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Hoekstra H.,Leiden University | Bartelmann M.,Institute For Theoretische Astrophysik | Dahle H.,University of Oslo | Israel H.,University of Bonn | And 3 more authors.
Space Science Reviews | Year: 2013

Despite consistent progress in numerical simulations, the observable properties of galaxy clusters are difficult to predict ab initio. It is therefore important to compare both theoretical and observational results to a direct measure of the cluster mass. This can be done by measuring the gravitational lensing effects caused by the bending of light by the cluster mass distribution. In this review we discuss how this phenomenon can be used to determine cluster masses and study the mass distribution itself. As sample sizes increase, the accuracy of the weak lensing mass estimates needs to improve accordingly. We discuss the main practical aspects of these measurements. We review a number of applications and highlight some recent results. © 2013 Springer Science+Business Media Dordrecht.

Limousin M.,Aix - Marseille University | Limousin M.,Copenhagen University | Morandi A.,Tel Aviv University | Sereno M.,Polytechnic University of Turin | And 6 more authors.
Space Science Reviews | Year: 2013

While clusters of galaxies are considered one of the most important cosmological probes, the standard spherical modelling of the dark matter and the intracluster medium is only a rough approximation. Indeed, it is well established both theoretically and observationally that galaxy clusters are much better approximated as triaxial objects. However, investigating the asphericity of galaxy clusters is still in its infancy. We review here this topic which is currently gathering a growing interest from the cluster community. We begin by introducing the triaxial geometry. Then we discuss the topic of deprojection and demonstrate the need for combining different probes of the cluster's potential. We discuss the different works that have been addressing these issues. We present a general parametric framework intended to simultaneously fit complementary data sets (X-ray, Sunyaev Zel'dovich and lensing data). We discuss in details the case of Abell 1689 to show how different models/data sets lead to different haloe parameters. We present the results obtained from fitting a 3D NFW model to X-ray, SZ, and lensing data for 4 strong lensing clusters. We argue that a triaxial model generally allows to lower the inferred value of the concentration parameter compared to a spherical analysis. This may alleviate tensions regarding, e.g. the over-concentration problem. However, we stress that predictions from numerical simulations rely on a spherical analysis of triaxial halos. Given that triaxial analysis will have a growing importance in the observational side, we advocate the need for simulations to be analysed in the very same way, allowing reliable and meaningful comparisons. Besides, methods intended to derive the three dimensional shape of galaxy clusters should be extensively tested on simulated multi-wavelength observations. © 2013 Springer Science+Business Media Dordrecht.

Joergens V.,Institute For Theoretische Astrophysik | Joergens V.,Max Planck Institute for Astronomy | Pohl A.,Max Planck Institute for Astronomy | Sicilia-Aguilar A.,Autonomous University of Madrid | Henning T.,Max Planck Institute for Astronomy
Astronomy and Astrophysics | Year: 2012

We show that the very young brown dwarf candidate ISO 217 (M6.25) is driving an intrinsically asymmetric bipolar outflow with a stronger and slightly faster red-shifted component based on spectro-astrometry of forbidden [S II] emission lines at 6716Å and 6731Å observed in UVES/VLT spectra taken in 2009. ISO 217 is only one of a handful of brown dwarfs and very low-mass stars (M5-M8) for which an outflow has been detected and that show that the T Tauri phase continues at the substellar limit. We measure a spatial extension of the outflow in [S II] of up to ± 190 mas (about ± 30 AU) and velocities of up to ± 40-50 km s -1. We find that the basic outflow properties (spatial extension, velocities, and outflow position angle) are of similar order as those determined in the discovery spectra from May 2007 of Whelan and coworkers. We show that the velocity asymmetry between both lobes is variable on timescales of a few years and that the strong asymmetry of a factor of two found in 2007 might be smaller than originally anticipated when using a more realistic stellar rest-velocity. We also detect forbidden line emission of [Fe II]λ7155Å, for which we propose as a potential origin the hot inner regions of the outflow. To comprehensively understand the ISO 217 system, we determine the properties of its accretion disk based on radiative transfer modeling of the SED from 0.66 to 24 μm. This disk model agrees very well with Herschel/PACS data at 70 μm. We find that the disk is flared and intermediately inclined (i ∼ 45°). The total disk mass of the best-fit model is 4 × 10 -6 M ·, which is low compared to the accretion and outflow rate of ISO 217 from the literature (∼). We propose that this discrepancy can be explained by either a higher disk mass than inferred from the model because of strong undetected grain growth and/or by an on average lower accretion rate and outflow rate than the determined values. We show that a disk inclination significantly exceeding 45°, as suggested from Hα modeling and from both lobes of the outflow being visible, is inconsistent with the SED data. Thus, despite its intermediate inclination angle, the disk of this brown dwarf does not appear to obscure the red outflow component in [S II], which is very rarely seen for T Tauri objects (only one other case). © 2012 ESO.

