10 research outputs found
Shape, spin and baryon fraction of clusters in the MareNostrum Universe
The MareNostrum Universe is one of the largest cosmological
SPH simulation done so far. It consists of dark and
gas particles in a box of 500 Mpc on a side. Here we study
the shapes and spins of the dark matter and gas components of the 10,000 most
massive objects extracted from the simulation as well as the gas fraction in
those objects. We find that the shapes of objects tend to be prolate both in
the dark matter and gas. There is a clear dependence of shape on halo mass, the
more massive ones being less spherical than the less massive objects. The gas
distribution is nevertheless much more spherical than the dark matter, although
the triaxiality parameters of gas and dark matter differ only by a few percent
and it increases with cluster mass. The spin parameters of gas and dark matter
can be well fitted by a lognormal distribution function. On average, the spin
of gas is 1.4 larger than the spin of dark matter. We find a similar behavior
for the spins at higher redshifts, with a slightly decrease of the spin ratios
to 1.16 at The cosmic normalized baryon fraction in the entire cluster
sample ranges from , at to at . At both
redshifts we find a slightly, but statistically significant decrease of
with cluster mass.Comment: 7 pages, 6 figures. Accepted for publication in The Astrophysical
Journa
The dynamical structure of dark matter haloes
Thanks to the ever increasing computational power and the development of more
sophisticated algorithms, numerical N-body simulations are now uncovering
several phenomenological relations between the physical properties of dark
matter haloes in position and velocity space. It is the aim of the present work
to investigate in detail the dynamical structure of dark matter haloes, as well
as its possible dependence on mass and its evolution with redshift up to z=5.
We use high-resolution cosmological simulations of individual objects to
compute the radially-averaged profiles of several quantities, scaled by the
radius Rmax at which the circular velocity attains its maximum value, Vmax. No
systematic dependence on mass or cosmic epoch are found within Rmax, and all
the different radial profiles are well fit by simple analytical models.
However, our results suggest that several properties are not `universal'
outside this radius. [Abridged]Comment: Accepted for publication in MNRAS (10 pages, 8 figures
Resolving the Formation of Protogalaxies. II. Central Gravitational Collapse
Numerous cosmological hydrodynamic studies have addressed the formation of
galaxies. Here we choose to study the first stages of galaxy formation,
including non-equilibrium atomic primordial gas cooling, gravity and
hydrodynamics. Using initial conditions appropriate for the concordance
cosmological model of structure formation, we perform two adaptive mesh
refinement simulations of ~10^8 M_sun galaxies at high redshift. The
calculations resolve the Jeans length at all times with more than 16 cells and
capture over 14 orders of magnitude in length scales. In both cases, the dense,
10^5 solar mass, one parsec central regions are found to contract rapidly and
have turbulent Mach numbers up to 4. Despite the ever decreasing Jeans length
of the isothermal gas, we only find one site of fragmentation during the
collapse. However, rotational secular bar instabilities transport angular
momentum outwards in the central parsec as the gas continues to collapse and
lead to multiple nested unstable fragments with decreasing masses down to
sub-Jupiter mass scales. Although these numerical experiments neglect star
formation and feedback, they clearly highlight the physics of turbulence in
gravitationally collapsing gas. The angular momentum segregation seen in our
calculations plays an important role in theories that form supermassive black
holes from gaseous collapse.Comment: Replaced with accepted version. To appear in ApJ v681 (July 1
Internal properties and environments of dark matter halos
We use seven high-resolution -body simulations to study the correlations
among different halo properties (assembly time, spin, shape and substructure),
and how these halo properties are correlated with the large-scale environment
in which halos reside. The large-scale tidal field estimated from halos above a
mass threshold is used as our primary quantity to characterize the large-scale
environment, while other parameters, such as the local overdensity and the
morphology of large-scale structure, are used for comparison. For halos at a
fixed mass, all the halo properties depend significantly on environment,
particularly the tidal field. The environmental dependence of halo assembly
time is primarily driven by local tidal field. The mass of the unbound fraction
in substructure is boosted in strong tidal force region, while the bound
fraction is suppressed. Halos have a tendency to spin faster in stronger tidal
field and the trend is stronger for more massive halos. The spin vectors show
significant alignment with the intermediate axis of the tidal field, as
expected from the tidal torque theory. Both the major and minor axes of halos
are strongly aligned with the corresponding principal axes of the tidal field.
