arXiv subjects

Matthias Bartelmann

Publications and source records attributed to Matthias Bartelmann.

At least 163 records · Page 9Linked to original sources

arXiv2001

Weak-Lensing Halo Numbers and Dark-Matter Profiles

Integral measures of weak gravitational lensing by dark-matter haloes, like the aperture mass, are sensitive to different physical halo properties dependent on the halo mass density profile. For isothermal profiles, the relation between aperture mass and virial mass is steeper than for haloes with the universal NFW profile. Consequently, the halo mass range probed by the aperture mass is much wider for NFW than for isothermal haloes. We use recent modifications to the Press-Schechter mass function in CDM models normalised to the local abundance of rich clusters, to predict the properties of the halo sample expected to be accessible with the aperture mass technique. While ~10 haloes should be detected per square degree if the haloes have NFW profiles, their number density is lower by approximately an order of magnitude if they have isothermal profiles. These results depend only very mildly on the cosmological background model. We conclude that counts of haloes with a significant weak-lensing signal are a powerful discriminator between different dark-matter profiles.

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arXiv2001

Giant cluster arcs as a constraint on the scattering cross-section of dark matter

We carry out ray tracing through five high resolution simulations of a galaxy cluster to study how its ability to produce giant gravitationally lensed arcs is influenced by the collision cross-section of its dark matter. In three cases typical dark matter particles in the cluster core undergo between 1 and 100 collisions per Hubble time; two more explore the long (``collisionless'') and short (``fluid'') mean free path limits. We study the size and shape distributions of arcs and compute the cross-section for producing ``extreme'' arcs of various sizes. Even a few collisions per particle modify the core structure enough to destroy the cluster's ability to produce long, thin arcs. For larger collision frequencies the cluster must be scaled up to unrealistically large masses before it regains the ability to produce giant arcs. None of our models with self-interacting dark matter (except the ``fluid'' limit) is able to produce radial arcs; even the case with the smallest scattering cross-section must be scaled to the upper limit of observed cluster masses before it produces radial arcs. Apparently the elastic collision cross-section of dark matter in clusters must be very small, below 0.1 cm^2 g^-1, to be compatible with the observed ability of clusters to produce both radial arcs and giant arcs.

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arXiv2001

Gravitational lensing: effects of cosmology and of lens and source profiles

We present detailed calculations of the magnification distribution, including both weak and strong lensing, using very recent solutions of the Dyer-Roeder (1973) equation for light propagation in a inhomogeneous universe with a cosmological constant and up-to-date models for the evolving cosmological distribution of dark matter halos. We address in particular the effect of a non-zero cosmological constant, of different density profiles of lenses, and of finite sizes of lensed sources. We show that, if dark matter fluctuations are normalized to the local cluster abundance, in the presence of a cosmological constant the optical depth for lensing {\em decreases} compared to the case of an Einstein-de Sitter universe, because halos in the relevant mass range are less abundant over a large redshift interval. We also discuss the differences in the magnification probability distributions produced by Navarro, Frenk & White (NFW) and by Singular Isothermal Sphere (SIS) density profiles of lenses. We find that the NFW lens is more efficient for moderate magnifications ($2\lsim A\lsim 4$), and less efficient for larger magnifications. Moreover, we discuss quantitatively the maximum magnification, $A_{\rm max}$, that can be achieved in the case of extended sources (galaxies) with realistic luminosity profiles, taking into account the possible ellipticity of the lens potential. Finally, we apply our results to a class of sources following the luminosity evolution typical for a unified scheme of QSO formation. We find that the lensed source counts at $850 μ$m can be larger than the unlensed ones by several orders of magnitude at flux densities $\gta100 $mJy.

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arXiv2001

The temperature-mass relation in magnetized galaxy clusters

We use cosmological, magneto-hydrodynamic simulations of galaxy clusters to quantify the dynamical importance of magnetic fields in these clusters. The set-up of initial magnetic field strengths at high redshifts is chosen such that observed Faraday-rotation measurements in low-redshift clusters are well reproduced in the simulations. We compute the radial profiles of the intracluster gas temperature and of the thermal and magnetic pressure in a set of clusters simulated in the framework of an Einstein-de Sitter and a low-density, spatially-flat CDM cosmological model. We find that, for a realistic range of initial magnetic field strengths, the temperature of the intracluster gas changes by less than $\approx5%$.

