969 research outputs found
Back-in-time dynamics of the cluster IE 0657-56 (the Bullet System)
We present a simplified dynamical model of the ``Bullet'' system of two
colliding clusters. The model constrains the masses of the system by requiring
that the orbits of the main and sub components satisfy the cosmological initial
conditions of vanishing physical separation a Hubble time ago. This is also
known as the timing argument. The model considers a system embedded in an
over-dense region. We argue that a relative speed of between
the two components is consistent with cosmological conditions if the system is
of a total mass of is embedded in a region of
a (mild) over-density of 10 times the cosmological background density.
Combining this with the lensing measurements of the projected mass, the model
yields a ratio of 3:1 for the mass of the main relative to that of the
subcomponent. The effect of the background weakens as the relative speed
between the two components is decreased. For relative speeds lower than , the timing argument yields masses which are too low to be
consistent with lensing.Comment: 5 pages, 3 figures, submitted to MNRA
Resolving Subhaloes' Lives with the Hierarchical Bound-Tracing Algorithm
We develop a new code, the Hierarchical Bound-Tracing (HBT for short) code,
to find and trace dark matter subhaloes in simulations based on the merger
hierarchy of dark matter haloes. Application of this code to a recent benchmark
test of finding subhaloes demonstrates that HBT stands as one of the best codes
to trace the evolutionary history of subhaloes. The success of the code lies in
its careful treatment of the complex physical processes associated with the
evolution of subhaloes and in its robust unbinding algorithm with an adaptive
source subhalo management. We keep a full record of the merger hierarchy of
haloes and subhaloes, and allow growth of satellite subhaloes through accretion
from its "satellite-of-satellites", hence allowing mergers among satellites.
Local accretion of background mass is omitted, while rebinding of stripped mass
is allowed. The justification of these treatments is provided by case studies
of the lives of individual subhaloes and by the success in finding the complete
subhalo catalogue. We compare our result to other popular subhalo finders and
show that HBT is able to well resolve subhaloes in high density environment and
keep strict physical track of subhaloes' merger history. This code is fully
parallelized and freely available upon request to the authors.Comment: 15 pages; accepted for publication by MNRA
The importance of the merging activity for the kinetic polarization of the Sunyaev-Zel'dovich signal from galaxy clusters
The polarization sensitivity of the upcoming millimetric observatories will
open new possibilities for studying the properties of galaxy clusters and for
using them as powerful cosmological probes. For this reason it is necessary to
investigate in detail the characteristics of the polarization signals produced
by their highly ionized intra-cluster medium (ICM). This work is focussed on
the polarization effect induced by the ICM bulk motions, the so-called kpSZ
signal, which has an amplitude proportional to the optical depth and to the
square of the tangential velocity. In particular we study how this polarization
signal is affected by the internal dynamics of galaxy clusters and what is its
dependence on the physical modelling adopted to describe the baryonic
component. This is done by producing realistic kpSZ maps starting from the
outputs of two different sets of high-resolution hydrodynamical N-body
simulations. The first set (17 objects) follows only non-radiative
hydrodynamics, while for each of 9 objects of the second set we implement four
different kinds of physical processes. Our results shows that the kpSZ signal
turns out to be a very sensitive probe of the dynamical status of galaxy
clusters. We find that major merger events can amplify the signal up to one
order of magnitude with respect to relaxed clusters, reaching amplitude up to
about 100 nuK. This result implies that the internal ICM dynamics must be taken
into account when evaluating this signal because simplicistic models, based on
spherical rigid bodies, may provide wrong estimates. Finally we find that the
dependence on the physical modelling of the baryonic component is relevant only
in the very inner regions of clusters.Comment: 13 pages, 7 figures, submitted to A&
Tracing early structure formation with massive starburst galaxies and their implications for reionization
Cosmological hydrodynamic simulations have significantly improved over the
past several years, and we have already shown that the observed properties of
Lyman-break galaxies (LBGs) at z=3 can be explained well by the massive
galaxies in the simulations. Here we extend our study to z=6 and show that we
obtain good agreement for the LBGs at the bright-end of the luminosity function
(LF). Our simulations also suggest that the cosmic star formation rate density
has a peak at z= 5-6, and that the current LBG surveys at z=6 are missing a
