Observations Of Gravitational Lenses Research Articles

Strong deflection limit analysis and gravitational lensing of an Ellis wormhole

Observations of gravitational lenses in strong gravitational fields give us a clue to understanding dark compact objects. In this paper, we extend a method to obtain a deflection angle in a strong deflection limit provided by Bozza [Phys. Rev. D 66, 103001 (2002)] to apply to ultrastatic spacetimes. We also discuss on the order of an error term in the deflection angle. Using the improved method, we consider gravitational lensing by an Ellis wormhole, which is an ultrastatic wormhole of the Morris-Thorne class.

COLD DARK MATTER SUBSTRUCTURES IN EARLY-TYPE GALAXY HALOS

ABSTRACT We present initial results from the “Ponos” zoom-in numerical simulations of dark matter substructures in massive ellipticals. Two very highly resolved dark matter halos with M vir = 1.2 × 1013 and M vir = 6.5 × 1012 and different (“violent” versus “quiescent”) assembly histories have been simulated down to z = 0 in a ΛCDM cosmology with a total of 921,651,914 and 408,377,544 particles, respectively. Within the virial radius, the total mass fraction in self-bound M sub > 106 subhalos at the present epoch is 15% for the violent host and 16.5% for the quiescent one. At z = 0.7, these fractions increase to 19% and 33%, respectively, as more recently accreted satellites are less prone to tidal destruction. In projection, the average fraction of surface mass density in substructure at a distance of R/R vir = 0.02 (∼5–10 kpc) from the two halo centers ranges from 0.6% to ≳2%, significantly higher than that measured in simulations of Milky Way-sized halos. The contribution of subhalos with M sub < 109 to the projected mass fraction is between one-fifth and one-third of the total, with the smallest share found in the quiescent host. We assess the impact of baryonic effects via twin, lower-resolution hydrodynamical simulations that include metallicity-dependent gas cooling, star formation, and a delayed-radiative-cooling scheme for supernova feedback. Baryonic contraction produces a super-isothermal total density profile and increases the number of massive subhalos in the inner regions of the main host. The host density profiles and projected subhalo mass fractions appear to be broadly consistent with observations of gravitational lenses.

Very long baseline interferometry and observations of gravitational lenses using intensity fluctuations: an analysis based on intensity autocorrelation

A novel interferometric technique that uses the spectrum of the current fluctuations of a quadratic detector, a type of detector commonly used in Astronomy, has recently been introduced. It has major advantages with respect to classical interferometry. It can be used to observe gravitational lenses that cannot be detected with standard techniques. It can be used to carry out very long baseline interferometry. Although the original theoretical analysis, that uses wave interaction effects, is rigorous, it is not easy to understand. The present article therefore carries out a simpler analysis, using the autocorrelation of intensity fluctuations, which is easier to understand. It is based on published experiments that were carried out to validate the original theory. The autocorrelation analysis also validates simple numerical techniques, based on the autocorrelation, to model the angular intensity distribution of a source. The autocorrelation technique also allows a much simpler detection of the signal. In practice, the gravitational lens applications are the ones that can readily be done with presently available telescopes. We describe a practical example that shows that presently available VLBI radio-astronomical data can be used to observe microlensisng and millilensing in macrolensed Quasars. They may give information on the dark matter substructures in the lensing galaxies.

The Importance of Einstein Rings

We develop a theory of Einstein rings and demonstrate it using the infrared Einstein ring images of the quasar host galaxies observed in PG 1115+080, B1608+656, and B1938+666. The shape of an Einstein ring accurately and independently determines the shape of the lens potential and the shape of the lensed host galaxy. We find that the host galaxies of PG 1115+080, B1608+656, and B1938+666 have axis ratios of 0.58 ± 0.02, 0.69 ± 0.02, and 0.62 ± 0.14, respectively, including the uncertainties in the lens models. The Einstein rings break the degeneracies in the mass distributions or Hubble constants inferred from observations of gravitational lenses. In particular, the Einstein ring in PG 1115+080 rules out the centrally concentrated mass distributions that lead to a high Hubble constant (H0 > 60 km s-1 Mpc-1) given the measured time delays. Deep, detailed observations of Einstein rings will be revolutionary for constraining mass models and determining the Hubble constant from time-delay measurements.

Gravitational lens frequencies and the cosmological constant: an examination of amplification bias and galaxy formation redshift

Previous investigations into the gravitational lens frequencies in quasar samples suggested that observations do not favor cosmological models with a large value of the cosmological constant. However, this conclusion depends critically on assumptions about the degree of amplification bias and the constant comoving number density of galaxies. We examine amplification bias selection effects under the condition that the darker image must be brighter than the limiting magnitude in the sample and show that the amplification bias selection effects are about five times smaller than the previous estimate. We also estimate the survey depth which is required for the darker image to recover the conventional estimates. This revision makes a cosmological model with a large cosmological constant consistent with observations of gravitational lenses. We also find that, when a model of the distribution of the galaxy formation redshifts is used, the lensing frequency is 15–45 percent lower than for the case of constant number density of galaxies.