168 research outputs found
A scaling relation between merger rate of galaxies and their close pair count
We study how to measure the galaxy merger rate from the observed close pair
count. Using a high-resolution N-body/SPH cosmological simulation, we find an
accurate scaling relation between galaxy pair counts and merger rates down to a
stellar mass ratio of about 1:30. The relation explicitly accounts for the
dependence on redshift (or time), on pair separation, and on mass of the two
galaxies in a pair. With this relation, one can easily obtain the mean merger
timescale for a close pair of galaxies. The use of virial masses, instead of
stellar masses, is motivated by the fact that the dynamical friction time scale
is mainly determined by the dark matter surrounding central and satellite
galaxies. This fact can also minimize the error induced by uncertainties in
modeling star formation in the simulation. Since the virial mass can be read
from the well-established relation between the virial masses and the stellar
masses in observation, our scaling relation can be easily applied to
observations to obtain the merger rate and merger time scale. For major merger
pairs (1:1-1:4) of galaxies above a stellar mass of 4*10^10 M_sun/h at z=0.1,
it takes about 0.31 Gyr to merge for pairs within a projected distance of 20
kpc/h with stellar mass ratio of 1:1, while the time taken goes up to 1.6 Gyr
for mergers with stellar mass ratio of 1:4. Our results indicate that a single
timescale usually used in literature is not accurate to describe mergers with
the stellar mass ratio spanning even a narrow range from 1:1 to 1:4.Comment: accepted for publication in Ap
Sampling Artifact in Volume Weighted Velocity Measurement.--- II. Detection in simulations and comparison with theoretical modelling
Measuring the volume weighted velocity power spectrum suffers from a severe
systematic error, due to imperfect sampling of the velocity field from
inhomogeneous distribution of dark matter particles/halos in simulations or
galaxies with velocity measurement. This "sampling artifact" depends on both
the mean particle number density and the intrinsic large scale
structure (LSS) fluctuation in the particle distribution. (1) We report robust
detection of this sampling artifact in N-body simulations. It causes %
underestimation of the velocity power spectrum at h/Mpc for samples with
(Mpc/h). This systematic underestimation
increases with decreasing and increasing . Its dependence on the
intrinsic LSS fluctuations is also robustly detected. (2) All these findings
are expected by our theoretical modelling in paper I \cite{Zhang14}. In
particular, the leading order theoretical approximation agrees quantitatively
well with simulation result for (Mpc/h). Furthermore, we provide an ansatz to take high order
terms into account. It improves the model accuracy to % at
h/Mpc over 3 orders of magnitude in and over typical
LSS clustering from to . (3) The sampling artifact is determined by
the deflection field, which is straightforwardly available in both
simulations and data of galaxy velocity. Hence the sampling artifact in the
velocity power spectrum measurement can be self-calibrated within our
framework. By applying such self-calibration in simulations, it becomes
promising to determine the {\it real} large scale velocity bias of
halos with % accuracy, and that of lower mass halos by
better accuracy. ...[abridged]Comment: 11 pages, 11 figures. More arguments added, match the PRD accepted
versio
The Growth and Structure of Dark Matter Haloes
In this paper, we analyse in detail the mass-accretion histories and
structural properties of dark haloes in high-resolution N-body simulations.
Modeling the density distribution in individual haloes with the NFW profile, we
find, for all main progenitors of a given halo, there is a tight correlation
between its inner scale radius and the mass within it, , which is
the basic reason why halo structural properties are closely related to their
mass-accretion histories. This correlation can be used to predict accurately
the structural properties of a dark halo at any time from its mass-accretion
history. We also test our model with a large sample of GIF haloes. The build-up
of dark haloes in CDM models generally consists of an early phase of fast
accretion and a late phase of slow accretion [where increases with time
approximately as the expansion rate]. These two phases are separated at a time
when the halo concentration parameter . Haloes in the two accretion
phases show systematically different properties, for example, the circular
velocity increases rapidly with time in the fast accretion phase but
remain almost constant in the slow accretion phase,the inner properties of a
halo, such as and increase rapidly with time in the fast accretion
phase but change only slowly in the slow accretion phase. The potential well
associated with a halo is built up mainly in the fast accretion phase, even
though a large amount of mass (over 10 times) can be accreted in the slow
accretion phase. We discuss our results in connection to the formation of dark
haloes and galaxies in hierarchical models.Comment: 26 pages, including 10 figures. v2: some conceptual changes. Accepted
for publication in MNRA
Kriging Interpolating Cosmic Velocity Field
[abridged] Volume-weighted statistics of large scale peculiar velocity is
preferred by peculiar velocity cosmology, since it is free of uncertainties of
galaxy density bias entangled in mass-weighted statistics. However, measuring
the volume-weighted velocity statistics from galaxy (halo/simulation particle)
velocity data is challenging. For the first time, we apply the Kriging
interpolation to obtain the volume-weighted velocity field. Kriging is a
minimum variance estimator. It predicts the most likely velocity for each place
based on the velocity at other places. We test the performance of Kriging
quantified by the E-mode velocity power spectrum from simulations. Dependences
on the variogram prior used in Kriging, the number of the nearby
particles to interpolate and the density of the observed sample are
investigated. First, we find that Kriging induces and systematics
at when
and , respectively. The deviation
increases for decreasing and increasing . When , a smoothing effect dominates small scales, causing
significant underestimation of the velocity power spectrum. Second, increasing
helps to recover small scale power. However, for cases, the recovery is limited. Finally, Kriging is
more sensitive to the variogram prior for lower sample density. The most
straightforward application of Kriging on the cosmic velocity field does not
show obvious advantages over the nearest-particle method (Zheng et al. 2013)
and could not be directly applied to cosmology so far. However, whether
potential improvements may be achieved by more delicate versions of Kriging is
worth further investigation.Comment: 11 pages, 5 figures, published in PR
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