A phenomenological model of galaxy clusters

We present a simple model to describe the dark matter density, the gas density, and the gas temperature profiles of galaxy clusters. Analytical expressions for these quantities are given in terms of only five free parameters with a clear physical meaning: the mass M of the dark matter halo (or the characteristic temperature T_0), the characteristic scale radius a, the cooling radius in units of a (0<alpha<1), the central temperature in units of T_0 (0<t<1), and the asymptotic baryon fraction in units of the cosmic value (f~1). It is shown that our model is able to reproduce the three-dimensional density and temperature profiles inferred from X-ray observations of real clusters within a 20 per cent accuracy over most of the radial range. Some possible applications are briefly discussed.Comment: 7 pages, 4 figures, submitted to MNRA

Effect of dark matter annihilation on gas cooling and star formation

In the current paradigm of cosmic structure formation, dark matter plays a key role on the formation and evolution of galaxies through its gravitational influence. On microscopic scales, dark matter particles are expected to annihilate amongst themselves into different products, with some fraction of the energy being transferred to the baryonic component. It is the aim of the present work to show that, in the innermost regions of dark matter halos, heating by dark matter annihilation may be comparable to the cooling rate of the gas. We use analytical models of the dark matter and gas distributions in order to estimate the heating and cooling rates, as well as the energy available from supernova explosions. Depending on the model parameters and the precise nature of dark matter particles, the injected energy may be enough to balance radiative cooling in the cores of galaxy clusters. On galactic scales, it would inhibit star formation more efficiently than supernova feedback. Our results suggest that dark matter annihilation prevents gas cooling and star formation within at least $0.01-1$ per cent of the virial radius

A Theoretical Perspective on Galaxy Clusters: Physical Properties of the Dark Matter and Baryons

Clusters of galaxies are studied from a theoretical point of view, comparing with observational results whenever possible. The problem is approached both analytically as well as by means of high-resoultion numerical simulations. The dark matter halo, the hot intracluster gas, and the stellar component are investigated separately. Numerical clusters are consistent with a relatively simple scenario, in which these objects form around local maxima of the primordial density field, smoothed on Mpc sacales. Hot diffuse gas is in approximate hydrostatic equilibrium with the dark matter potential, and it is well described by a polytropic equation of state. Global X-ray properties are closely interrelated, and deviations from self-similarity are expected even when radiative processes are not considered. Star formation is severely reduced in the inner regions of clusters, due to the fact that infalling galaxies loose their hot gas reservoirs during the first orbit.Comment: Ph.D. Thesis, Universidad Autonoma de Madrid. 210 pages, 46 figure

Do galaxies form a spectroscopic sequence?

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2011 RAS © The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We identify a spectroscopic sequence of galaxies, analogous to the Hubble sequence of morphological types, based on the Automatic Spectroscopic K-means-based (ASK) classification. Considering galaxy spectra as multidimensional vectors, the majority of the spectral classes are distributed along a well-defined curve going from the earliest to the latest types, suggesting that the optical spectra of normal galaxies can be described in terms of a single affine parameter. Optically bright active galaxies, however, appear as an independent, roughly orthogonal branch that intersects the main sequence exactly at the transition between early and late typesThis work has been funded by projects AYA2007-67965-C03-03, AYA 2007-67965-C03-01, AYA2007-67752-C03-01 and CSD 2006-00070 (Spanish Ministry of Science, Technology and Innovation

The structure of the ICM from High Resolution SPH simulations

We present results from a set of high (512^3 effective resolution), and ultra-high (1024^3) SPH adiabatic cosmological simulations of cluster formation aimed at studying the internal structure of the intracluster medium (ICM). We derive a self-consistent analytical model of the structure of the intracluster medium (ICM). We discuss the radial structure and scaling relations expected from purely gravitational collapse, and show that the choice of a particular halo model can have important consequences on the interpretation of observational data. The validity of the approximations of hydrostatic equilibrium and a polytropic equation of state are checked against results of our simulations. The properties of the ICM are fully specified when a 'universal' profile is assumed for either the dark or the baryonic component. We also show the first results from an unprecedented large-scale simulation of 500 Mpc/h and 2 times 512^3 gas and dark matter particles. This experiment will make possible a detailed study of the large-scale distribution of clusters as a function of their X-ray properties.Comment: 5 pages, 3 figures, to appear in the Proceedings of IAU Colloquium 195: "Outskirts of Galaxy Clusters: intense life in the suburbs", Torino Italy, March 200

Numerical estimation of densities

[Abridged] We present a novel technique, dubbed FiEstAS, to estimate the underlying density field from a discrete set of sample points in an arbitrary multidimensional space. FiEstAS assigns a volume to each point by means of a binary tree. Density is then computed by integrating over an adaptive kernel. As a first test, we construct several Monte Carlo realizations of a Hernquist profile and recover the particle density in both real and phase space. At a given point, Poisson noise causes the unsmoothed estimates to fluctuate by a factor ~2 regardless of the number of particles. This spread can be reduced to about 1 dex (~26 per cent) by our smoothing procedure. [...] We conclude that our algorithm accurately measure the phase-space density up to the limit where discreteness effects render the simulation itself unreliable. Computationally, FiEstAS is orders of magnitude faster than the method based on Delaunay tessellation that Arad et al. employed, making it practicable to recover smoothed density estimates for sets of 10^9 points in 6 dimensions.Comment: 12 pages, 18 figures, submitted to MNRAS. The code is available upon reques