Stacy A.,University of Texas at Austin | Greif T.H.,Institute For Theoretische Astrophysik | Greif T.H.,University of Heidelberg | Bromm V.,University of Texas at Austin
Monthly Notices of the Royal Astronomical Society | Year: 2010

We investigate the formation of metal-free, Population III (Pop III), stars within a minihalo at z ≃ 20 with a smoothed particle hydrodynamics (SPH) simulation, starting from cosmological initial conditions. Employing a hierarchical, zoom-in procedure, we achieve sufficient numerical resolution to follow the collapsing gas in the centre of the minihalo up to number densities of 1012 cm-3. This allows us to study the protostellar accretion on to the initial hydrostatic core, which we represent as a growing sink particle, in improved physical detail. The accretion process, and in particular its termination, governs the final masses that were reached by the first stars. The primordial initial mass function, in turn, played an important role in determining to what extent the first stars drove early cosmic evolution. We continue our simulation for 5000 yr after the first sink particle has formed. During this time period, a disc-like configuration is assembled around the first protostar. The disc is gravitationally unstable, develops a pronounced spiral structure and fragments into several other protostellar seeds. At the end of the simulation, a small multiple system has formed, dominated by a binary with masses ~40 and ~10 M⊙ If Pop III stars were to form typically in binaries or small multiples, the standard model of primordial star formation, where single, isolated stars are predicted to form in minihaloes, would have to be modified. This would have crucial consequences for the observational signature of the first stars, such as their nucleosynthetic pattern, and the gravitational wave emission from possible Pop III black hole binaries. © 2010 The Authors. Journal compilation © 2010 RAS.

Walch S.,University of Cardiff | Whitworth A.P.,University of Cardiff | Girichidis P.,University of Cardiff | Girichidis P.,Institute For Theoretische Astrophysik
Monthly Notices of the Royal Astronomical Society | Year: 2012

The collapse of weakly turbulent pre-stellar cores is a critical stage in the process of star formation. Being highly non-linear and stochastic, the outcome of collapse can only be explored theoretically by performing large ensembles of numerical simulations. Standard practice is to quantify the initial turbulent velocity field in a core in terms of the amount of turbulent energy (or some equivalent) and the exponent in the power spectrum (n≡-dlogP k/dlogk). In this paper, we present a numerical study of the influence of the details of the turbulent velocity field on the collapse of an isolated, weakly turbulent, low-mass pre-stellar core. We show that, as long asn≳ 3 (as is usually assumed), a more critical parameter thannis the maximum wavelength in the turbulent velocity field, λ MAX. This is because λ MAX carries most of the turbulent energy, and thereby influences both the amountandthe spatial coherence of the angular momentum in the core. We show that the formation of dense filaments during collapse depends critically on λ MAX, and we explain this finding using a force balance analysis. We also show that the core has only a high probability of fragmenting if λ MAX >R CORE/2 (whereR CORE is the core radius), the dominant mode of fragmentation involves the formation and break-up of filaments and although small protostellar discs (with radiusR DISC≲ 20 au) form routinely, more extended discs are rare. In turbulent, low-mass cores of the type we simulate here, the formation of large, fragmenting protostellar discs is suppressed by early fragmentation in the filaments. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Andrae R.,Max Planck Institute for Astronomy | Jahnke K.,Max Planck Institute for Astronomy | Melchior P.,Institute For Theoretische Astrophysik
Monthly Notices of the Royal Astronomical Society | Year: 2011

