The rates and time-delay distribution of multiply imaged supernovae behind lensing clusters

Time delays of gravitationally lensed sources can be used to constrain the mass model of a deflector and determine cosmological parameters. We here present an analysis of the time-delay distribution of multiply imaged sources behind 17 strong lensing galaxy clusters with well-calibrated mass models. We find that for time delays less than 1000 days, at z=3.0, their logarithmic probability distribution functions are well represented by P (log \Delta t)=5.3 x 10^-4 \Delta t^\beta M_250^-2\beta, with \beta=0.77, where M_250 is the projected cluster mass inside 250 kpc (in 10^14 M_sun), and \beta is the power-law slope of the distribution. The resultant probability distribution function enables us to estimate the time-delay distribution in a lensing cluster of known mass. For a cluster with M_250=2 x 10^14 M_sun, the fraction of time delays less than 1000 days is approximately 3%. Taking Abell 1689 as an example, its dark halo and brightest galaxies, with central velocity dispersions larger than 500 km/s, mainly produce large time delays, while galaxy-scale mass clumps are responsible for generating smaller time delays. We estimate the probability of observing multiple images of a supernova in the known images of Abell 1689. A two-component model of estimating the supernova rate is applied in this work. For a magnitude threshold of m_AB=26.5, the yearly rate of Type Ia (core-collapse) supernovae with time delays less than 1000 days is 0.004 +- 0.002 (0.029 +- 0.001). If the magnitude threshold is lowered to m_AB ~ 27.0, the rate of core-collapse supernovae suitable for time delay observation is 0.044 +- 0.015 per year.Comment: 23 pages, 7 figures, JCAP in pres

An X-ray/optical study of the geometry and dynamics of MACS J0140.0-0555, a massive post-collision cluster merger

We investigate the physical properties, geometry and dynamics of the massive cluster merger MACS J0140.0-0555 (z=0.451) using X-ray and optical diagnostics. Featuring two galaxy overdensities separated by about 250 kpc in projection on the sky, and a single peak in the X-ray surface brightness distribution located between them, MACS J0140.0-0555 shows the tell-tale X-ray/optical morphology of a binary, post-collision merger. Our spectral analysis of the X-ray emission, as measured by our Chandra ACIS-I observation of the system, finds the intra-cluster medium to be close to isothermal (~8.5 keV) with no clear signs of cool cores or shock fronts. Spectroscopic follow-up of galaxies in the field of MACS J0140.0-0555 yields a velocity dispersion of 875 (+70/-100) km/s (n_z=66) and no significant evidence of bimodality or substructure along the line of sight. In addition, the difference in radial velocity between the brightest cluster galaxies of the two sub-clusters of 144+/-25 km/s is small compared to typical collision velocities of several 1000 km/s. A strongly lensed background galaxy at z=0.873 (which features variable X-ray emission from an active nucleus) provides the main constraint on the mass distribution of the system. We measure M(<75 kpc) = (5.6+/- 0.5)*10^13 M_sun for the north-western cluster component and a much less certain estimate of (1.5-3)*10^13 M_sun for the south-eastern subcluster. These values are in good agreement with our X-ray mass estimates which yield a total mass of MACS J0140.0-0555 of M(<r_500) ~ (6.8-9.1)*10^14 M_sun. ......Comment: 11 pages, 8 figures, and 2 tables. Accepted for publication in MNRA

VLT/X-Shooter Near-Infrared Spectroscopy and HST Imaging of Gravitationally-Lensed z~2 Compact Quiescent Galaxies

Quiescent massive galaxies at z~2 are thought to be the progenitors of present-day massive ellipticals. Observations revealed them to be extraordinarily compact. The determination of stellar ages, star formation rates and dust properties via spectroscopic measurements has up to now only been feasible for the most luminous and massive specimens (~3x M*). Here we present a spectroscopic study of two near-infrared selected galaxies which are close to the characteristic stellar mass M* (~0.9x M* and ~1.3x M*) and whose observed brightness has been boosted by the gravitational lensing effect. We measure the redshifts of the two galaxies to be z=1.71\pm0.02 and z=2.15\pm0.01. By fitting stellar population synthesis models to their spectro-photometric SEDs we determine their ages to be 2.4^{+0.8}_{-0.6} Gyr and 1.7\pm0.3 Gyr, respectively, which implies that the two galaxies have higher mass-to-light ratios than most quiescent z~2 galaxies in other studies. We find no direct evidence for active star-formation or AGN activity in either of the two galaxies, based on the non-detection of emission lines. Based on the derived redshifts and stellar ages we estimate the formation redshifts to be z=4.3^{+3.4}_{-1.2} and z=4.3^{+1.0}_{-0.6}, respectively. We use the increased spatial resolution due to the gravitational lensing to derive constraints on the morphology. Fitting Sersic profiles to the de-lensed images of the two galaxies confirms their compactness, with one of them being spheroid-like, and the other providing the first confirmation of a passive lenticular galaxy at a spectroscopically derived redshift z~2.Comment: accepted for publication in Ap