significant number of faint galaxies that are dimmer than the current magnitude
limit. Together, our results suggest that the universe could be reionized at
z=6 by the Pop II stars in ordinary galaxies. We also estimate the LF of
Lyman-alpha emitters (LAEs) at z=6 by relating the star formation rate in the
simulation to the Ly-alpha luminosity. We find that the simulated LAE LFs agree
with the observed data provided that the net escape fraction of Ly-alpha photon
is f_{Ly-alpha} <= 0.1. We investigate two possible scenarios for this effect:
(1) all sources in the simulation are uniformly dimmer by a factor of 10
through attenuation, and (2) one out of ten LAEs randomly lights up at a given
moment. We show that the correlation strength of the LAE spatial distribution
can possibly distinguish the two scenarios.Comment: 9 pages, 4 figures. Summary of the talk given at the "First Light &
Reionization" workshop at UC Irvine, May 2005. The published article is
available from http://dx.doi.org/10.1016/j.newar.2005.11.00
Detecting neutral hydrogen in emission at redshift z ~ 1
We use a large N-body simulation to examine the detectability of HI in
emission at redshift z ~ 1, and the constraints imposed by current observations
on the neutral hydrogen mass function of galaxies at this epoch. We consider
three different models for populating dark matter halos with HI, designed to
encompass uncertainties at this redshift. These models are consistent with
recent observations of the detection of HI in emission at z ~ 0.8. Whilst
detection of 21 cm emission from individual halos requires extremely long
integrations with existing radio interferometers, such as the Giant Meter Radio
Telescope (GMRT), we show that the stacked 21 cm signal from a large number of
halos can be easily detected. However, the stacking procedure requires accurate
redshifts of galaxies. We show that radio observations of the field of the
DEEP2 spectroscopic galaxy redshift survey should allow detection of the HI
mass function at the 5-12 sigma level in the mass range 10^(11.4) M_sun/h <
M_halo < 10^(12.5)M_sun/h, with a moderate amount of observation time. Assuming
a larger noise level that corresponds to an upper bound for the expected noise
for the GMRT, the detection significance for the HI mass function is still at
the 1.7-3 sigma level. We find that optically undetected satellite galaxies
enhance the HI emission profile of the parent halo, leading to broader wings as
well as a higher peak signal in the stacked profile of a large number of halos.
We show that it is in principle possible to discern the contribution of
undetected satellites to the total HI signal, even though cosmic variance
limitation make this challenging for some of our models.Comment: 14 pages, 9 figures, Submitted To MNRA
Hydrodynamical Simulations of the IGM at High Mach Numbers
We present a new approach to doing Eulerian computational fluid dynamics that
is designed to work at high Mach numbers encountered in hydrodynamical
simulations of the IGM. In conventional Eulerian CFD, the thermal energy is
poorly tracked in supersonic bulk flows where local fluid variables cannot be
accurately separated from the much larger bulk flow components. We described a
method in which local fluid quantities can be directly tracked and the Eulerian
fluid equations solved in a local frame moving with the flow. The new algorithm
has been used to run large hydrodynamical simulations on a 1024^3 grid to study
the kinetic SZ effect. The KSZ power spectrum is broadly peaked at l~10^4 with
temperature fluctuations on micro Kelvin levels.Comment: 6 pages, to appear in the Proc. from the IGM/Galaxy Connection
conferenc
Magnetic field structure due to the global velocity field in spiral galaxies
We present a set of global, self-consistent N-body/SPH simulations of the
dynamic evolution of galactic discs with gas and including magnetic fields. We
have implemented a description to follow the evolution of magnetic fields with
the ideal induction equation in the SPH part of the Vine code. Results from a
direct implementation of the field equations are compared to a representation
by Euler potentials, which pose a div(B)-free description, an constraint not
fulfilled for the direct implementation. All simulations are compared to an
implementation of magnetic fields in the Gadget code which includes also
cleaning methods for div(B).
Starting with a homogeneous seed field we find that by differential rotation
and spiral structure formation of the disc the field is amplified by one order
of magnitude within five rotation periods of the disc. The amplification is
stronger for higher numerical resolution. Moreover, we find a tight connection
of the magnetic field structure to the density pattern of the galaxy in our
simulations, with the magnetic field lines being aligned with the developing
spiral pattern of the gas. Our simulations clearly show the importance of
non-axisymmetry for the evolution of the magnetic field.Comment: 17 pages, 18 figure
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