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arXiv2001

Lensing by Lyman Limit Systems: Determining the Mass to Gas Ratio

We present a new method to determine the total mass-to-neutral gas ratio in Lyman-limits systems. The method exploits the relation between the neutral hydrogen column density and the magnification of background sources due to the weak gravitational lensing that these systems induce. Because weak lensing does not provide a direct measure of mass, one must use this relation in a statistical sense to solve for the average mass-to-gas ratio and its distribution. We use a detailed mock catalog of quasars (sources) and Lyman-limit systems (lenses) to demonstrate the applicability of this approach through our ability to recover the parameter. This mock catalog also allows us to check for systematics in the method and to sketch its limitations. For a universal constant mass-to-gas ratio and a sample of N quasars, we obtain an unbiased estimate of its value with 95% confidence limits (independent of its actual value) of +/- 140 {10^5/N)^0.5.

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arXiv2000

Lensing Sunyaev-Zel'dovich Clusters

Full-sky microwave surveys like the upcoming Planck satellite mission will detect of order 10^4 galaxy clusters through their thermal Sunyaev-Zel'dovich effect. I investigate the properties of the gravitationally lensing subsample of these clusters. The main results are: (1) The combined sample comprises >~70% of the complete sample. (2) It is confined to redshifts 0.2+-0.1, and to masses (5+-3) x 10^14 solar masses. (3) Using a particular measure for the weak lensing effect, viz. the aperture mass, cluster masses can be determined with a relative accuracy of ~20% if their density profile is known. Consequently, the mass function of the combined sample can accurately be measured. (4) For low-density universes, I predict a sharp peak in the measured (aperture) mass function near 5 x 10^14 solar masses and explain its origin, showing that the peak will be absent in high-density universes. (5) The location of the peak and the exponential decrease of the mass function on its high-mass side will allow the determination of the amplitude of the dark-matter power spectrum on the cluster scale and the baryon fraction in clusters, and constrain the thermal history of the intracluster gas.

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arXiv2000

Cluster galaxies: contribution to the arc statistics

We present the results of a set of numerical simulations aiming at evaluating the effects of cluster galaxies on the arc statistics. At this goal we use nine different galaxy clusters obtained from N-body simulations. We mimic the presence of a population of galaxies inside each cluster, trying to reproduce the observed luminosity function and the spatial distribution. We compare the statistical distributions of the properties of the gravitational arcs produced by our clusters with and without galaxies. We find that the cluster galaxies do not introduce perturbations strong enough to significantly change the number of arcs and the distributions of the properties of long arcs.

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arXiv2000

Effects of cluster galaxies on arc statistics

We present the results of a set of numerical simulations evaluating the effect of cluster galaxies on arc statistics. We perform a first set of gravitational lensing simulations using three independent projections for each of nine different galaxy clusters obtained from N-body simulations. The simulated clusters consist of dark matter only. We add a population of galaxies to each cluster, mimicking the observed luminosity function and the spatial galaxy distribution, and repeat the lensing simulations including the effects of cluster galaxies, which themselves act as individual lenses. Each galaxy is represented by a spherical Navarro, Frenk & White (1997) density profile. We consider the statistical distributions of the properties of the gravitational arcs produced by our clusters with and without galaxies. We find that the cluster galaxies do not introduce perturbations strong enough to significantly change the number of arcs and the distributions of lengths, widths, curvature radii and length-to-width ratios of long arcs. We find some changes to the distribution of short-arc properties in presence of cluster galaxies. The differences appear in the distribution of curvature radii for arc lengths smaller than 12'', while the distributions of lengths, widths and length-to-width ratios are significantly changed only for arcs shorter than 4''.

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arXiv1999

Weak Gravitational Lensing

We review theory and applications of weak gravitational lensing. After summarising Friedmann-Lemaitre cosmological models, we present the formalism of gravitational lensing and light propagation in arbitrary space-times. We discuss how weak-lensing effects can be measured. The formalism is then applied to reconstructions of galaxy-cluster mass distributions, gravitational lensing by large-scale matter distributions, QSO-galaxy correlations induced by weak lensing, lensing of galaxies by galaxies, and weak lensing of the cosmic microwave background. Contents: Introduction - Cosmological Background - Gravitational Light Deflection - Principles of Weak Gravitational Lensing - Weak Lensing by Galaxy Clusters - Weak Cosmological Lensing - QSO Magnification Bias and Large-Scale Structure - Galaxy-Galaxy Lensing - The Impact of Weak Gravitational Lensing on the Microwave Background Radiation - Summary and Outlook

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arXiv1999

Cluster deprojection with joint lensing, X-ray, and Sunyaev-Zeldovich data

We propose a new cluster deprojection algorithm for recovering cluster structure along the line of sight (los) based on the Richardson-Lucy (RL) algorithm, which recovers a nonnegative theoretical distribution from a given projection. To optimize our reconstruction we combine X-ray and Sunyaev-Zeldovich (SZ) data with gravitational lensing maps. The combination of these three data types constrains the structure of rich clusters of galaxies. To validate our new approach, we apply the algorithm to gas-dynamical cluster simulations. A reconstruction using the lensing potential is shown.