The Abundance of Low-luminosity Lyman alpha Emitters at High Redshift

We derive the luminosity function of high-redshift Lyman alpha emitting sources from a deep, blind, spectroscopic survey that utilized strong-lensing magnification by intermediate-redshift clusters of galaxies. We observed carefully selected regions near 9 clusters, consistent with magnification factors generally greater than 10 for the redshift range 4.5<z<6.7. Eleven emission-line candidates were located in the range 2.2<z<5.6 whose identification we justify as Lyman alpha, in most cases via further spectroscopic observations. The selection function we constructed for our survey takes into account our varying intrinsic Lyman alpha line sensitivity as a function of wavelength and sky position. By virtue of the strong magnification factor, we provide constraints on the Lyman alpha luminosity function to unprecedented limits of 10^40 erg/s, corresponding to a star-formation rate of 0.01 Msun/yr. Our cumulative z=5 Lyman alpha luminosity function is consistent with a power law form, n(>L) proportional to L^-1 over 10^41 to 10^42.5 erg/s. When combined with the results of other surveys, limited at higher luminosities, our results suggest evidence for the suppression of star formation in low-mass halos, as predicted in popular models of galaxy formation.Comment: 22 pages, 14 figures, submitted to Ap

Privatization, Investment and Ownership Efficiency

We provide a model that explains the following empirical observations: i) private ownership is more efficient than public ownership, ii) privatizations are associated with increases in efficiency and iii) the increase in efficiency predates the privatization. The two key mechanisms explaining the results are: (i) a government owner keeping control takes into account the negative effect on employment of investment and (ii) a privatizing government has a stronger incentive to invest than an acquiring firm: the government exploits the fact that investments increase the sales price not only due to the increase in the acquirer's profit, but also due to a reduced profit for the non-acquirer.Privatization; Asset Ownership; Restructuring; Oligopoly

Novel Particle Model for the Prediction of Stability and Episodic Collapse of Coastal Cliffs and Levees

This thesis investigates the WCSPH model by considering fluid entry and exit, and integrates the WCSPH method into a new, novel, particle-based Bluff Morphology Model (BMM). Using the BMM, this thesis investigates the stability, collapse and equilibrium position of soft coastal bluffs (cliffs). Fluid and floating object interaction using a novel adaptation of the WCSPH method is investigated by incorporating a floating object model. In particular, this thesis examines the water impact, hydrodynamic forces, fluid motions, and movement of objects in the conventional case studies of object entry and exit from still water. A two-dimensional wedge drop analysis was examined, and the hydrodynamic forces show acceptable agreement with published experimental and numerical results. Simulations for water entry and exit of a buoyant and neutral density cylinder compares well with the previous experimental, numerical and empirical studies. These results provide a good foundation to evaluate the accuracy and stability of WCSPH for modelling complex flows, and therefore offers a platform for the use of WCSPH in a Bluff Morphology Model. The BMM combines a multiple wedge displacement method with an adapted Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) method. At first the wedge method is applied to compute the stability of the bluff. Once the critical failure mechanism of the bluff slope has been identified, if the Factor of Safety for the mechanism is less than 1, the adapted WCSPH method is used to predict the failure movement and residual shape of the slope. The model is validated against benchmark test cases of bluff stability for purely frictional, purely cohesive, and mixed strength bluff materials including 2D static water tables. The model predictions give a good correlation with the expected values, with medium resolution models producing errors of typically less than 2.0%. In addition, the prediction of lateral movement of a surveyed cliff and the dynamic collapse of a vertical bluff are computed, and compare well with published literature. This model is further extended to then investigate the effect of two dimensional seepage on the stability and collapse of soil slopes and levees. To incorporate the seepage in the model, Darcy’s Law is applied to the interactions among neighbouring soil particles and ghost particles are introduced along the enclosed soil boundary to ensure that no fluid crosses the boundary. The contribution of partially saturated soils and matric suction, as well as the change in hydraulic conductivity due to seepage, are predicted well by this model. The predicted time evolution of slope stability and seepage induced collapse are in reasonable agreement with the experimental results for homogeneous frictional sand and multiple layered cohesive soils. Rapid drawdown over a sand soil is also investigated, and the location and time of the levee collapse occurrence are captured well. A toe erosion model is incorporated within the numerical model, and the location and quantity of erosion caused by lateral seepage is well predicted. The interplay of erosion, seepage and slope instability is examined