41 research outputs found
The orientation of galaxy dark matter haloes around cosmic voids
Using the Millennium N-body Simulation we explore how the shape and angular momentum of galaxy dark matter haloes surrounding the largest cosmological voids are oriented. We find that the major and intermediate axes of the haloes tend to lie parallel to the surface of the voids, whereas the minor axis points preferentially in the radial direction. We have quantified the strength of these alignments at different radial distances from the void centres. The effect of these orientations is still detected at distances as large as 2.2 Rvoid from the void centre. Taking a subsample of haloes expected to contain disc-dominated galaxies at their centres we detect, at the 99.9 per cent confidence level, a signal that the angular momentum of those haloes tends to lie parallel to the surface of the voids. Contrary to the alignments of the inertia axes, this signal is only detected in shells at the void surface (1 < R < 1.07 Rvoid) and disappears at larger distances. This signal, together with the similar alignment observed using real spiral galaxies, strongly supports the prediction of the Tidal Torque theory that both dark matter haloes and baryonic matter have acquired, conjointly, their angular momentum before the moment of turnaround
The influence of environment on galaxy formation
The dynamical evolution of the matter content of the universe is modelled throughout this study as that of self and mutually gravitating Lagrangian fluids in the so called ΛCDM-Concordance cosmological framework which leads to the Hierarchical Clustering paradigm for the formation of cosmic structures. As a numerical tool for investigating galaxy formation scenarios in this context, we employed GADGET2 (see Springel 2005) and the more recent GADGET3 (see Springel et al. 2008): we describe the numerical solvers implemented in the code and test their behaviour in both gravitational and hydrodynamical setups of relevance for cosmological calculations (Tasker et al. 2008). Using the outputs of the MILLENNIUM simulation and the relative Semi Analytical galaxy catalogues produced by Croton et al. 2006, we developed an algorithm aimed at the identification of large spherical underdense regions in the simulated Large Scale Structure (LSS), at z = 0. Focusing on this peculiar environment, we found a confirmation in numerical simulations for the observations by Trujillo, Carretero & Patiri (2006). The Tidal Torque Theory can predict the spatial distribution of the orientation of both the angular momentum vector of Milky Way size galaxies located on the surface of large spherical voids, and of their host DM halos. We re–simulated the 5 GIMIC regions (Crain et al. 2009) following the gravitational evolution of the CDM component only. We then applied a Semi Analytical Model (SAM) of galaxy formation (De Lucia & Blaizot 2007) obtaining the galaxy catalogues and merger histories for the 5 different volumes simulated. It is not yet well understood if and how the LSS environment can influence the Star Formation (SF) histories of galaxies. Starting from the stellar mass content of semi–analytical galaxies at z = 0, we defined characteristic epochs for their build up and, as a preliminary study, investigated how these distribute as a function of different LSS environments
The influence of environment on galaxy formation
The dynamical evolution of the matter content of the universe is modelled throughout this study as that of self and mutually gravitating Lagrangian fluids in the so called ΛCDM-Concordance cosmological framework which leads to the Hierarchical Clustering paradigm for the formation of cosmic structures. As a numerical tool for investigating galaxy formation scenarios in this context, we employed GADGET2 (see Springel 2005) and the more recent GADGET3 (see Springel et al. 2008): we describe the numerical solvers implemented in the code and test their behaviour in both gravitational and hydrodynamical setups of relevance for cosmological calculations (Tasker et al. 2008). Using the outputs of the MILLENNIUM simulation and the relative Semi Analytical galaxy catalogues produced by Croton et al. 2006, we developed an algorithm aimed at the identification of large spherical underdense regions in the simulated Large Scale Structure (LSS), at z = 0. Focusing on this peculiar environment, we found a confirmation in numerical simulations for the observations by Trujillo, Carretero & Patiri (2006). The Tidal Torque Theory can predict the spatial distribution of the orientation of both the angular momentum vector of Milky Way size galaxies located on the surface of large spherical voids, and of their host DM halos. We re–simulated the 5 GIMIC regions (Crain et al. 2009) following the gravitational evolution of the CDM component only. We then applied a Semi Analytical Model (SAM) of galaxy formation (De Lucia & Blaizot 2007) obtaining the galaxy catalogues and merger histories for the 5 different volumes simulated. It is not yet well understood if and how the LSS environment can influence the Star Formation (SF) histories of galaxies. Starting from the stellar mass content of semi–analytical galaxies at z = 0, we defined characteristic epochs for their build up and, as a preliminary study, investigated how these distribute as a function of different LSS environments
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
Hydrodynamic simulations with the Godunov SPH
We present results based on an implementation of the Godunov Smoothed
Particle Hydrodynamics (GSPH), originally developed by Inutsuka (2002), in the
GADGET-3 hydrodynamic code. We first review the derivation of the GSPH
discretization of the equations of moment and energy conservation, starting
from the convolution of these equations with the interpolating kernel. The two
most important aspects of the numerical implementation of these equations are
(a) the appearance of fluid velocity and pressure obtained from the solution of
the Riemann problem between each pair of particles, and (b the absence of an
artificial viscosity term. We carry out three different controlled
hydrodynamical three-dimensional tests, namely the Sod shock tube, the
development of Kelvin-Helmholtz instabilities in a shear flow test, and the
"blob" test describing the evolution of a cold cloud moving against a hot wind.