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arXiv1999

High-redshift quasars lensed by spiral galaxies

Given its extraordinary spatial resolution and sensitivity, the projected Next Generation Space Telescope (NGST) is likely to detect a large number of high-redshift QSOs lensed by spiral galaxies. Using realistic models for the QSO and spiral populations, we calculate the expected number density of detectable QSOs multiply imaged by spirals, and investigate the influence of various evolution effects on that number density. It is shown that NGST will probably find of order ten lensed QSOs per square degree at 26th magnitude in the V and L bands, and that various observable quantities like the total number density of lensed QSOs in these two bands, the ratio between the number densities of lensed QSOs in the V and L bands, the fraction of QSOs with more than two images and so forth can be used to constrain the evolution of the QSOs, the spirals, the dust in spirals, and the masses of spiral disks.

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arXiv1999

SPH simulations of magnetic fields in galaxy clusters (proceedings)

We perform cosmological, hydrodynamic simulations of magnetic fields in galaxy clusters. The computational code combines the special-purpose hardware Grape for calculating gravitational interaction, and smooth-particle hydrodynamics for the gas component. We employ the usual MHD equations for the evolution of the magnetic field in an ideally conducting plasma. As a first application, we focus on the question what kind of initial magnetic fields yield final field configurations within clusters which are compatible with Faraday-rotation measurements. Our main results can be summarised as follows: (i) Initial magnetic field strengths are amplified by approximately three orders of magnitude in cluster cores, one order of magnitude above the expectation from spherical collapse. (ii) Vastly different initial field configurations (homogeneous or chaotic) yield results that cannot significantly be distinguished. (iii) Micro-Gauss fields and Faraday-rotation observations are well reproduced in our simulations starting from initial magnetic fields of \~10^-9 G strength. Our results show that (i) shear flows in clusters are crucial for amplifying magnetic fields beyond simple compression, (ii) final field configurations in clusters are dominated by the cluster collapse rather than by the initial configuration, and (iii) initial magnetic fields of order 10^-9 G are required to match Faraday-rotation observations in real clusters.

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arXiv1999

SPH simulations of magnetic fields in galaxy clusters

We perform cosmological, hydrodynamic simulations of magnetic fields in galaxy clusters. The computational code combines the special-purpose hardware Grape for calculating gravitational interaction, and smooth-particle hydrodynamics for the gas component. We employ the usual MHD equations for the evolution of the magnetic field in an ideally conducting plasma. As a first application, we focus on the question what kind of initial magnetic fields yield final field configurations within clusters which are compatible with Faraday-rotation measurements. Our main results can be summarised as follows: (i) Initial magnetic field strengths are amplified by approximately three orders of magnitude in cluster cores, one order of magnitude above the expectation from spherical collapse. (ii) Vastly different initial field configurations (homogeneous or chaotic) yield results that cannot significantly be distinguished. (iii) Micro-Gauss fields and Faraday-rotation observations are well reproduced in our simulations starting from initial magnetic fields of \~ 10^-9 G strength at redshift 15. Our results show that (i) shear flows in clusters are crucial for amplifying magnetic fields beyond simple compression, (ii) final field configurations in clusters are dominated by the cluster collapse rather than by the initial configuration, and (iii) initial magnetic fields of order 10^-9 G are required to match Faraday-rotation observations in real clusters.

