arXiv subjects

M. Lombardi

Publications and source records attributed to M. Lombardi.

81 records · Page 5Linked to original sources

arXiv2004

Mass-sheet degeneracy: Fundamental limit on the cluster mass reconstruction from statistical (weak) lensing

Weak gravitational lensing is considered to be one of the most powerful tools to study the mass and the mass distribution of galaxy clusters. However, weak lensing mass reconstructions are plagued by the so-called mass-sheet degeneracy--the surface mass density κof the cluster can be determined only up to a degeneracy transformation κ\to κ' = λκ+ (1 -λ), where λis an arbitrary constant. This transformation fundamentally limits the accuracy of cluster mass determinations if no further assumptions are made. We describe here a method to break the mass-sheet degeneracy in weak lensing mass maps using distortion and redshift information of background galaxies and illustrate this by two simple toy models. Compared to other techniques proposed in the past, it does not rely on any assumptions on cluster potential; it can be easily applied to non-parametric mass-reconstructions and no assumptions on boundary conditions have to be made. In addition it does not make use of weakly constrained information (such as the source number counts, used in the magnification effect). Our simulations show that we are effectively able to break the mass-sheet degeneracy for supercritical lenses, but that for undercritical lenses the mass-sheet degeneracy is very difficult to be broken, even under idealised conditions.

astro-ph↗

arXiv2004

The signature of substructure on gravitational lensing in the LCDM cosmological model

We present a study of the lens properties of quadruply imaged systems, lensed by numerically simulated galaxies. We investigate a simulated elliptical and disc galaxy drawn from high resolution simulations of galaxy formation in a concordance $Λ$CDM universe. The simulations include the effects of gas dynamics, star formation and feedback processes. Flux-ratio anomalies observed in strong gravitational lensing potentially provide an indicator for the presence of mass substructure in lens galaxies as predicted from CDM simulations. We particularly concentrate on the prediction that, for an ideal cusp caustic, the sum of the signed magnifications of the three highly magnified images should vanish when the source approaches the cusp. Strong violation of this cusp relation indicates the presence of substructure, regardless of the global, smooth mass model of the lens galaxy. We draw the following conclusions: (1) the level of substructure present in simulations produces violations of the cusp relation comparable to those observed, (2) higher-order catastrophes (e.g. swallowtails) can also cause changes of the order of 0.6 in the cusp relation as predicted by a smooth model, (3) the flux anomaly distribution depends on the image parity and flux and both the brightest minimum and saddle-point images are more affected by substructure than the fainter images. In addition, the brightest saddle point is demagnified w.r.t. the brightest minimum. Our results are fully numerical and properly include all mass scales, without making semi-analytic assumptions. They are ultimately limited by the mass resolution of single particles in the simulation determined by current computational limits, however show that our results are not affected by shot-noise due to the finite number of particles.

astro-ph↗

arXiv2003

Massive star formation in a gravitationally-lensed HII-galaxy at z=3.357

The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic follow-up of the z=0.570 cluster RX J0848+4456 from the ROSAT Deep Cluster Survey. The arc is characterized by a very red R-K color and strong, narrow emission lines. Analysis of HST WFPC2 imaging and Keck optical and infrared spectroscopy shows that the arc is an \hii galaxy magnified by a factor of ~10 by a complex cluster environment. The high intrinsic luminosity, the emission line spectrum, the absorption components seen in Lyαand C IV, and the restframe ultraviolet continuum are all consistent with a simple HII region model containing ~ 10^6 hot O stars. The best fit parameters for this model imply a very hot ionizing continuum (T_BB ~ 80,000 K), high ionization parameter (log U ~ -1), and low nebular metallicity (Z / Z_\odot ~ 0.05). The narrowness of the emission lines requires a low mass-to-light ratio for the ionizing stars, suggestive of an extremely low metallicity stellar cluster. The apparent overabundance of silicon in the nebula could indicate enrichment by past pair instability supernovæ, requiring stars more massive than ~140 M_\odot.

astro-ph↗

arXiv2002

X-ray and weak lensing measurements of the mass profile of MS1008.1-1224: Chandra and VLT data

We analyse the Chandra dataset of the galaxy cluster MS1008.1-1224 to recover an estimate of the gravitating mass as function of the radius and compare these results with the weak lensing reconstruction of the mass distribution obtained from deep FORS1-VLT multicolor imaging. Even though the X-ray morphology is disturbed with a significant excess in the northern direction suggesting that the cluster is not in a relaxed state, we are able to match the two mass profiles both in absolute value and in shape within 1 sigma uncertainty and up to 1100 h50^-1 kpc. The recovered X-ray mass estimate does not change by using either the azimuthally averaged gas density and temperature profiles or the results obtained in the northern sector alone where the signal-to-noise ratio is higher.