The results of our tests confirm and extend in a number of aspects those
recently obtained by Cha (2010): (i) GSPH provides a much improved description
of contact discontinuities, with respect to SPH, thus avoiding the appearance
of spurious pressure forces; (ii) GSPH is able to follow the development of
gas-dynamical instabilities, such as the Kevin--Helmholtz and the
Rayleigh-Taylor ones; (iii) as a result, GSPH describes the development of curl
structures in the shear-flow test and the dissolution of the cold cloud in the
"blob" test.
We also discuss in detail the effect on the performances of GSPH of changing
different aspects of its implementation. The results of our tests demonstrate
that GSPH is in fact a highly promising hydrodynamic scheme, also to be coupled
to an N-body solver, for astrophysical and cosmological applications.
[abridged]Comment: 19 pages, 13 figures, MNRAS accepted, high resolution version can be
obtained at
http://adlibitum.oats.inaf.it/borgani/html/papers/gsph_hydrosim.pd
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.Comment: 11 pages, 6 figures, submitted to MNRA
Massive and refined: a sample of large galaxy clusters simulated at high resolution. I:Thermal gas and shock waves properties
We present a sample of 20 massive galaxy clusters with total virial masses in
the range of 6 10^14 M_sol<M(vir)< 2 10^15M_sol, re-simulated with a customized
version of the 1.5. ENZO code employing Adaptive Mesh Refinement. This
technique allowed us to obtain unprecedented high spatial resolution (25kpc/h)
up to the distance of 3 virial radii from the clusters center, and makes it
possible to focus with the same level of detail on the physical properties of
the innermost and of the outermost cluster regions, providing new clues on the
role of shock waves and turbulent motions in the ICM, across a wide range of
scales.
In this paper, a first exploratory study of this data set is presented. We
report on the thermal properties of galaxy clusters at z=0. Integrated and
morphological properties of gas density, gas temperature, gas entropy and
baryon fraction distributions are discussed, and compared with existing
outcomes both from the observational and from the numerical literature.
Our cluster sample shows an overall good consistency with the results
obtained adopting other numerical techniques (e.g. Smoothed Particles
Hydrodynamics), yet it provides a more accurate representation of the accretion
patterns far outside the cluster cores. We also reconstruct the properties of
shock waves within the sample by means of a velocity-based approach, and we
study Mach numbers and energy distributions for the various dynamical states in
clusters, giving estimates for the injection of Cosmic Rays particles at
shocks. The present sample is rather unique in the panorama of cosmological
simulations of massive galaxy clusters, due to its dynamical range, statistics
of objects and number of time outputs. For this reason, we deploy a public
repository of the available data, accessible via web portal at
http://data.cineca.it.Comment: 26 pages, 20 figures, New Astronomy accepted. Reference list updated.
Higher quality versions of the paper can be found at:
http://www.ira.inaf.it/~vazza/papers A public archive of galaxy clusters data
is accessible at http://data.cineca.it
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
Fossil evidence for spin alignment of SDSS galaxies in filaments
We search for and find fossil evidence that the distribution of the spin axes
of galaxies in cosmic web filaments relative to their host filaments are not
randomly distributed. This would indicate that the action of large scale tidal
torques effected the alignments of galaxies located in cosmic filaments. To
this end, we constructed a catalogue of clean filaments containing edge-on
galaxies. We started by applying the Multiscale Morphology Filter (MMF)
technique to the galaxies in a redshift-distortion corrected version of the
Sloan Digital Sky Survey DR5. From that sample we extracted those 426 filaments
that contained edge-on galaxies (b/a < 0.2). These filaments were then visually
classified relative to a variety of quality criteria. Statistical analysis
using "feature measures" indicates that the distribution of orientations of
these edge-on galaxies relative to their parent filament deviate significantly
from what would be expected on the basis of a random distribution of
orientations. The interpretation of this result may not be immediately
apparent, but it is easy to identify a population of 14 objects whose spin axes
are aligned perpendicular to the spine of the parent filament (\cos \theta <
0.2). The candidate objects are found in relatively less dense filaments. This
might be expected since galaxies in such locations suffer less interaction with
surrounding galaxies, and consequently better preserve their tidally induced
orientations relative to the parent filament. The technique of searching for
fossil evidence of alignment yields relatively few candidate objects, but it
does not suffer from the dilution effects inherent in correlation analysis of
large samples.Comment: 20 pages, 19 figures, slightly revised and upgraded version, accepted
for publication by MNRAS. For high-res version see
http://www.astro.rug.nl/~weygaert/SpinAlignJones.rev.pd
The Aspen-Amsterdam void finder comparison project
Despite a history that dates back at least a quarter of a century, studies of voids in the large-scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void-finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of 13 different void finders constructed using particles, haloes, and semi-analytical model galaxies extracted from a subvolume of the Millennium simulation. This study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 per cent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than mB=−20 is found to be smaller than about −0.8 by all our void finding algorithm