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arXiv1999

The Core Structure of Galaxy Clusters from Gravitational Lensing

We examine gravitational lensing constraints on the structure of galaxy clusters and compare them with the results of cosmological N-body simulations of cluster formation in cold dark matter (CDM) dominated universes. We find that cluster core masses, as measured by the observed location of giant tangential arcs, generally exceed those of dark matter halos of similar velocity dispersion. The magnitude of the discrepancy is a strong function of cluster mass. Arc properties in the most massive clusters in the sample (i.e. those with velocity dispersion, σ\sim 1500-2000 km/s) are essentially consistent with the N-body predictions. On the other hand, giant arcs in σ\sim 1000 km/s clusters can only be reconciled with CDM cluster halos if their lensing power has been increased substantially by the presence of a massive (\sim 3 \times 10^{12} h^{-1} M_{\odot}) central galaxy and of significant substructure. Best agreement is found if the mass of the central galaxy and the effects of substructure are approximately independent of cluster mass. Massive central galaxies with steep inner density profiles are also needed to explain a clear trend observed in our dataset between the radial thickness of giant tangential arcs and the velocity dispersion of the cluster lens. The position and redshift of radial arcs may be used as independent tests of these results, but at present the dataset available is too limited to have a significant impact on these conclusions. Our results depend only weakly on the cosmological model adopted, and suggest that structural parameters of clusters derived from strong lensing studies cannot usefully constrain the values of cosmological parameters.

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arXiv1999

Power spectrum from weak-shear data

We demonstrate that the aperture mass as a measure for cosmic shear closely approximates (to better than ~5%) the scaled and shifted power spectrum of the projected mass density. This cosmological weak-lensing information can thus be used to directly infer the projected matter power spectrum with high accuracy. As an application, we show that aperture-mass observations can be used to constrain the cosmic density parameter and the power-spectrum amplitude. We show that, for a particular example, it should be possible to constrain Omega_0 to within ~ +/- 27%, and sigma_8 to within ~ +/- 8% using weak-shear data on a square-shaped field of 8 degrees side length.

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arXiv1999

Projection effects in mass-selected galaxy-cluster samples

Selection of galaxy clusters by mass is now possible due to weak gravitational lensing effects. It is an important question then whether this type of selection reduces the projection effects prevalent in optically selected cluster samples. We address this question using simulated data, from which we construct synthetic cluster catalogues both with Abell's criterion and an aperture-mass estimator sensitive to gravitational tidal effects. The signal-to-noise ratio of the latter allows to some degree to control the properties of the cluster sample. For the first time, we apply the cluster-detection algorithm proposed by Schneider to large-scale structure simulations. We find that selection of clusters through weak gravitational lensing is more reliable in terms of completeness and spurious detections. Choosing the signal-to-noise threshold appropriately, the completeness can be increased up to 100%, and the fraction of spurious detections can significantly be reduced compared to Abell-selected cluster samples. We also investigate the accuracy of mass estimates in cluster samples selected by both luminosity and weak-lensing effects. We find that mass estimates from gravitational lensing, for which we employ the zeta-statistics by Kaiser et al., are significantly more accurate than those obtained from galaxy kinematics via the virial theorem.

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arXiv1998

Entropy-regularized Maximum-Likelihood cluster mass reconstruction

We present a new method for reconstructing two-dimensional mass maps of galaxy clusters from the image distortion of background galaxies. In contrast to most previous approaches, which directly convert locally averaged image ellipticities to mass maps (direct methods), our entropy-regularized maximum-likelihood method is an inverse approach. Albeit somewhat more expensive computationally, our method allows high spatial resolution in those parts of the cluster where the lensing signal is strong enough. Furthermore, it allows to straightforwardly incorporate additional constraints, such as magnification information or strong-lensing features. Using synthetic data, we compare our new approach to direct methods and find indeed a substantial improvement especially in the reconstruction of mass peaks. The main differences to previously published inverse methods are discussed.

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arXiv1997

Effects of Disks on Gravitational Lensing by Spiral Galaxies

Gravitational lensing of a quasar by a spiral galaxy should often be accompanied by damped Lyman-alpha absorption and dust extinction due to the intervening gaseous disk. In nearly edge-on configurations, the surface mass density of the gas and stars in the disk could by itself split the quasar image and contribute significantly to the overall lensing cross section. We calculate the lensing probability of a disk+halo mass model for spiral galaxies, including cosmic evolution of the lens parameters. A considerable fraction of the lens systems contains two images with sub-arcsecond separation, straddling a nearly edge-on disk. Because of that, extinction by dust together with observational selection effects (involving a minimum separation and a maximum flux ratio for the lensed images), suppress the detection efficiency of spiral lenses in optical wavebands by at least an order of magnitude. The missing lenses could be recovered in radio surveys. In modifying the statistics of damped Lyman-alpha absorbers, the effect of extinction dominates over the magnification bias due to lensing.

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