arXiv subjects

G. Yepes

Publications and source records attributed to G. Yepes.

At least 73 records · Page 4Linked to original sources

arXiv2011

CLARA's view on the escape fraction of Lyman-Alpha photons in high redshift galaxies

Using CLARA (Code for Lyman Alpha Radiation Analysis) we constrain the escape fraction of Lyman-Alpha radiation in galaxies in the redshift range 5 ~5. The LAE Luminosity Function (LF) based on the extended clumpy model reproduces broadly the bright end of the LF derived from observations at z \sim 5 and z \sim 6. At z \sim 7 our model over-predicts the LF by roughly a factor of four, presumably because the effects of the neutral intergalactic medium are not taken into account. The remaining tension between the observed and simulated faint end of the LF, both in the UV-continuum and Lyman-Alpha at redshifts z \sim 5 and z \sim 6 points towards an overabundance of simulated LAEs hosted in haloes of masses 1.0x10^10h-1Msol < Mh < 4.0x10^10h-1Msol. Given the difficulties in explaining the observed overabundance by dust absorption, a probable origin of the mismatch are the high star formation rates in the simulated haloes around the quoted mass range. A more efficient supernova feedback should be able to regulate the star formation process in the shallow potential wells of these haloes.

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arXiv2011

Comparison of an X-ray selected sample of massive lensing clusters with the MareNostrum Universe LCDM simulation

A long-standing problem of strong lensing by galaxy clusters regards the observed high rate of giant gravitational arcs as compared to the predictions in the framework of the "standard" cosmological model. Recently, few other inconsistencies between theoretical expectations and observations have been claimed which regard the large size of the Einstein rings and the high concentrations of few clusters with strong lensing features. All of these problems consistently indicate that observed galaxy clusters may be gravitational lenses stronger than expected. We use clusters extracted from the MareNostrum Universe to build up mock catalogs of galaxy clusters selected through their X-ray flux. We use these objects to estimate the probability distributions of lensing cross sections, Einstein rings, and concentrations for the sample of 12 MACS clusters at $z>0.5$ presented in Ebeling et al. (2007) and discussed in Zitrin et al. (2010). We find that simulated clusters produce $\sim 50%$ less arcs than observed clusters do. The medians of the distributions of the Einstein ring sizes differ by $\sim 25%$ between simulations and observations. We estimate that, due to cluster triaxiality and orientation biases affecting the lenses with the largest cross sections, the concentrations of the individual MACS clusters inferred from the lensing analysis should be up to a factor of $\sim 2$ larger than expected from the $Λ$CDM model. The arc statistics, the Einstein ring, and the concentration problems in strong lensing clusters are mitigated but not solved on the basis of our analysis. Nevertheless, due to the lack of redshifts for most of the multiple image systems used for modeling the MACS clusters, the results of this work will need to be verified with additional data. The upcoming CLASH program will provide an ideal sample for extending our comparison (abridged).

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arXiv2011

Radio relics in the MareNostrum Universe

We identify shocked gas in simulated galaxy clusters extracted from the MareNostrum Universe simulation (Gottloeber et al. 2006) assuming that shock waves are regions of electron acceleration. We perform flux number counts within the framework of the non-thermal emission model developed by Hoeft et al. (2008). Results are presented at two different observing frequencies, i.e. 1.4 GHz and 120 MHz, posing interesting constraints for LOFAR and upcoming radio telescopes.

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arXiv2010

Bullet Clusters in the MareNostrum Universe

We estimate the expected distribution of displacements between the dark matter and gas cores in simulated clusters. We use the MareNostrum Universe, one of the largest non radiative, SPH Lambda CDM cosmological simulations. We find that projected 2-D displacements between dark matter and gas, equal or larger than the observed in the Bullet Cluster, are expected in 1% to 2% of the clusters with masses larger than 10^{14} Msun. The 2-D displacement distribution is roughly the same between redshifts 0<z<0.5 when multiplied by a factor of (1+z)^{-1/2}. We conclude that the separations between dark matter and gas as observed in the bullet cluster can be easily found in a Lambda CDM universe. Furthermore we find that the displacement distribution is not very sensitive to the normalization of the power spectrum. Upcoming surveys could extend the measurements of these displacements between dark matter and gas into large samples of hundreds of clusters, providing a potential test for Lambda CDM.

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arXiv2010

Strong lensing in the MareNostrum Universe II: scaling relations and optical depths

The strong lensing events that are observed in compact clusters of galaxies can, both statistically and individually, return important clues about the structural properties of the most massive structures in the Universe. Substantial work is ongoing in order to understand the degree of similarity between the lensing cluster population and the population of clusters as a whole, with members of the former being likely more massive, compact, and substructured than members of the latter. In this work we exploit synthetic clusters extracted from the {\sc MareNostrum Universe} cosmological simulation in order to estimate the correlation between the strong lensing efficiency and other bulk properties of lensing clusters, such as the virial mass and the bolometric X-ray luminosity. We found that a positive correlation exist between all these quantities, with the substantial scatter being smaller for the luminosity-cross section relation. We additionally used the relation between the lensing efficiency and the virial mass in order to construct a synthetic optical depth that agrees well with the true one, while being extremely faster to be evaluated. We finally estimated what fraction of the total giant arc abundance is recovered when galaxy clusters are selected according to their dynamical activity or their X-ray luminosity. Our results show that there is a high probability for high-redshift strong lensing clusters to be substantially far away from dynamical equilibrium, and that $30-40\%$ of the total amount of giant arcs are lost if looking only at very X-ray luminous objects.

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arXiv2010

The Relation Between Halo Shape, Velocity Dispersion and Formation Time

We use dark matter haloes identified in the MareNostrum Universe and galaxy groups identified in the Sloan Data Release 7 galaxy catalogue, to study the relation between halo shape and halo dynamics, parametrizing out the mass of the systems. A strong shape-dynamics, independent of mass, correlation is present in the simulation data, which we find it to be due to different halo formation times. Early formation time haloes are, at the present epoch, more spherical and have higher velocity dispersions than late forming-time haloes. The halo shape-dynamics correlation, albeit weaker, survives the projection in 2D (ie., among projected shape and 1-D velocity dispersion). A similar shape-dynamics correlation, independent of mass, is also found in the SDSS DR7 groups of galaxies and in order to investigate its cause we have tested and used, as a proxy of the group formation time, a concentration parameter. We have found, as in the case of the simulated haloes, that less concentrated groups, corresponding to late formation times, have lower velocity dispersions and higher elongations than groups with higher values of concentration, corresponding to early formation times.

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arXiv2009

Simulated vs. observed UV emission at high redshift: a hint for a clumpy ISM?

We discuss the rest-frame UV emission between 5< z < 7 from the MareNostrum High-z Universe, a SPH simulation done with more than 2 billion particles. Cosmological simulations of galaxy formation generally overpredict the UV restframe luminosity function at high redshift, both at the bright and faint ends. In this Letter we explore a dust attenuation model where a larger extinction is applied to star populations younger than a given age, mimicking the effect of a clumpy interstellar medium. We show that this scenario fits reasonably well both the UV luminosity functions and the UV-continuum slopes derived from observations. The model assumes a large obscuration for stars younger than 25 Myr from the gas clouds where they should be embedded at their formation time. We find that the optical depth in these clouds should be between 30 and 100 times larger than the mean optical depth for the homogeneous part of the interstellar medium. These values are one order of magnitude larger than those estimated in local galaxies. Therefore, we conclude that LambdaCDM predictions for the high-z UV emission can accommodate the current observations if we consider a dust extinction model based on the assumption of a clumpy environment at high redshift.

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arXiv2009

The Evolution of Central Group Galaxies in Hydrodynamical Simulations

We trace the evolution of central galaxies in three ~10^13 M_sun galaxy groups simulated at high resolution in cosmological hydrodynamical simulations. The evolution in the group potential leads, at z=0, to central galaxies that are massive, gas-poor early-type systems supported by stellar velocity dispersion resembling either elliptical or S0 galaxies. Their z~2-2.5 main progenitors are massive M* ~ 3-10 x 10^10 M_sun, star forming (20-60 M_sun/yr) galaxies which host substantial reservoirs of cold gas (~5 x 10^9 M_sun) in extended gas disks. Our simulations thus show that star forming galaxies observed at z~2 are likely the main progenitors of central galaxies in galaxy groups at z=0. Their central stellar densities stay approximately constant from z~1.5 down to z=0. Instead, the galaxies grow inside-out, by acquiring a stellar envelope outside the innermost ~2 kpc. Consequently the density within the effective radius decreases by up to two orders of magnitude. Both major and minor mergers contribute to most of the mass accreted outside the effective radius and thus drive the evolution of the half-mass radii. In one of the three simulated groups the short central cooling time leads to a dramatic rejuvenation of the central group galaxy at z<1, affecting its morphology, kinematics and colors. This episode is eventually terminated by a group-group merger. Our simulations demonstrate that, in galaxy groups, the interplay between halo mass assembly, galaxy merging and gas accretion has a substantial influence on the star formation histories and z=0 morphologies of central galaxies.[Abridged]

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arXiv2009

The Sunyaev-Zeldovich effect in superclusters of galaxies using gasdynamical simulations: the case of Corona Borealis

[Abridged] We study the thermal and kinetic Sunyaev-Zel'dovich (SZ) effect associated with superclusters of galaxies using the MareNostrum Universe SPH simulation. We consider superclusters similar to the Corona Borealis Supercluster (CrB-SC). This paper is motivated by the detection at 33GHz of a strong temperature decrement in the CMB towards the core of this supercluster. Multifrequency observations with VSA and MITO suggest the existence of a thermal SZ effect component in the spectrum of this cold spot, which would account for roughly 25% of the total observed decrement. We identify nine regions containing superclusters similar to CrB-SC, obtain the associated SZ maps and calculate the probability of finding such SZ signals arising from hot gas within the supercluster. Our results show that WHIM produces a thermal SZ effect much smaller than the observed value. Neither can summing the contribution of small clusters and galaxy groups in the region explain the amplitude of the SZ signal. When we take into account the actual posterior distribution from the observations, the probability that WHIM can cause a thermal SZ signal like the one observed is <1%, rising up to a 3.2% when the contribution of small clusters and galaxy groups is included. If the simulations provide a suitable description of the gas physics, then we conclude that the thermal SZ component of the CrB spot most probably arises from an unknown galaxy cluster along the line of sight. The simulations also show that the kinetic SZ signal associated with the supercluster cannot provide an explanation for the remaining 75% of the observed cold spot in CrB.

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arXiv2009

The sizes of mini-voids in the local universe: an argument in favor of a warm dark matter model?

Using high-resolution simulations within the Cold and Warm Dark Matter models we study the evolution of small scale structure in the Local Volume, a sphere of 8 Mpc radius around the Local Group. We compare the observed spectrum of mini-voids in the Local Volume with the spectrum of mini-voids determined from the simulations. We show that the \LWDM model can easily explain both the observed spectrum of mini-voids and the presence of low-mass galaxies observed in the Local Volume, provided that all haloes with circular velocities greater than 20 km/s host galaxies. On the contrary within the LCDM model the distribution of the simulated mini-voids reflects the observed one if haloes with maximal circular velocities larger than 35 km/s host galaxies. This assumption is in contradiction with observations of galaxies with circular velocities as low as 20 km/s in our Local Universe. A potential problem of the LWDM model could be the late formation of the haloes in which the gas can be efficiently photo-evaporated. Thus star formation is suppressed and low-mass haloes might not host any galaxy at all.

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arXiv2009

The velocity function in the local environment from LCDM and LWDM constrained simulations

Using constrained simulations of the local Universe for generic cold dark matter and for 1keV warm dark matter, we investigate the difference in the abundance of dark matter halos in the local environment. We find that the mass function within 20 Mpc/h of the Local Group is ~2 times larger than the universal mass function in the 10^9-10^13 M_odot/h mass range. Imposing the field of view of the on-going HI blind survey ALFALFA in our simulations, we predict that the velocity function in the Virgo-direction region exceeds the universal velocity function by a factor of 3. Furthermore, employing a scheme to translate the halo velocity function into a galaxy velocity function, we compare the simulation results with a sample of galaxies from the early catalog release of ALFALFA. We find that our simulations are able to reproduce the velocity function in the 80-300 km/s velocity range, having a value ~10 times larger than the universal velocity function in the Virgo-direction region. In the low velocity regime, 35-80 km/s, the warm dark matter simulation reproduces the observed flattening of the velocity function. On the contrary, the simulation with cold dark matter predicts a steep rise in the velocity function towards lower velocities; for V_max=35 km/s, it forecasts ~10 times more sources than the ones observed. If confirmed by the complete ALFALFA survey, our results indicate a potential problem for the cold dark matter paradigm or for the conventional assumptions about energetic feedback in dwarf galaxies.

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arXiv2008

A Dynamical Classification of the Cosmic Web

A dynamical classification of the cosmic web is proposed. The large scale environment is classified into four web types: voids, sheets, filaments and knots. The classification is based on the evaluation of the deformation tensor, i.e. the Hessian of the gravitational potential, on a grid. The classification is based on counting the number of eigenvalues above a certain threshold, lambda_th at each grid point, where the case of zero, one, two or three such eigenvalues corresponds to void, sheet, filament or a knot grid point. The collection of neighboring grid points, friends-of-friends, of the same web attribute constitutes voids, sheets, filaments and knots as web objects. A simple dynamical consideration suggests that lambda_th should be approximately unity, upon an appropriate scaling of the deformation tensor. The algorithm has been applied and tested against a suite of (dark matter only) cosmological N-body simulations. In particular, the dependence of the volume and mass filling fractions on lambda_th and on the resolution has been calculated for the four web types. Also, the percolation properties of voids and filaments have been studied. Our main findings are: (a) Already at lambda_th = 0.1 the resulting web classification reproduces the visual impression of the cosmic web. (b) Between 0.2 < lambda_th < 0.4, a system of percolated voids coexists with a net of interconected filaments. This suggests a reasonable choice for lambda_th as the parameter that defines the cosmic web. (c) The dynamical nature of the suggested classification provides a robust framework for incorporating environmental information into galaxy formation models, and in particular the semi-analytical ones.

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arXiv2008

Diffuse radio emission from clusters in the MareNostrum Universe simulation

Large-scale diffuse radio emission is observed in some clusters of galaxies. There is ample of evidence that the emission has its origin in synchrotron losses of relativistic electrons, accelerated in the course of clusters mergers. In a cosmological simulation we locate the structure formation shocks and estimate their radio emission. We proceed as follows: Introducing a novel approach to identify strong shock fronts in an SPH simulation, we determine the Mach number as well as the downstream density and temperature in the MareNostrum Universe simulation which has 2x1024^3 particles in a 500 Mpc/h box and was carried out with non-radiative physics. Then, we estimate the radio emission using the formalism derived in Hoeft & Brueggen (2007) and produce artificial radio maps of massive clusters. Several of our clusters show radio objects with similar morphology to large-scale radio relics found in the sky, whereas about half of the clusters show only very little radio emission. In agreement with observational findings, the maximum diffuse radio emission of our clusters depends strongly on their X-ray temperature. We find that the so-called accretion shocks cause only very little radio emission. We conclude that a moderate efficiency of shock acceleration, namely xi_e <= 0.005, and moderate magnetic fields in the region of the relics, namely 0.07 to 0.8 muGauss are sufficient to reproduce the number density and luminosity of radio relics.

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arXiv2008

Comparison of clustering properties of observed objects and dark matter halos on different mass and spatial scales

We investigate the large-scale distribution of galaxy clusters taken from several X-ray catalogs. Different statistics of clustering like the conditional correlation function (CCF) and the minimal spanning tree (MST) as well as void statistics were used. Clusters show two distinct regimes of clustering: 1) on scales of superclusters (~40/h Mpc) the CCF is represented by a power law; 2) on larger scales a gradual transition to homogeneity (~100/h Mpc) is observed. We also present the correlation analysis of the galaxy distribution taken from DR6 SDSS main galaxy database. In case of galaxies the limiting scales of the different clustering regimes are 1)10-15/h Mpc; 2) 40-50/h Mpc. The differences in the characteristic scales and scaling exponents of the cluster and galaxy distribution can be naturally explained within the theory of biased structure formation. We compared the density contrasts of inhomogeneities in the cluster and galaxy distributions in the SDSS region. The estimation of the relative cluster-galaxy bias gives the value b = 5 +/- 2. The distribution of real clusters is compared to that of simulated (model) clusters (the MareNostrum Universe simulations). We selected a cluster sample from 500/h Mpc simulation box with WMAP3 cosmological parameters and sigma_8 = 0.8. We found a general agreement between the distribution of observed and simulated clusters. The differences are mainly due to the presences of the Shapley supercluster in the observed sample. On the basis of SDSS galaxy sample we study properties of the power law behavior showed by the CCF on small scales. We show that this phenomenon is quite complex, with significant scatter in scaling properties, and characterized by a non-trivial dependence on galaxy properties and environment.