Given the enormous galaxy data bases of modern sky surveys, parametrizing galaxy morphologies is a very challenging task due to the huge number and variety of objects. We assess the different problems faced by existing parametrization schemes (CAS, Gini, M20, Sérsic profile, shapelets) in an attempt to understand why parametrization is so difficult and in order to suggest improvements for future parametrization schemes. We demonstrate that morphological observables (e.g. steepness of the radial light profile, ellipticity, asymmetry) are intertwined and cannot be measured independently of each other. We present strong arguments in favour of model-based parametrization schemes, namely reliability assessment, disentanglement of morphological observables and point spread function modelling. Furthermore, we demonstrate that estimates of the concentration and Sérsic index obtained from the Zurich Structure & Morphology catalogue are in excellent agreement with theoretical predictions. We also demonstrate that the incautious use of the concentration index for classification purposes can cause a severe loss of the discriminative information contained in a given data sample. Moreover, we show that, for poorly resolved galaxies, concentration index and M20 suffer from strong discontinuities, i.e. similar morphologies are not necessarily mapped to neighbouring points in the parameter space. This limits the reliability of these parameters for classification purposes. Two-dimensional Sérsic profiles accounting for centroid and ellipticity are identified as the currently most reliable parametrization scheme in the regime of intermediate signal-to-noise ratios and resolutions, where asymmetries and substructures do not play an important role. We argue that basis functions provide good parametrization schemes in the regimes of high signal-to-noise ratios and resolutions. Concerning Sérsic profiles, we show that scale radii cannot be compared directly for profiles of different Sérsic indices. Furthermore, we show that parameter spaces are typically highly non-linear. This implies that significant caution is required when distance-based classification methods are used. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

Melchior P.,Institute For Theoretische Astrophysik | Bohnert A.,University of Bonn | Bohnert A.,European Southern Observatory | Lombardi M.,European Southern Observatory | Bartelmann M.,Institute For Theoretische Astrophysik
Astronomy and Astrophysics | Year: 2010

Aims: We seek to understand the impact on shape estimators obtained from circular and elliptical shapelet models under two realistic conditions: (a) only a limited number of shapelet modes are available for the model, and (b) the intrinsic galactic shapes are not restricted to shapelet models. Methods: We create a set of simplistic simulations, in which the galactic shapes follow a Sérsic profile. By varying the Sérsic index and applied shear, we quantify the amount of bias on shear estimates that arises from insufficient modeling. Additional complications from PSF convolution, pixelation, and pixel noise are also discussed. Results: Highly elliptical galaxy shapes cannot be accurately modeled within the circular shapelet basis system and are biased towards less elongated shapes. This problem can be cured by allowing elliptical basis functions. Another problem occurs for galaxies whose radial profile differs strongly from the Gaussian weighting function employed in both the circular and the elliptical shapelet bases. For galaxies with large Sérsic indices, shear estimates from circular shapelets are biased low even for small apparent ellipticities because of the preference for round models, and shear estimates from elliptical shapelets depend critically on accurate ellipticity priors. Independent of the particular form of the estimator, the bias depends on the true intrinsic galaxy morphology, but also on the size and shape of the PSF. Conclusions: As long as the issues discussed here are not solved, we question that the shapelet method can provide weak-lensing measurements for all observable galaxies (independent of their Sérsic index) with an accuracy demanded by upcoming missions and surveys. An accurate and reliable calibration, specific for the dataset under investigation, would be required but is difficult to infer consistently from the data. © 2010 ESO.

