Yago Ascasibar

Bio: Yago Ascasibar is an academic researcher from Autonomous University of Madrid. The author has contributed to research in topics: Galaxy & Star formation. The author has an hindex of 36, co-authored 109 publications receiving 4431 citations. Previous affiliations of Yago Ascasibar include University of Oxford & Spanish National Research Council.

The Origin of Cold Fronts in the Cores of Relaxed Galaxy Clusters

Abstract: Chandra X-ray observations revealed the presence of cold fronts (sharp contact discontinuities between gas regions with different temperatures and densities) in the centers of many, if not most, relaxed clusters with cool cores. We use high-resolution simulations of idealized cluster mergers to show that they are due to sloshing of the cool gas in the central gravitational potential, which is easily set off by minor mergers and can persist for gigayears. The only necessary condition is a steep entropy profile, as observed in cooling flow clusters. Even if the infalling subcluster has no gas during core passage, the gravitational disturbance sets the main mass peak (gas and dark matter together) in motion relative to the surrounding gas. A rapid change in the direction of motion causes a change in ram pressure, which pushes the cool gas away from the dark matter peak and triggers sloshing. For nonzero impact parameters, the cool gas acquires angular momentum, resulting in a characteristic spiral pattern of cold fronts. There is little visible disturbance outside the cool core in such a merger. If the subcluster retains its gas during core passage, the cool central gas of the main cluster is more easily decoupled from the dark matter peak. Subsequently, some of that gas, and often the cool gas from the subcluster, falls back to the center and starts sloshing. However, in such a merger, global disturbances are readily visible in X-rays for a long time. We conclude that cold fronts at the centers of relaxed clusters, often spiral or concentric arc in shape, are probably caused by encounters with small subhalos stripped of all of their gas at the early infall stages.

Haloes gone MAD: The Halo-Finder Comparison Project

Abstract: We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including friends-of-friends, spherical-overdensity and phase-space-based algorithms. We

Haloes gone MAD: The Halo-Finder Comparison Project

Abstract: [abridged] We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including friends-of-friends (FOF), spherical-overdensity (SO) and phase-space based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allows halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large-scale structure of the universe. All the halo finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Via a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30-40 particles. However, also here the phase space finders excelled by resolving substructure down to 10-20 particles. By comparing the halo finders using a high resolution cosmological volume we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity, and peak of the rotation curve).

CALIFA, the Calar Alto Legacy Integral Field Area survey - III. Second public data release

Abstract: This paper describes the Second Public Data Release (DR2) of the Calar Alto Legacy Integral Field Area (CALIFA) survey. The data for 200 objects are made public, including the 100 galaxies of the First Public Data Release (DR1). Data were obtained with the integral-field spectrograph PMAS /PPak mounted on the 3.5 m telescope at the Calar Alto observatory. Two different spectral setups are available for each galaxy, (i) a low-resolution V500 setup covering the wavelength range 3745-7500 A with a spectral resolution of 6.0 A (FWHM); and (ii) a medium-resolution V1200 setup covering the wavelength range 3650-4840 A with a spectral resolution of 2.3 A (FWHM). The sample covers a redshift range between 0.005 and 0.03, with a wide range of properties in the color-magnitude diagram, stellar mass, ionization conditions, and morphological types. All the cubes in the data release were reduced with the latest pipeline, which includes improved spectrophotometric calibration, spatial registration, and spatial resolution. The spectrophotometric calibration is better than 6% and the median spatial resolution is 2´´ 4. In total, the second data release contains over 1.5 million spectra.

Structure finding in cosmological simulations: the state of affairs

Abstract: The ever increasing size and complexity of data coming from simulations of cosmic structure formation demand equally sophisticated tools for their analysis. During the past decade, the art of object finding in these simulations has hence developed into an important discipline itself. A multitude of codes based upon a huge variety of methods and techniques have been spawned yet the question remained as to whether or not they will provide the same (physical) information about the structures of interest. Here we summarize and extent previous work of the 'halo finder comparison project': we investigate in detail the (possible) origin of any deviations across finders. To this extent, we decipher and discuss differences in halo-finding methods, clearly separating them from the disparity in definitions of halo properties. We observe that different codes not only find different numbers of objects leading to a scatter of up to 20 per cent in the halo mass and V-max function, but also that the particulars of those objects that are identified by all finders differ. The strength of the variation, however, depends on the property studied, e.g. the scatter in position, bulk velocity, mass and the peak value of the rotation curve is practically below a few per cent, whereas derived quantities such as spin and shape show larger deviations. Our study indicates that the prime contribution to differences in halo properties across codes stems from the distinct particle collection methods and - to a minor extent - the particular aspects of how the procedure for removing unbound particles is implemented. We close with a discussion of the relevance and implications of the scatter across different codes for other fields such as semi-analytical galaxy formation models, gravitational lensing and observables in general.

Pattern Recognition and Machine Learning

Abstract: Probability Distributions.- Linear Models for Regression.- Linear Models for Classification.- Neural Networks.- Kernel Methods.- Sparse Kernel Machines.- Graphical Models.- Mixture Models and EM.- Approximate Inference.- Sampling Methods.- Continuous Latent Variables.- Sequential Data.- Combining Models.