astro-ph↗

arXiv2002

Using VLBI Data to Investigate the Galaxy Structure in the Gravitationally Lensed System B1422+231

Gravitationally lensed systems with multiply imaged quasars are an excellent tool for studying the properties of distant galaxies. In particular, they provide the most accurate mass measures for the lensing galaxy. The system B1422+231 is a well studied example of a quadruply imaged quasar, with high-quality VLBI data available. Very accurate data on image positions, fluxes and deconvolved image sizes provide good constraints for lensing models. We discuss here the failure of smooth models in fitting the data. Since it is intuitively clear that the mass of a lens galaxy is not a smooth entity, we have investigated how deviation from a smooth model can influence lensing phenomena, especially the image flux ratios. To explore expectations about the level of substructure in galaxies and its influence on strong lensing, N-body simulations of a model galaxy are employed. By using the mass distribution of this model galaxy as a lens, synthetic data sets of different four image system configurations are generated. Their analysis can possibly provide evidence for the presence and strong influence of substructure in the primary lens galaxy.

astro-ph↗

arXiv2002

B1422+231: The influence of mass substructure on strong lensing

In this work we investigate the gravitationally lensed system B1422+231. High--quality VLBI image positions, fluxes and shapes as well as an optical HST lens galaxy position are used. First, two simple and smooth models for the lens galaxy are applied to fit observed image positions and fluxes; no even remotely acceptable model was found. Such models also do not accurately reproduce the image shapes. In order to fit the data successfully, mass substructure has to be added to the lens, and its level is estimated. To explore expectations about the level of substructure in galaxies and its influence on strong lensing, N-body simulation results of a model galaxy are employed. By using the mass distribution of this model galaxy as a lens, synthetic data sets of different four image system configurations are generated and simple lens models are again applied to fit them. The difficulties in fitting these lens systems turn out to be similar to the case of some real gravitationally lensed systems, thus possibly providing evidence for the presence and strong influence of substructure in the primary lens galaxy.

astro-ph↗

arXiv2001

A Diffusion Model for Classical Chaotic Compound Scattering

We consider the classical map proposed previously to be the exact classical analogue of Rydberg Molecules calculated with the approximations relevant to the multi-channel quantum defect theory. The resulting classical map is analyzed at energies above the threshold for the Rydberg electron. At energies very near to this threshold we find the possibility of bounded motion for positive energy due to conserved tori as well as the possibility of forming a compound system, i.e. a system where the particle is trapped for long times before emerging again to the continuum. The compound scattering displays unusual features for short time behavior. A diffusion model explains these features.

nlin.CD↗

arXiv2000

Weak lensing mass reconstruction of MS1008.1-1224

We present an in-depth weak lensing analysis of the cluster MS1008 based on deep multicolor imaging obtained during the Science Verification of FORS1 at the VLT. The image quality (half arcsec seeing) and depth of the VLT images allow the shear signal to be mapped with high signal-to-noise and to be traced out to 1.2 h_50^-1 Mpc, near the edge of the 6'.8x6'.8 field of view. Using BVRI color information, as well as 81 redshifts in the field from the CNOC survey, background galaxies can be effectively separated from cluster and foreground objects. PSF distorsions are found to be moderate across the FORS images and thus easily removed. Due to the small statistical errors in the mass reconstruction, this dataset provides a testing ground where several systematic effects (e.g. mass-sheet degeneracy, redshift distribution of the background sources, cluster galaxy contamination) can be quantified. Several methods are used to remove the mass-sheet degeneracy which is found to dominate the systematic error budget. We measure a lower limit to the mass of 2.6 10^14 h_50^-1 M_sun within 1 h_50^-1 Mpc and a ``total'' mass of 5.3 10^14 h_50^-1 M_sun by fitting a softened isothermal sphere. The availability of the CNOC redshift data and X-ray observations on this cluster allow a comparison of different determinations of the mass radial profile. We find the lensing and X-ray measurements in excellent agreement, while the mass derived from the virial analysis is marginally (1-2 sigma) in agreement at radii where both methods are reliable.

astro-ph↗

arXiv1999

Multichannel Quantum Defect Theory: a Quantum Poincare Map

The multichannel quantum defect theory (MQDT) can be reinterpreted as a quantum Poincare map in representation of angular momentum. This has two important implications: we have a paradigm of a true quantum Poincare map without semi-classical input and we get an entirely new insight into the significance of MQDT.

chao-dyn↗