Andrae R.,Max Planck Institute for Astronomy | Melchior P.,Institute For Theoretische Astrophysik | Bartelmann M.,Institute For Theoretische Astrophysik
Astronomy and Astrophysics | Year: 2010

Context. The huge and still rapidly growing amount of galaxies in modern sky surveys raises the need for an automated and objective classification method. Unsupervised learning algorithms are of particular interest, since they discover classes automatically. Aims. We briefly discuss the pitfalls of oversimplified classification methods and outline an alternative approach called "clustering analysis". Methods. We have categorised different classification methods according to their capabilities. Based on this categorisation, we present a probabilistic classification algorithm that automatically detects the optimal classes preferred by the data. We explored the reliability of this algorithm in systematic tests. Using a sample of 1520 bright galaxies from the SDSS, we demonstrate the performance of this algorithm in practice. We are able to disentangle the problems of classification and parametrisation of galaxy morphologies in this case. Results. We give physical arguments that a probabilistic classification scheme is necessary. When applied to a small set of 84 galaxies visually classified as face-on discs, edge-on discs, and ellipticals, the clustering algorithm discovers precisely these classes and produces excellent object-to-class assignments. The resulting grouping of the galaxies outperforms a principal components analysis applied to the same data set. Applying the algorithm to a sample of 1520 SDSS galaxies, we find morphologically distinct classes when the number of classes are 3 and 8. Conclusions. Although interpreting clustering results is a challenging task, the algorithm we present produces reasonable morphological classes and object-to-class assignments without any prior assumptions. © 2010 ESO.

Andrae R.,Max Planck Institute for Astronomy | Melchior P.,Institute For Theoretische Astrophysik | Melchior P.,Ohio State University | Jahnke K.,Max Planck Institute for Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2011

Parametrization of galaxy morphologies is a challenging task, for instance in shear measurements of weak gravitational lensing or investigations of formation and evolution of galaxies. The huge variety of different morphologies requires a parametrization scheme that is highly flexible and that accounts for certain morphological observables, such as ellipticity, steepness of the radial light profile and azimuthal structure. In this article, we revisit the method of sérsiclets, where galaxy morphologies are decomposed into a set of polar basis functions that are based on the Sérsic profile. This approach is justified by the fact that the Sérsic profile is the first-order Taylor expansion of any real light profile. We show that sérsiclets indeed overcome the modelling failures of shapelets in the case of early-type galaxies. However, sérsiclets implicate an unphysical relation between the steepness of the light profile and the spatial scale of the polynomial oscillations, which is not necessarily obeyed by real galaxy morphologies and can therefore give rise to modelling failures. Moreover, we demonstrate that sérsiclets are prone to undersampling, which restricts sérsiclet modelling to highly resolved galaxy images. Analysing data from the weak-lensing great08 challenge, we demonstrate that sérsiclets should not be used in weak-lensing studies. We conclude that although the sérsiclet approach appears very promising at first glance, it suffers from conceptual and practical problems that severely limit its usefulness. In particular, sérsiclets do not provide high-precision results in weak-lensing studies. Finally, we show that the Sérsic profile can be enhanced by higher order terms in the Taylor expansion, which can drastically improve model reconstructions of galaxy images. When orthonormalized, these higher order profiles can overcome the problems of sérsiclets, while preserving their mathematical justification. However, this method is computationally expensive. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Melchior P.,Institute For Theoretische Astrophysik | Viola M.,Institute For Theoretische Astrophysik | Schafer B.M.,Astronomisches Rechen Institute | Bartelmann M.,Institute For Theoretische Astrophysik
Monthly Notices of the Royal Astronomical Society | Year: 2011

We introduce a novel method for weak-lensing measurements, which is based on a mathematically exact deconvolution of the moments of the apparent brightness distribution of galaxies from the telescope's point spread function (PSF). No assumptions on the shape of the galaxy or the PSF are made. The (de)convolution equations are exact for unweighted moments only, while in practice a compact weight function needs to be applied to the noisy images to ensure that the moment measurement yields significant results. We employ a Gaussian weight function, whose centroid and ellipticity are iteratively adjusted to match the corresponding quantities of the source. The change of the moments caused by the application of the weight function can then be corrected by considering higher order weighted moments of the same source. Because of the form of the deconvolution equations, even an incomplete weighting correction leads to an excellent shear estimation if galaxies and PSF are measured with a weight function of identical size. We demonstrate the accuracy and capabilities of this new method in the context of weak gravitational lensing measurements with a set of specialized tests and show its competitive performance on the GREAT08 Challenge data. A complete c++ implementation of the method can be requested from the authors. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

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