The Cosmological simulation code GADGET-2

Abstract: We discuss the cosmological simulation code GADGET-2, a new massively parallel TreeSPH code, capable of following a collisionless fluid with the N-body method, and an ideal gas by means of smoothed particle hydrodynamics (SPH). Our implementation of SPH manifestly conserves energy and entropy in regions free of dissipation, while allowing for fully adaptive smoothing lengths. Gravitational forces are computed with a hierarchical multipole expansion, which can optionally be applied in the form of a TreePM algorithm, where only short-range forces are computed with the ‘tree’ method while long-range forces are determined with Fourier techniques. Time integration is based on a quasi-symplectic scheme where long-range and short-range forces can be integrated with different time-steps. Individual and adaptive short-range time-steps may also be employed. The domain decomposition used in the parallelization algorithm is based on a space-filling curve, resulting in high flexibility and tree force errors that do not depend on the way the domains are cut. The code is efficient in terms of memory consumption and required communication bandwidth. It has been used to compute the first cosmological N-body simulation with more than 10 10 dark matter particles, reaching a homogeneous spatial dynamic range of 10 5 per dimension in a three-dimensional box. It has also been used to carry out very large cosmological SPH simulations that account for radiative cooling and star formation, reaching total particle numbers of more than 250 million. We present the algorithms used by the code and discuss their accuracy and performance using a number of test problems. GADGET-2 is publicly released to the research community. Ke yw ords: methods: numerical ‐ galaxies: interactions ‐ dark matter.

Monthly Notices of the Royal Astronomical Society

Abstract: Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

Cosmological smoothed particle hydrodynamics simulations: a hybrid multiphase model for star formation

Abstract: We present a model for star formation and supernova feedback that describes the multiphase structure of star-forming gas on scales that are typically not resolved in cosmological simulations. Our approach includes radiative heating and cooling, the growth of cold clouds embedded in an ambient hot medium, star formation in these clouds, feedback from supernovae in the form of thermal heating and cloud evaporation, galactic winds and outflows, and metal enrichment. Implemented using smoothed particle hydrodynamics, our scheme is a significantly modified and extended version of the grid-based method of Yepes et al., and enables us to achieve a high dynamic range in simulations of structure formation. We discuss properties of the feedback model in detail and show that it predicts a self-regulated, quiescent mode of star formation, which, in particular, stabilizes the star-forming gaseous layers of disc galaxies. The parametrization of this mode can be reduced to a single free quantity that determines the overall time-scale for star formation. We fix this parameter numerically to match the observed rates of star formation in local disc galaxies. When normalized in this manner, cosmological simulations employing our model nevertheless overproduce the observed cosmic abundance of stellar material. We are thus motivated to extend our feedback model to include galactic winds associated with star formation. Using small-scale simulations of individual star-forming disc galaxies, we show that these winds produce either galactic fountains or outflows, depending on the depth of the gravitational potential. In low-mass haloes, winds can greatly suppress the overall efficiency of star formation. When incorporated into cosmological simulations, our combined model for star formation and winds predicts a cosmic star formation density that is consistent with observations, provided that the winds are sufficiently energetic. Moreover, outflows from galaxies in these simulations drive chemical enrichment of the intergalactic medium – in principle, accounting for the presence of metals in the Lyman α forest.

E pur si muove: Galilean-invariant cosmological hydrodynamical simulations on a moving mesh

Abstract: Hydrodynamic cosmological simulations at present usually employ either the Lagrangian smoothed particle hydrodynamics (SPH) technique or Eulerian hydrodynamics on a Cartesian mesh with (optional) adaptive mesh refinement (AMR). Both of these methods have disadvantages that negatively impact their accuracy in certain situations, for example the suppression of fluid instabilities in the case of SPH, and the lack of Galilean invariance and the presence of overmixing in the case of AMR. We here propose a novel scheme which largely eliminates these weaknesses. It is based on a moving unstructured mesh defined by the Voronoi tessellation of a set of discrete points. The mesh is used to solve the hyperbolic conservation laws of ideal hydrodynamics with a finite-volume approach, based on a second-order unsplit Godunov scheme with an exact Riemann solver. The mesh-generating points can in principle be moved arbitrarily. If they are chosen to be stationary, the scheme is equivalent to an ordinary Eulerian method with second-order accuracy. If they instead move with the velocity of the local flow, one obtains a Lagrangian formulation of continuum hydrodynamics that does not suffer from the mesh distortion limitations inherent in other mesh-based Lagrangian schemes. In this mode, our new method is fully Galilean invariant, unlike ordinary Eulerian codes, a property that is of significant importance for cosmological simulations where highly supersonic bulk flows are common. In addition, the new scheme can adjust its spatial resolution automatically and continuously, and hence inherits the principal advantage of SPH for simulations of cosmological structure growth. The high accuracy of Eulerian methods in the treatment of shocks is also retained, while the treatment of contact discontinuities improves. We discuss how this approach is implemented in our new code arepo, both in 2D and in 3D, and is parallelized for distributed memory computers. We also discuss techniques for adaptive refinement or de-refinement of the unstructured mesh. We introduce an individual time-step approach for finite-volume hydrodynamics, and present a high-accuracy treatment of self-gravity for the gas that allows the new method to be seamlessly combined with a high-resolution treatment of collisionless dark matter. We use a suite of test problems to examine the performance of the new code and argue that the hydrodynamic moving-mesh scheme proposed here provides an attractive and competitive alternative to current SPH and Eulerian techniques.