10,361 research outputs found
Triggering Active Galactic Nuclei in Hierarchical Galaxy Formation: Disk instability vs. Interactions
Using a semi analytic model for galaxy formation we investigate the effects
of Black Hole accretion triggered by disk instabilities (DI) in isolated
galaxies on the evolution of AGN. Specifically, we took on, developed and
expanded the Hopkins & Quataert (2011) model for the mass inflow following disk
perturbations, and compare the corresponding evolution of the AGN population
with that arising in a scenario where galaxy interactions trigger AGN (IT
mode). We extended and developed the DI model by including different disk
surface density profiles, to study the maximal contribution of DI to the
evolution of the AGN population. We obtained the following results: i) for
luminosities corresponding to the DI mode can provide the
BH accretion needed to match the observed AGN luminosity functions up to ; in such a luminosity range and redshift, it can compete with the
IT scenario as the main driver of cosmological evolution of AGN; ii) The DI
scenario cannot provide the observed abundance of high-luminosity QSO with
AGN, as well as the abundance of high-redhshift QSOs with , while the IT scenario provides
an acceptable match up to , as found in our earliest works; iii)
The dispersion of the distributions of Eddington ratio for low- and
intermediate-luminosity AGN (bolometric = -
erg/s) is predicted to be much smaller in the DI scenario compared to the IT
mode; iv) The above conclusions are robust with respect to the explored
variants of the Hopkins & Quataert (2011) model. We discuss the physical origin
of our findings, and how it is possible to pin down the dominant fueling
mechanism in the low-intermediate luminosity range where
both the DI and the IT modes are viable candidates as drivers for the AGN
evolution.Comment: Accepted for publication in Astronomy & Astrophysics, 24 pages, 8
figures; updated reference
Energy harvesting from train vibrations
In this paper, linear mechanical oscillators are designed to harvest energy from train-induced vibrations. The harvested energy could be used, for example, to charge sensors mounted on the rail track for structural health monitoring. The dominant frequencies due to a passing train are determined for a specific train and speed from a recorded acceleration time-history. Using a simple model of an oscillator, the total energy harvested for the passage of one train is calculated. The stiffness, and hence the tuning frequency of the device, is varied in simulations to determine the optimum frequency at which to tune the device for a constant value of mass and damping in the device. Further simulations are conducted to investigate the power that could be harvested from multiple oscillators tuned at several dominant frequencies, and their performances are analysed and compared. The constraint for maximum relative displacement is considered in the design of each harvester, and this is adopted to assure that the amplitude of the oscillation is finite and does not exceed the physical size of the device. The robustness of the harvester is also analysed for different train speeds
Asset Prices and Monetary Policy: A New View of the Cost Channel
Should the central bank act to prevent "excessive" asset price dynamics or should it wait until the boom spontaneously turns into a crash and intervene afterwards to attenuate the fallout on the real economy? The standard "three equation" New Keynesian framework is inadequate to analyse this issue for the very simple reason that asset prices are not explicitly included in the model. There are two straightforward ways to take into account asset price dynamics in this framework. First of all, the objective function of the central bank - usually defined in terms of inflation and the output gap - could be "augmented" to take into account asset price inflation. Second, expected asset price inflation can affect the IS curve through a wealth effect. In this paper we follow a different route. In our model in fact, the expected asset price dynamics will be eventually incorporated into the NK Phillips curve. This is due to the assumption of a cost channel for monetary policy which is activated whenever monetary policy affects asset prices and dividends. In fact they determine the cost of external finance in the simple "equity only" financing model we consider, abstracting for simplicity from internal funds and the credit market.
Heterogeneity and Aggregation in a Financial Accelerator Model
In this paper we present a macroeconomic model in which changes in the variance (and higher moments of the distribution) of firm's financial conditions - i.e. "distributive shocks" - are bound to play a crucial role in the determination of output fluctuations. Firms differ by degree of financial robustness, which affects (optimal) investment in a bankruptcy risk context (àla Greenwald-Stiglitz). As to households, for the sake of simplicity, we assume that they are homogeneous in every respect so that we can adopt the representative agent hypothesis. We can explore the properties of the macro-dynamic model either via the study of the two-dimensional map defining the laws of motion of the average equity ratio and of the variance of the distribution or via simulations in a multiagent framework.
Palladium-Catalyzed Asymmetric Conjugate Addition of Arylboronic Acids to Five-, Six-, and Seven-Membered β-Substituted Cyclic Enones: Enantioselective Construction of All-Carbon Quaternary Stereocenters
The first enantioselective Pd-catalyzed construction of all-carbon quaternary stereocenters via 1,4-addition of arylboronic acids to β-substituted cyclic enones is reported. Reaction of a wide range of arylboronic acids and cyclic enones using a catalyst prepared from Pd(OCOCF_3)_2 and a chiral pyridinooxazoline ligand yields enantioenriched products bearing benzylic stereocenters. Notably, this transformation is tolerant to air and moisture, providing a practical and operationally simple method of synthesizing enantioenriched all-carbon quaternary stereocenters
Investigation of the reinforcement of ductule metals with strong, high modulus discontinuous, brittle fibers Quarterly report, 1 May - 1 Aug. 1968
Factors affecting tensile strength of ductile metals reinforced with short, brittle fiber
Some fundamental fracture mechanisms applicable to advanced filament reinforced composites
Stress analysis and fracture mechanisms of advanced fiber reinforced composite
Backscattering Differential Ghost Imaging in Turbid Media
In this Letter we present experimental results concerning the retrieval of
images of absorbing objects immersed in turbid media via differential ghost
imaging (DGI) in a backscattering configuration. The method has been applied,
for the first time to our knowledge, to the imaging of small thin black objects
located at different depths inside a turbid solution of polystyrene nanospheres
and its performances assessed via comparison with standard imaging techniques.
A simple theoretical model capable of describing the basic optics of DGI in
turbid media is proposed.Comment: 5 pages, 6 figure
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