Improving the precision matrix for precision cosmology

The estimation of cosmological constraints from observations of the large scale structure of the Universe, such as the power spectrum or the correlation function, requires the knowledge of the inverse of the associated covariance matrix, namely the precision matrix, $\mathbf{\Psi}$. In most analyses, $\mathbf{\Psi}$ is estimated from a limited set of mock catalogues. Depending on how many mocks are used, this estimation has an associated error which must be propagated into the final cosmological constraints. For future surveys such as Euclid and DESI, the control of this additional uncertainty requires a prohibitively large number of mock catalogues. In this work we test a novel technique for the estimation of the precision matrix, the covariance tapering method, in the context of baryon acoustic oscillation measurements. Even though this technique was originally devised as a way to speed up maximum likelihood estimations, our results show that it also reduces the impact of noisy precision matrix estimates on the derived confidence intervals, without introducing biases on the target parameters. The application of this technique can help future surveys to reach their true constraining power using a significantly smaller number of mock catalogues.Comment: 9 pages, 7 figures, minor changes to match version accepted by MNRA

Large scale anisotropies on halo infall

We perform a statistical analysis of the peculiar velocity field around dark matter haloes in numerical simulations. We examine different properties of the infall of material onto haloes and its relation to central halo shapes and the shape of the large scale surrounding regions (LSSR). We find that the amplitude of the infall velocity field along the halo shape minor axis is larger than that along the major axis. This is consistent for general triaxial haloes, and for both prolate and oblate systems. We also report a strong anisotropy of the velocity field along the principal axes of the LSSR. The infall velocity field around dark matter haloes reaches a maximum value along the direction of the minor axis of the LSSR, whereas along the direction of its major axis, it exhibits the smallest velocities. We also analyse the dependence of the matter velocity field on the local environment. The amplitude of the infall velocity at high local density regions is larger than at low local density regions. The velocity field tends to be more laminar along the direction towards the minor axis of the LSSR, where the mean ratio between flow velocity and velocity dispersion is of order unity and nearly constant up to scales of 15 Mpc/h. We also detect anisotropies in the outflowing component of the velocity field, showing a maximum amplitude along the surrounding LSSR major axis.Comment: 12 pages, 9 figures, accepted for publication in Monthly Notices of the Royal Astronomical Societ

Construcción de un catálogo de cúmulos de galaxias en proceso de colisión

En este trabajo presentamos los primeros resultados de la identificación de cúmulos de galaxias en colisión en catálogos de galaxias con mediciones de corrimiento al rojo (SDSS, 2DF) e introducimos la metodología. Calibramos un método mediante el estudio de los árboles de fusión de cúmulos en un catálogo simulado basado en un modelo semianalítico completo de formación de galaxias además de la simulación cosmológica Millenium. También discutimos acciones futuras para estudiar nuestra muestra de cúmulos de galaxias en colisión, incluidas las observaciones de rayos X y la reconstrucción masiva obtenida mediante el uso de lentes gravitacionales débiles.In this work we present first results of the identification of colliding galaxy clusters in galaxy catalogs with redshift measurements (SDSS, 2DF), and introduce the methodology. We calibrated a method by studying the merger trees of clusters in a mock catalog based on a full-blown semi-analytic model of galaxy formation on top of the Millenium cosmological simulation. We also discuss future actions for studding our sample of colliding galaxy clusters, including x-ray observations and mass reconstruction obtained by using weak gravitational lenses.Fil: de Los Rios, Martín Emilio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; ArgentinaFil: Dominguez, M. J.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; ArgentinaFil: Paz, Dante Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; Argentin

Guess the cheese flavour by the size of its holes: A cosmological test using the abundance of Popcorn voids

We present a new definition of cosmic void and a publicly available code with the algorithm that implements it. Underdense regions are defined as free-form objects, called popcorn voids, made from the union of spheres of maximum volume with a given joint integrated underdensity contrast.The method is inspired by the excursion-set theory and consequently no rescaling processing is needed, the removal of overlapping voids and objects with sizes below the shot noise threshold is inherent in the algorithm. The abundance of popcorn voids in the matter field can be fitted using the excursion-set theory provided the relationship between the linear density contrast of the barrier and the threshold used in void identification is modified relative to the spherical evolution model. We also analysed the abundance of voids in biased tracer samples in redshift space. We show how the void abundance can be used to measure the geometric distortions due to the assumed fiducial cosmology, in a test similar to an Alcock-Paczy\'nski test. Using the formalism derived from previous works, we show how to correct the abundance of popcorn voids for redshift-space distortion effects. Using this treatment, in combination with the excursion-set theory, we demonstrate the feasibility of void abundance measurements as cosmological probes. We obtain unbiased estimates of the target parameters, albeit with large degeneracies in the parameter space. Therefore, we conclude that the proposed test in combination with other cosmological probes has potential to improve current cosmological parameter constraints.Comment: Updated manuscript sent to the MNRAS after referee report: 16 pages, 8 figures. Corrections were made to Fig. 4, some related conclusions were modified. The main conclusions remain unchange

Angular momentum-Large-scale structure alignments in LCDM models and the SDSS

We study the alignments between the angular momentum of individual objects and the large-scale structure in cosmological numerical simulations and real data from the Sloan Digital Sky Survey, Data Release 6. To this end we measure anisotropies in the two point cross-correlation function around simulated halos and observed galaxies, studying separately the 1- and 2-halo regimes. The alignment of the angular momentum of dark-matter haloes in LCDM simulations is found to be dependent on scale and halo mass. At large distances (2-halo regime), the spins of high mass haloes are preferentially oriented in the direction perpendicular to the distribution of matter; lower mass systems show a weaker trend that may even reverse to show an angular momentum in the plane of the matter distribution. In the 1-halo term regime, the angular momentum is aligned in the direction perpendicular to the matter distribution; the effect is stronger than for the 1-halo term and increases for higher mass systems. On the observational side, we focus our study on galaxies in the Sloan Digital Sky Survey, Data Release 6 (SDSS-DR6) with elongated apparent shapes, and study alignments with respect to the major semi-axis. We find an excess of structure in the direction of the major semi-axis for all samples; the red sample shows the highest alignment (2.7+-0.08%) and indicates that the angular momentum of flattened spheroidals tends to be perpendicular to the large-scale structure. (Abridged)Comment: 10 pages, 6 figures, accepted for publication in MNRAS; the definitive version is available at www.blackwell-synergy.co

Alignments of Galaxy Group Shapes with Large Scale Structure

In this paper we analyse the alignment of galaxy groups with the surrounding large scale structure traced by spectroscopic galaxies from the Sloan Digital Sky Survey Data Release 7. We characterise these alignments by means of an extension of the classical two-point cross-correlation function, developed by Paz et al. 2008 (arXiv:0804.4477, MNRAS 389 1127). We find a strong alignment signal between the projected major axis of group shapes and the surrounding galaxy distribution up to scales of 30 Mpc/h. This observed anisotropy signal becomes larger as the galaxy group mass increases, in excellent agreement with the corresponding predicted alignment obtained from mock catalogues and LCDM cosmological simulations. These measurements provide new direct evidence of the adequacy of the gravitational instability picture to describe the large-scale structure formation of our Universe.Comment: 12 pages,7 figures, Accepted for publication in MNRA

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements