arXiv subjects

Gustavo Yepes

Publications and source records attributed to Gustavo Yepes.

At least 199 records · Page 11Linked to original sources

arXiv2013

The kinematics of the Local Group in a cosmological context

Recent observations constrained the tangential velocity of M31 with respect to the Milky Way (MW) to be v_tan<34.4 km/s and the radial velocity to be in the range v_rad=-109+/- 4.4 km/s (van der Marel et al. 2012). In this study we use a large volume high resolution N-body cosmological simulation (Bolshoi) together with three constrained simulations to statistically study this kinematics in the context of the LCDM. The comparison of the ensembles of simulated pairs with the observed LG at the 1-sigma level in the uncertainties has been done with respect to the radial and tangential velocities, the reduced orbital energy (e_tot), angular momentum (l_orb) and the dimensionless spin parameter, lambda. Our main results are: (i) the preferred radial and tangential velocities for pairs in LCDM are v_rad=-80+/-20 km/s, v_tan=50+/-10 km/s, (ii) pairs around that region are 3 to 13 times more common than pairs within the observational values, (iii) 15%to 24% of LG-like pairs in LCDM have energy and angular momentum consistent with observations while (iv) 9% to 13% of pairs in the same sample show similar values in the inferred dimensionless spin parameter. It follows that within current observational uncertainties the quasi-conserved quantities that characterize the orbit of the LG, i.e. e_tot, r_orb and lambda, do not challenge the standard LCDM model, but the model is in tension with regard to the actual values of the radial and tangential velocities. This might hint to a problem of the LCDM model to reproduce the observed LG.

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arXiv2013

Size matters: the non-universal density profile of subhaloes in SPH simulations and implications for the Milky Way's dSphs

We use dark matter only and full hydrodynamical Constrained Local UniversE Simulations (CLUES) of the formation of the Local Group to study the density profile of subhaloes of the simulated Milky Way and Andromeda galaxies. We show that the Einasto model provides the best description of the subhaloes' density profile, as opposed to the more commonly used NFW profile or any generalisation of it. We further find that the Einasto shape parameter \nEin\ is strongly correlated with the total subhalo mass, pointing towards the notion of a non-universality of the subhaloes' density profile. We observe that the effect of mass loss due to tidal stripping, in both the dark matter only and the hydrodynamical run, is the reduction of the shape parameter \nEin\ between the infall and the present time. Assuming now that the dSphs of our Galaxy follow the Einasto profile and using the maximum and minimum values of \nEin\ from our hydrodynamical simulation as a gauge, we can improve the observational constraints on the \Rmax-\Vmax\ pairs obtained for the brightest satellite galaxies of the Milky Way. When considering only the subhaloes with $-13.2\lesssim M_V\lesssim-8.8$, i.e. the range of luminosity of the classical dwarfs, we find that all our simulated objects are consistent with the observed dSphs if their haloes follow the Einasto model with $1.6\lesssim n_{\rm E} \lesssim5.3$. The numerically motivated Einasto profile for the observed dSphs will alleviate the recently presented "massive failures" problem.

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arXiv2012

The MUSIC of Galaxy Clusters I: Baryon properties and Scaling Relations of the thermal Sunyaev-Zel'dovich Effect

We introduce the Marenostrum-MultiDark SImulations of galaxy Clusters (MUSIC) Dataset, one of the largest sample of hydrodynamically simulated galaxy clusters with more than 500 clusters and 2000 groups. The objects have been selected from two large N-body simulations and have been resimulated at high resolution using SPH together with relevant physical processes (cooling, UV photoionization, star formation and different feedback processes). We focus on the analysis of the baryon content (gas and star) of clusters in the MUSIC dataset both as a function of aperture radius and redshift. The results from our simulations are compared with the most recent observational estimates of the gas fraction in galaxy clusters at different overdensity radii. When the effects of cooling and stellar feedbacks are included, the MUSIC clusters show a good agreement with the most recent observed gas fractions quoted in the literature. A clear dependence of the gas fractions with the total cluster mass is also evident. The impact of the aperture radius choice, when comparing integrated quantities at different redshifts, is tested: the standard definition of radius at a fixed overdensity with respect to critical density is compared with a definition based on the redshift dependent overdensity with respect to background density. We also present a detailed analysis of the scaling relations of the thermal SZ (Sunyaev Zel'dovich) Effect derived from MUSIC clusters. The integrated SZ brightness, Y, is related to the cluster total mass, M, as well as, the M-Y counterpart, more suitable for observational applications. Both laws are consistent with predictions from the self-similar model, showing a very low scatter. The effects of the gas fraction on the Y-M scaling and the presence of a possible redshift dependence on the Y-M scaling relation are also explored.

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arXiv2012

Dwarf Galaxies and the Cosmic Web

We use a cosmological simulation of the formation of the Local Group of Galaxies to identify a mechanism that enables the removal of baryons from low-mass halos without appealing to feedback or reionization. As the Local Group forms, matter bound to it develops a network of filaments and pancakes. This moving web of gas and dark matter drifts and sweeps a large volume, overtaking many halos in the process. The dark matter content of these halos is unaffected but their gas can be efficiently removed by ram-pressure. The loss of gas is especially pronounced in low-mass halos due to their lower binding energy and has a dramatic effect on the star formation history of affected systems. This "cosmic web stripping" may help to explain the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in ΛCDM and the large diversity of star formation histories and morphologies characteristic of faint galaxies. Although our results are based on a single high-resolution simulation, it is likely that the hydrodynamical interaction of dwarf galaxies with the cosmic web is a crucial ingredient so far missing from galaxy formation models.

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arXiv2012

Towards a panchromatic picture of galaxy evolution during the reionization epoch

There are thousands of confirmed detections of star forming galaxies at high redshift (z > 4). These observations rely primarily on the detection of the spectral Lyman Break and the Lyman-alpha emission line. Theoretical modelling of these sources helps to interpret the observations in the framework of the standard cosmological paradigm. We present results from the High-z MareNostrum Project, aimed at constructing a panchromatic picture of the high redshift galaxy evolution that will improve our understanding of young star forming galaxies. Our simulation successfully reproduces the observational constraints from Lyman Break Galaxies and Lyman-alpha emitters at 5 < z < 7 . Based on this model we make predictions on the expected Far Infrared (FIR) emission that should be observed for LAEs. These predictions will help to settle down the question on the dust content of massive high-z galaxies, an issue that will be feasible to probe observationally with the Atacama Large Millimetre Array (ALMA).

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arXiv2012

A kinematic classification of the cosmic web

A new approach for the classification of the cosmic web is presented. In extension of the previous work of Hahn et al. (2007) and Forero-Romero et al. (2009) the new algorithm is based on the analysis of the velocity shear tensor rather than the gravitational tidal tensor. The procedure consists of the construction of the the shear tensor at each (grid) point in space and the evaluation of its three eigenvectors. A given point is classified to be either a void, sheet, filament or a knot according to the number of eigenvalues above a certain threshold, 0, 1, 2, or 3 respectively. The threshold is treated as a free parameter that defines the web. The algorithm has been applied to a dark matter only, high resolution simulation of a box of side-length 64$h^{-1}$Mpc and N = $1024^3$ particles with the framework of the WMAP5/LCDM model. The resulting velocity based cosmic web resolves structures down to <0.1$h^{-1}$Mpc scales, as opposed to the ~1$h^{-1}$Mpc scale of the tidal based web. The under-dense regions are made of extended voids bisected by planar sheets, whose density is also below the mean. The over-dense regions are vastly dominated by the linear filaments and knots. The resolution achieved by the velocity based cosmic web provides a platform for studying the formation of halos and galaxies within the framework of the cosmic web.

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arXiv2012

Applying scale-free mass estimators to the Local Group in Constrained Local Universe Simulations

We use the recently proposed scale-free mass estimators to determine the masses of the Milky Way (MW) and Andromeda (M31) galaxy in a dark matter only Constrained Local UniversE Simulation (CLUES). While these mass estimators work rather well for isolated spherical host systems, we examine here their applicability to a simulated binary system with a unique satellite population similar to the observed satellites of MW and M31. We confirm that the scale-free estimators work also very well in our simulated Local Group galaxies with the right number of satellites which follow the observed radial distribution. In the isotropic case and under the assumption that the satellites are tracking the total gravitating mass, the power-law index of the radial satellite distribution $N(<r)\propto r^{3-γ}$ is directly related to the host's mass profile $M(<r)\propto r^{1-α}$ as $α=γ-2$. The use of this relation for any given $γ$ leads to highly accurate mass estimations which is a crucial point for observer, since they do not know a priori the mass profile of the MW and M31 haloes. We discuss possible bias in the mass estimators and conclude that the scale-free mass estimators can be satisfactorily applied to the real MW and M31 system.

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arXiv2012

The cosmic web and the orientation of angular momenta

We use a 64$h^{-1}$Mpc dark matter (DM) only cosmological simulation to examine the large scale orientation of haloes and substructures with respect the cosmic web. A web classification scheme based on the velocity shear tensor is used to assign to each halo in the simulation a web type: knot, filament, sheet or void. Using $\sim10^6$ haloes that span ~3 orders of magnitude in mass the orientation of the halo's spin and the orbital angular momentum of subhaloes with respect to the eigenvectors of the shear tensor is examined. We find that the orbital angular momentum of subhaloes tends to align with the intermediate eigenvector of the velocity shear tensor for all haloes in knots, filaments and sheets. This result indicates that the kinematics of substructures located deep within the virialized regions of a halo is determined by its infall which in turn is determined by the large scale velocity shear, a surprising result given the virilaized nature of haloes. The non-random nature of subhalo accretion is thus imprinted on the angular momentum measured at z = 0. We also find that haloes' spin axis is aligned with the third eigenvector of the velocity shear tensor in filaments and sheets: the halo spin axis points along filaments and lies in the plane of cosmic sheets.

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arXiv2011

Baryon and Sunyaev-Zel'dovich effect properties of MareNostrum and MultiDark simulated clusters (MUSIC)

We report the first results of the MUSIC project. It consists of two data sets of resimulates clusters extracted from two large dark matter only simulations: Marenostrum Universe and Multidark. In total, the MUSIC contains more than 400 clusters resimulated with high resolution both with radiative and non-radiative physics included. Here we present the first results on the properties of the baryon content and the Sunyaev Zeldovich scaling relations.

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arXiv2011

Halo Contraction Effect in Hydrodynamic Simulations of Galaxy Formation

The condensation of gas and stars in the inner regions of dark matter halos leads to a more concentrated dark matter distribution. While this effect is based on simple gravitational physics, the question of its validity in hierarchical galaxy formation has led to an active debate in the literature. We use a collection of several state-of-the-art cosmological hydrodynamic simulations to study the halo contraction effect in systems ranging from dwarf galaxies to clusters of galaxies, at high and low redshift. The simulations are run by different groups with different codes and include hierarchical merging, gas cooling, star formation, and stellar feedback. We show that in all our cases the inner dark matter density increases relative to the matching simulation without baryon dissipation, at least by a factor of several. The strength of the contraction effect varies from system to system and cannot be reduced to a simple prescription. We present a revised analytical model that describes the contracted mass profile to an rms accuracy of about 10%. The model can be used to effectively bracket the response of the dark matter halo to baryon dissipation. The halo contraction effect is real and must be included in modeling of the mass distribution of galaxies and galaxy clusters.

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arXiv2011

Too small to succeed? Lighting up massive dark matter subhaloes of the Milky Way

Using Constrained Local UniversE Simulations (CLUES) of the formation of the Local Group in a cosmological context we investigate the recently highlighted problem that the majority of the most massive dark subhaloes of the Milky Way are too dense to host any of its bright satellites. In particular, we examine the influence of baryonic processes and find that they leave a twofold effect on the relation between the peak of the rotation curve and its position (Vmax and Rmax). Satellites with a large baryon fraction experience adiabatic contraction thus decreasing Rmax while leaving Vmax more or less unchanged. Subhaloes with smaller baryon fractions undergo a decrease in Vmax possibly due to outflows of material. Furthermore, the situation of finding subhaloes in simulations that lie outside the confidence interval for possible hosts of the bright MW dwarf spheroidals, appears to be far more prominent in cosmologies with a high sigma8 normalisation and depends on the mass of the host. We conclude that the problem cannot be simply solved by including baryonic processes and hence demands further investigations.

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arXiv2011

Disentangling the dark matter halo from the stellar halo

The outer haloes of the Milky Way and Andromeda galaxies contain as much important information on their assembly and formation history as the properties of the discs resident in their centres. In this paper we have used the Constrained Local UniversE Simulation project to disentangle the stellar and DM component of three galaxies that resemble the MW, M31 and M33 using both DM only and DM + gas-dynamical simulations. Stars that are accreted in substructures and then stripped follow a completely different radial distribution than the stripped DM: the stellar halo is much more centrally concentrated than DM. In order to understand how the same physical process can lead to different z=0 radial profiles, we examined the potential at accretion of each stripped particle. We found that star particles sit at systematically higher potentials than DM, making them harder to strip. We then searched for a threshold in the potential of accreted particles ϕ_[th], above which DM particles behave as star particles. We found such a threshold at >16 ϕ_{subhalo}, where ϕ_{subhalo} is the potential at a subhaloes edge at the time of accretion. Thus a rule as simple as selecting particles according to their potential at accretion is able to reproduce the effect that the complicated physics of star formation has on the stellar distribution. This niversal result reproduces the stellar halo to an accuracy of within ~2%. Studies which make use of DM particles as a proxy for stars will undoubtedly miscalculate their proper radial distribution and structure unless particles are selected according to their potential at accretion. Furthermore, we have examined the time it takes to strip a given star or DM particle after accretion. We find that, owing to their higher binding energies, stars take longer to be stripped than DM. The stripped DM halo is thus considerably older than the stripped stellar halo.

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arXiv2011

Renegade Subhaloes in the Local Group

Using a dark matter only Constrained Local UniversE Simulation (CLUES) we examine the existence of subhaloes that change their affiliation from one of the two prominent hosts in the Local Group (i.e. the Milky Way and the Andromeda galaxy) to the other, and call these objects "renegade subhaloes". In light of recent claims that the two Magellanic Clouds (MCs) may have originated from another region (or even the outskirts) of the Local Group or that they have been spawned by a major merger in the past of the Andromeda galaxy, we investigate the nature of such events. However, we cannot confirm that renegade subhaloes enter as deep into the potential well of their present host nor that they share the most simplest properties with the MCs, namely mass and relative velocity. Our simulation rather suggests that these renegade subhaloes appear to be flying past one host before being pulled into the other. A merger is not required to trigger such an event, it is rather the distinct environment of our simulated Local Group facilitating such behavior. Since just a small fraction of the full z=0 subhalo population are renegades, our study indicates that it will be intrinsically difficult to distinguish them despite clear differences in their velocity, radial distribution, shape and spin parameter distributions.

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arXiv2011

Large scale environmental bias of the QSO line of sight proximity effect

We analyse the proximity zone of the intergalactic matter around high-redshift quasars in a cosmological environment. In a box of 64 h-1 Mpc base length we employ dark matter only simulations. For estimating the hydrogen temperature and density distribution we use the effective equation of state. Hydrogen is assumed to be in photoionisation equilibrium with a model background flux which is fit to recent observations of the mean optical depth and transmission flux statistics. At redshifts z = 3, 4, and 4.8, we select model quasar positions at the centre of the 20 most massive halos and 100 less massive halos identified in the simulation. From each assumed quasar position we cast 100 random lines of sight for two box length including the changes in the ionisation fractions by the QSO flux field and derive mock Lyα spectra. The proximity effect describes the dependence of the mean normalised optical depth ξ = τeff, QSO/τeff, Lyα as a function of the ratio of the ionisation rate by the QSO and the background field, ω = ΓQSO/ΓUVB, i.e. the profile ξ = (1 + ω/a)-0.5, where a strength parameter a is introduced. The strength parameter measures the deviation from the theoretical background model and is used to quantify any influence of the environmental density field. We reproduce an unbiased measurement of the proximity effect which is not affected by the host halo mass. The scatter between different lines of sight and different quasar host positions increases with decreasing redshift. Around the host halos, we find only a slight average overdensity in the proximity zone at comoving radii of 1 < rc < 10h-1 Mpc. However, a clear power-law correlation of the strength parameter with the average overdensity in rc is found, showing an overestimation of the ionising background in overdense regions and an underestimation in underdense regions.