In general, a halo that can accrete more material after the formation of its
main halo on average is younger, is more elongated, spins faster, and contains
a larger amount of substructure. Higher density environments not only provide
more material for halo to accrete, but also are places of stronger tidal field
that tends to suppress halo accretion. The environmental dependencies are the
results of these two competing effects. The tidal field based on halos can be
estimated from observation, and we discuss the implications of our results for
the environmental dependence of galaxy properties.Comment: Accepted for publication in MNRA
Preheating by Previrialization and its Impact on Galaxy Formation
We use recent observations of the HI-mass function to constrain galaxy
formation. The data conflicts with the standard model where most of the gas in
a low-mass dark matter halo is assumed to settle into a disk of cold gas that
is depleted by star formation and supernova-driven outflows until the disk
becomes gravitationally stable. A consistent model can be found if low-mass
haloes are embedded in a preheated medium, with a specific gas entropy ~ 10Kev
cm^2. Such a model simultaneously matches the faint-end slope of the galaxy
luminosity function. We propose a preheating model where the medium around
low-mass haloes is preheated by gravitational pancaking. Since gravitational
tidal fields suppress the formation of low-mass haloes while promoting that of
pancakes, the formation of massive pancakes precedes that of the low-mass
haloes within them. We demonstrate that the progenitors of present-day dark
matter haloes with M<10^{12}h^{-1}\msun were embedded in pancakes of masses
~5x10^{12}h^{-1}\msun at z~2. The formation of such pancakes heats the gas to
a temperature of 5x10^5K and compresses it to an overdensity of ~10. Such gas
has a cooling time that exceeds the age of the Universe at z~2, and has a
specific entropy of ~15Kev cm^2, almost exactly the amount required to explain
the stellar and HI mass functions. (Abridged)Comment: 13 pages, 3 figures. Accepted for publication in MNRA
3D Spectroscopy with VLT/GIRAFFE - IV: Angular Momentum and Dynamical Support of Intermediate Redshift Galaxies
[Abridged] One of the most outstanding problems related to numerical models
of galaxy formation is the so-called ``angular momentum catastrophe''. We study
the evolution of the angular momentum from z~0.6 to z=0 to further our
understanding of the mechanisms responsible for the large angular momenta of
disk galaxies observed today. This study is based on a complete sample of 32,
0.4<z<0.75 galaxies observed with FLAMES/GIRAFFE at the VLT. Their kinematics
had been classified as rotating disks, perturbed rotators, or complex
kinematics .We have computed the specific angular momentum of disks (j_disk)
and the dynamical support of rotating disks through the V/sigma ratio. To study
how angular momentum can be acquired dynamically, we have compared the
properties of distant and local galaxies. We find that distant rotating disks
have essentially the same properties (j_disk and R_d) as local disks, while
distant galaxies with more complex kinematics have a significantly higher
scatter in the j_disk--V_max and R_d--V_max planes. On average, distant
galaxies show lower values of V/sigma than local galaxies. We found
observational evidence for a non-linear random walk evolution of the angular
momentum in galaxies during the last 8 Gyr. The evolution related to galaxies
with complex kinematics can be attributed to mergers. If galaxies observed at
intermediate redshift are related to present-day spirals, then our results fit
quite well with the ``spiral rebuilding'' scenario proposed by Hammer et al.
(2005)Comment: 12 pages, 8 figures. Accepted for publication in A&