47 research outputs found
Mass function of haloes: scale invariant models
Press-Schechter theory gives a simple, approximate functional form of the
mass function of dark matter haloes. Sheth and Tormen (ST) refined this mass
function to give an improved analytical fit to results of N-body simulations.
These forms of the halo mass function are universal (independent of cosmology
and power spectrum) when scaled in suitable variables. Using large suites of
LCDM N-body simulations, studies in the last few years have shown that this
universality is only approximate. We explore whether some of the deviations
from universality can be attributed to the power spectrum by computing the mass
function in N-body simulations of various scale-free models in an Einstein-de
Sitter cosmology. This choice of cosmology does not introduce any scale into
the problem. These models have the advantage of being self-similar, hence
stringent checks can be imposed while running these simulations. This set of
numerical experiments is designed to isolate any power spectrum dependent
departures from universality of mass functions. We show explicitly that the
best fit ST parameters have a clear dependence on power spectrum. Our results
also indicate that an improved analytical theory with more parameters is
required in order to provide better fits to the mass function.Comment: 8 pages, four figure
High redshift supermassive blackholes: accretion through cold flows
We use zoom-in techniques to re-simulate three high-redshift (z > 5.5) halos
which host 10^9 solar mass blackholes from the ~ Gpc volume, MassiveBlack
cosmological hydrodynamic simulation. We examine a number of factors
potentially affecting supermassive blackhole growth at high redshift in
cosmological simulations. These include numerical resolution, feedback
prescriptions and formulation of smoothed particle hydrodynamics. We find that
varying the size of the region over which feedback energy is deposited
directly, either for fixed number of neighbours or fixed volume makes very
little difference to the accretion history of blackholes. Changing mass
resolution by factors of up to 64 also does not change the blackhole growth
history significantly. We find that switching from the density-entropy
formulation to the pressure-entropy formulation of smoothed particle
hydrodynamics slightly increases the accretion rate onto blackholes. In general
numerical details appear to have small effects on the main fueling mechanism
for blackholes at these high redshifts. We examine the fashion by which this
occurs, finding that the insensitivity to simulation technique seems to be a
hallmark of the cold flow feeding picture of these high-z supermassive
blackholes. We show that the gas that participates in critical accretion
phases, in these massive objects at z > 6~7 is in all cases colder, denser, and
forms more coherent streams than the average gas in the halo. This is also
mostly the case when the blackhole accretion is feedback regulated (z < 6),
however the distinction is less prominent. For our resimulated halos, cold
flows appear to be a viable mechanism for forming the most massive blackholes
in the early universe, occurring naturally in LambdaCDM models of structure
formation. Not requiring fine tuning of numerical parameters, they seem to be
physically inevitable in these objects.Comment: 15 pages, 12 figure
Halo mass function in scale invariant models
Sheth-Tormen mass function has been widely used to quantify the abundance of
dark matter halos. It is a significant improvement over the Press-Schechter
mass function as it uses ellipsoidal collapse in place of spherical collapse.
Both of these mass functions can be written in a form that is universal, i.e.,
independent of cosmology and power spectrum when scaled in suitable variables.
However, cosmological simulations have shown that this universality is
approximate. In this paper, we investigate the power spectrum dependence of
halo mass function through a suite of dark-matter-only N-body simulations of
seven power-law models in an Einstein-de Sitter cosmology. This choice of
cosmology and a power-law power spectrum ensures the self-similar evolution of
dark matter distribution, allowing us to isolate the power spectrum dependence
of mass function. We find that the mass function shows a clear
non-universality. We present fits for the parameters of the Sheth-Tormen mass
function for a range of power-law power-spectrum indices. We find a mild
evolution in the overall shape of the mass function with the epoch. Finally, we
extend our result to LCDM cosmology. We show that the Sheth-Tormen mass
function with parameter values derived from a matched power-law EdS cosmology
provides a better fit to the LCDM mass function than the standard Sheth-Tormen
mass function. Our results indicate that an improved analytical theory is
required to provide better fits to the mass function.Comment: 11 pages, 10 figures. This is a much expanded and upgraded version of
0908.2702. Submitted to MNRA
Galaxy Shapes and Intrinsic Alignments in The MassiveBlack-II Simulation
The intrinsic alignment of galaxy shapes with the large-scale density field
is a contaminant to weak lensing measurements, as well as being an interesting
signature of galaxy formation and evolution (albeit one that is difficult to
predict theoretically). Here we investigate the shapes and relative
orientations of the stars and dark matter of halos and subhalos (central and
satellite) extracted from the MassiveBlack-II simulation, a state-of-the-art
high resolution hydrodynamical cosmological simulation which includes stellar
and AGN feedback in a volume of . We consider
redshift evolution from to and mass evolution within the range of
subhalo masses, . The shapes of
the dark matter distributions are generally more round than the shapes defined
by stellar matter. The projected root-mean-square (RMS) ellipticity per
component for stellar matter is measured to be at
for , which compares favourably with
observational measurements. We find that the shapes of stellar and dark matter
are more round for less massive subhalos and at lower redshifts. By directly
measuring the relative orientation of the stellar matter and dark matter of
subgroups, we find that, on average, the misalignment between the two
components is larger for less massive subhalos. The mean misalignment angle
varies from for and shows a weak dependence on redshift. We also compare the
misalignment angles in central and satellite subhalos at fixed subhalo mass,
and find that centrals are more misaligned than satellites. We present fitting
formulae for the shapes of dark and stellar matter in subhalos and also the
probability distributions of misalignment angles.Comment: 18 pages, 18 figures, submitted to MNRA
Intrinsic alignments of galaxies in the MassiveBlack-II simulation: analysis of two-point statistics
The intrinsic alignment of galaxies with the large-scale density field is an
important astrophysical contaminant in upcoming weak lensing surveys. We
present detailed measurements of the galaxy intrinsic alignments and associated
ellipticity-direction (ED) and projected shape () correlation functions
for galaxies in the cosmological hydrodynamic MassiveBlack-II (MB-II)
simulation. We carefully assess the effects on galaxy shapes, misalignment of
the stellar component with the dark matter shape and two-point statistics of
iterative weighted (by mass and luminosity) definitions of the (reduced and
unreduced) inertia tensor. We find that iterative procedures must be adopted
for a reliable measurement of the reduced tensor but that luminosity versus
mass weighting has only negligible effects. Both ED and correlations
increase in amplitude with subhalo mass (in the range of ), with a weak redshift dependence (from to
) at fixed mass. At , we predict a that is in
reasonable agreement with SDSS LRG measurements and that decreases in amplitude
by a factor of --18 for galaxies in the LSST survey. We also compared
the intrinsic alignments of centrals and satellites, with clear detection of
satellite radial alignments within their host halos. Finally, we show that
(using subhalos as tracers of density) and (using dark
matter density) predictions from the simulations agree with that of non-linear
alignment models (NLA) at scales where the 2-halo term dominates in the
correlations (and tabulate associated NLA fitting parameters). The 1-halo term
induces a scale dependent bias at small scales which is not modeled in the NLA
model.Comment: 25 pages, 27 figures, revised after referee comments, accepted for
publication in MNRA