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arXiv2011

Haloes gone MAD: The Halo-Finder Comparison Project

[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).

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arXiv2010

The luminosities of backsplash galaxies in constrained simulations of the Local Group

We study the differences and similarities in the luminosities of bound, infalling and the so-called backsplash (Gill et al. 2005) galaxies of the Milky Way and M31 using a hydrodynamical simulation performed within the Constrained Local UniversE Simulation (CLUES) project. The simulation models the formation of the Local Group within a self-consistent cosmological framework. We find that even though backsplash galaxies passed through the virial radius of their host halo and hence may have lost a (significant) fraction of their mass, their stellar populations are hardly affected. This leaves us with comparable luminosity functions for infalling and backsplash galaxies and hence little hope to decipher their past (and different) formation and evolutionary histories by luminosity measurements alone. Nevertheless, due to the tidal stripping of dark matter we find that the mass-to-light ratios have changed when comparing the various populations against each other: they are highest for the infalling galaxies and lowest for the bound satellites with the backsplash galaxies in-between.

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arXiv2010

The preferred direction of infalling satellite galaxies in the Local Group

Using a high resolution DM simulation of the Local Group (LG), conducted within the framework of the Constrained Local UniversE Simulation (CLUES) project, we investigate the nature of how satellites of the MW and M31 are accreted. Satellites of these galaxies are accreted anisotropically, entering the virial radius of their hosts from specific "spots" with respect to the large scale structure. Furthermore, material which is tidally stripped from these satellites is also, at z=0, distributed anisotropically and is characterized by an ellipsoidal sub-volume embedded in the halo. The angular pattern created by the locus of satellite infall points and the projected stripped DM is investigated within a coordinate system determined by the location of the LG companion and the simulated Virgo cluster across concentric shells ranging from 0.1 to 5 r_vir. Remarkably, the principal axis of the ellipsoidal sub-volume shows a coherent alignment extending from well within the halo to a few r_vir. A spherical harmonics transform applied to the angular distributions confirms the visual impression: namely, the angular distributions of both the satellites entry points and stripped DM is dominated by the l=2 quadrupole term, whose major principal axis is aligned across the shells considered. It follows that the structure of the outer (r>0.5 r_vir) regions of the main halos is closely related to the cosmic web within which it is embedded. Given the hypothesis that a large fraction of the stellar halo of the Milky Way has been accreted from satellites, the present results can be applied to the stellar halo of the MW and M31. We predict that the remnants of tidally stripped satellites should be embedded in streams of material composed of dark matter and stars. The present results can therefore shed light on the existence of satellites embedded within larger streams of matter, such as the Segue 2 satellite.

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arXiv2010

Reionization of the Local Group of Galaxies

We present the first detailed structure formation and radiative transfer simulations of the reionization history of our cosmic neighbourhood. To this end, we follow the formation of the Local Group of galaxies and nearby clusters by means of constrained simulations, which use the available observational constraints to construct a representation of those structures which reproduces their actual positions and properties at the present time. We find that the reionization history of the Local Group is strongly dependent on the assumed photon production efficiencies of the ionizing sources, which are still poorly constrained. If sources are relatively efficient, i.e. the process is 'photon-rich', the Local Group is primarily ionized externally by the nearby clusters. Alternatively, if the sources are inefficient, i.e. reionization is 'photon-poor' the Local Group evolves largely isolated and reionizes itself. The mode of reionization, external vs. internal, has important implications for the evolution of our neighbourhood, in terms of e.g. its satellite galaxy populations and primordial stellar populations. This therefore provides an important avenue for understanding the young universe by detailed studies of our nearby structures.

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