Quasar H II Regions During Cosmic Reionization

Cosmic reionization progresses as HII regions form around sources of ionizing radiation. Their average size grows continuously until they percolate and complete reionization. We demonstrate how this typical growth can be calculated around the largest, biased sources of UV emission, such as quasars, by further developing an analytical model based on the excursion set formalism. This approach allows us to calculate the sizes and growth of the HII regions created by the progenitors of any dark matter halo of given mass and redshift with a minimum of free parameters. Statistical variations in the size of these pre-existing HII regions are an additional source of uncertainty in the determination of very high redshift quasar properties from their observed HII region sizes. We use this model to demonstrate that the transmission gaps seen in very high redshift quasars can be understood from the radiation of only their progenitors and associated clustered small galaxies. The fit sets a lower limit on the redshift of overlap at z = 5.8 +/- 0.1. This interpretation makes the transmission gaps independent of the age of the quasars observed. If this interpretation were correct it would raise the prospects of using radio interferometers currently under construction to detect the epoch of reionization.Comment: 6 pages, 3 figures, accepted by MNRAS, revised to match published versio

Fixed term employment contracts in an equilibrium search model

Fixed term employment contracts have been introduced in number of European countries as a way to provide flexibility to economies with high employment protection levels. We introduce these contracts into the equilibrium search model in Alvarez and Veracierto (1999), a version of the Lucas and Prescott island model, adapted to have undirected search and variable labor force participation. We model a contract of length J as a tax on separations of workers with tenure higher than J. We show a version of the welfare theorems, and characterize the efficient allocations. This requires solving a control problem, whose solution is characterized by two dimensional inaction sets. For J = 1 these contracts are equivalent to the case of firing taxes, and for large J they are equivalent to the laissez- faire case. In a calibrated version of the model, we evaluate to what extent contract lengths similar to those observed in Europe, close the gap between these two extremes.Labor contract ; Employment (Economic theory)

Labor market policies in an equilibrium search model

We explore to what extent differences in employment and unemployment across economies can be generated by differences in labor market policies. We use a version of the Lucas-Prescott equilibrium search model with undirected search and endogenous labor-force participation. Minimum wages, degree of unionization, firing taxes, and unemployment benefits are introduced and their effects analyzed. When the model is calibrated to US observations it reproduces several of the elasticities of employment and unemployment with respect to changes in policies reported in the empirical literature. We find that: i) minimum wages have small effects; ii) firing taxes have similar effects to those found in frictionless general equilibrium models; iii) unions have large and negative effects on employment, unemployment, and welfare; and iv) unemployment benefits substantially increase unemployment and reduce welfare.Labor market ; Labor supply ; Wages

UV driven evaporation of close-in planets: energy-limited; recombination-limited and photon-limited flows

We have investigated the evaporation of close-in exoplanets irradiated by ionizing photons. We find that the properties of the flow are controlled by the ratio of the recombination time to the flow time-scale. When the recombination time-scale is short compared to the flow time-scale the the flow is in approximate local ionization equilibrium with a thin ionization front, where the photon mean free path is short compared to flow scale. In this "recombination limited" flow the mass-loss scales roughly with the square root of the incident flux. When the recombination time is long compared to the flow time-scale the ionization front becomes thick and encompasses the entire flow, with the mass-loss rate scaling linearly with flux. If the planet's potential is deep the flow is approximately "energy-limited"; however, if the planet's potential is shallow we identify a new limiting mass-loss regime, which we term "photon-limited". In this scenario the mass-loss rate is purely limited by the incoming flux of ionizing photons. We have developed a new numerical approach that takes into account the frequency dependence of the incoming ionizing spectrum and performed a large suite of 1D simulations to characterise UV driven mass-loss around low mass planets. We find the flow is "recombination-limited" at high fluxes but becomes "energy-limited" at low fluxes; however, the transition is broad occurring over several order of magnitude in flux. Finally, we point out the transitions between the different flow types does not occur at a single flux value, but depends on the planet's properties, with higher mass planets becoming "energy-limited" at lower fluxes.Comment: Published in Ap

The Evolution of Dark-Matter Dominated Cosmological Halos

Adaptive SPH and N-body simulations were carried out to study the evolution of the equilibrium structure of dark matter halos that result from the gravitational instability and fragmentation of cosmological pancakes. Such halos resemble those formed by hierarchical clustering from realistic initial conditions in a CDM universe and, therefore, serve as a test-bed model for studying halo dynamics. The dark matter density profile is close to the universal halo profile identified previously from N-body simulations of structure formation in CDM, with a total mass and concentration parameter which grow linearly with scale factor a. When gas is included, this concentration parameter is slightly larger than the pure N-body result. We also find that the dark matter velocity distribution is less isotropic and more radial than found by N-body simulations of CDM.Comment: To appear in "The 20th Texas Symposium on Relativistic Astrophysics", eds. H. Martel and J.C. Wheeler, AIP, in press (2001) (3 pages, 2 figures