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arXiv2007

Spin alignment of dark matter haloes in the shells of the largest voids

Using the high resolution cosmological N-body simulation MareNostrum Universe we study the orientation of shape and angular momentum of galaxy-size dark matter haloes around large voids. We find that haloes located on the shells of the largest cosmic voids have angular momenta that tend to be preferentially perpendicular to the direction that joins the centre of the halo and the centre of the void. This alignment has been found in spiral galaxies around voids using galaxy redshift surveys. We measure for the first time the strength of this alignment, showing how it falls off with increasing distance to the centre of the void. We also confirm the correlation between the intensity of this alignment and the halo mass. The analysis of the orientation of the halo main axes confirms the results of previous works. Moreover, we find a similar alignment for the baryonic matter inside dark matter haloes, which is much stronger in their inner parts.

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arXiv2007

Entropy of gas and dark matter in galaxy clusters

On the basis of a large scale 'adiabatic', namely non-radiative and non-dissipative, cosmological smooth particle hydrodynamic simulation we compare the entropy profiles of the gas and the dark matter (DM) in galaxy clusters. The quantity K_g = T_g ρ_g^{-2/3} provides a measure for the entropy of the intra-cluster gas. By analogy with the thermodynamic variables of the gas the velocity dispersion of the DM is associated with a formal temperature and thereby K_DM = σ_DM^2 ρ_DM^{-2/3} is defined. This DM entropy is related to the DM phase space density by K_DM \propto Q_DM^{-2/3}. In accord with other studies the DM phase space density follows a power law behaviour, Q_DM \propto r^{-1.82}, which corresponds to K_DM \propto r^{1.21}. The simulated intra-cluster gas has a flat entropy core within (0.8 \pm 0.4) R_s, where R_s is the NFW scale radius. The outer profile follows the DM behaviour, K_g \propto r^{1.21}, in close agreement with X-ray observations. Upon scaling the DM and gas densities by their mean cosmological values we find that outside the entropy core a constant ratio of K_g / K_{DM} = 0.71 \pm 0.18 prevails. By extending the definition of the gas temperature to include also the bulk kinetic energy the ratio of the DM and gas extended entropy is found to be unity for r > 0.8 R_s. The constant ratio of the gas thermal entropy to that of the DM implies that observations of the intra-cluster gas can provide an almost direct probe of the DM.

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arXiv2006

Dark and baryonic matter in the MareNostrum Universe

We report some results from one of the largest hydrodynamical cosmological simulations of large scale structures that has been done up to date. The MareNostrum Universe SPH simulation consists of 2 billion particles (2 times 1024^3) in a cubic box of 500 h^-1 Mpc on a side. This simulation has been done in the MareNostrum parallel supercomputer at the Barcelona SuperComputer Center. Due to the large simulated volume and good mass resolution, our simulated catalog of dark matter halos comprises more than half a million objects with masses larger than a typical Milky Way galaxy halo. From this dataset we have studied several statistical properties such as the evolution of the halo mass function, the void distribution, the shapes of dark and gas halos and the large scale distribution of baryons.

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arXiv2006

The MareNostrum Universe

The MareNostrum Universe is one of the biggest SPH cosmological simulations done so far. It contains more than 2 billion particles (2 times 1024^3) in a 500 Mpc/h cubic volume. This simulation has been performed on the MareNostrum supercomputer at the Barcelona Supercomputer Center. We have obtained more than 0.5 million halos with masses greater than a typical Milky Way galaxy halo. We report results about the halo mass function, the shapes of dark matter and gas distributions in halos, the baryonic fraction in galaxy clusters and groups, baryon oscillations in the dark matter and the halo power spectra as well as the distribution and evolution of the gas fraction at large scales.

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