be attained even in time 2^{n^{1-epsilon}}, unless the Exponential Time Hypothesis fails

Use of a municipal sewage sludge as a source of fertilizer nitrogen : nitrogen availability and cadmium-zinc interactions

A mixture of primary and aerobic secondary municipal sewage sludge was obtained from Maryville, Tennessee to determine its suitability as a fertilizer for corn (Zea mays L.). In the field experiment, a check, an inorganic N fertilization rate (168 kg ha-1 as NH4NO3), and three application rates of sludge (17, 34, and 51 Mg ha-1 at 0.18 kg kg-1 solids were compared for crop response at two levels of soil pH (pH \u3c 5.5 and pH ≥ 6.0). Sludge additions increased silage and grain yields due to increased available N supplied by the sludge. The silage yields from the sludge application of 51 mg ha-1 were similar to the inorganic N application rate of 168 kg ha-1. Grain yields from the highest sludge application were not as great as from the inorganic N application, probably due to a lower availability of N from the organic source. When considering potential toxicity to the crop as well as to potential consumers, there were no harmful levels of Cu, Ni, Cd, and Zn present in leaf tissues or grain even with a moderately acidic soil (pH 5.2). A greenhouse experiment was initiated to analyze the interaction of Cd and Zn in a soil, sludge, and plant system. Sludge obtained from Maryville, Tennessee was spiked with four rates of Cd and Zn in a completely randomized factorial arrangement and applied to soil at application rates of 7, 229, 451, and 673 μmol Cd kg-1 and 0.9, 4.7, 8.5, and 12.3 mmol Zn kg-1 The highest Cd and Zn additions resulted in yield increments of 11% and 20% respectively. The modest yield reduction due to Cd was attributed to the high percentage of fixed by the sludge and sequestered in the roots. Zinc effectively competed with Cd uptake as evidenced by a decrease in Cd content of roots with increased Zn additions. While decreasing Cd uptake, Zn had an effect of increasing Cd translocation from roots to tops. Cadmium, on the other hand, decreased Zn translocation from roots to tops. Estimations of Cd+2 and Zn+2 activities in soil solution were attempted using chelator and total soil solution methods. Calculated Cd+2 and Zn+2 activities proved to be overestimates of actual activities in soil solutions. Total soil solution Zn concentration and DTPA extractable Zn were good measurements of soil Zn availability

Seeing the light : experimental signatures of emergent electromagnetism in a quantum spin ice

The "spin ice" state found in the rare earth pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7 offers a beautiful realisation of classical magnetostatics, complete with magnetic monopole excitations. It has been suggested that in "quantum spin ice" materials, quantum-mechanical tunnelling between different ice configurations could convert the magnetostatics of spin ice into a quantum spin liquid which realises a fully dynamical, lattice-analogue of quantum electromagnetism. Here we explore how such a state might manifest itself in experiment, within the minimal microscopic model of a such a quantum spin ice. We develop a lattice field theory for this model, and use this to make explicit predictions for the dynamical structure factor which would be observed in neutron scattering experiments on a quantum spin ice. We find that "pinch points", seen in quasi-elastic scattering, which are the signal feature of a classical spin ice, fade away as a quantum ice is cooled to its zero-temperature ground state. We also make explicit predictions for the ghostly, linearly dispersing magnetic excitations which are the "photons" of this emergent electromagnetism. The predictions of this field theory are shown to be in quantitative agreement with Quantum Monte Carlo simulations at zero temperature.Comment: 26 pages, 18 figures, minor revision