401 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
A physical and concise halo model based on the depletion radius
We develop a self-consistent and accurate halo model by partitioning matter
according to the depletion radii of haloes. Unlike conventional models that
define haloes with the virial radius while relying on a separate exclusion
radius or ad-hoc fixes to account for halo exclusion, our model distributes
mass across all scales self-consistently. Using a cosmological simulation, we
show that our halo definition leads to very simple and intuitive model
components, with the one-halo term given by the Einasto profile with no
truncation needed, and the halo-halo correlation function following a universal
power-law form down to the halo boundary. The universal halo-halo correlation
also allows us to easily model the distribution of unresolved haloes as well as
diffuse matter. Convolving the halo profile with the halo-halo correlation
function, we obtain a complete description of the halo-matter correlation
across all scales, which self-consistently accounts for halo exclusion on the
transition scale. Mass conservation is explicitly maintained in our model, and
the scale dependence of the classical halo bias is easily reproduced. Our model
can successfully reconstruct the halo-matter correlation function with percent
level accuracy for halo virial masses in the range of at , and covers
the radial range of . We also show
that our model profile can accurately predict the characteristic depletion
radius at the minimum bias and the splash-back radius at the steepest density
slope locations.Comment: 19 pages, 19 figure
The mass of our Milky Way
We perform an extensive review of the numerous studies and methods used to
determine the total mass of the Milky Way. We group the various methods into
seven broad classes, including: i) estimating Galactic escape velocity using
high velocity objects; ii) measuring the rotation curve through terminal and
circular velocities; iii) modeling halo stars, globular clusters and satellite
galaxies with the Spherical Jeans equation and iv) with phase-space
distribution functions; v) simulating and modeling the dynamics of stellar
streams and their progenitors; vi) modeling the motion of the Milky Way, M31
and other distant satellites under the framework of Local Group timing
argument; and vii) measurements made by linking the brightest Galactic
satellites to their counterparts in simulations. For each class of methods, we
introduce their theoretical and observational background, the method itself,
the sample of available tracer objects, model assumptions, uncertainties,
limits and the corresponding measurements that have been achieved in the past.
Both the measured total masses within the radial range probed by tracer objects
and the extrapolated virial masses are discussed and quoted. We also discuss
the role of modern numerical simulations in terms of helping to validate model
assumptions, understanding systematic uncertainties and calibrating the
measurements. While measurements in the last two decades show a factor of two
scatters, recent measurements using \textit{Gaia} DR2 data are approaching a
higher precision. We end with a detailed discussion of future developments,
especially as the size and quality of the observational data will increase
tremendously with current and future surveys. In such cases, the systematic
uncertainties will be dominant and thus will necessitate a much more rigorous
testing and characterization of the various mass determination methods.Comment: invited review published by Science China Physics, Mechanics &
Astronom
The multidimensional dependence of halo bias in the eye of a machine: a tale of halo structure, assembly and environment
We develop a novel approach in exploring the joint dependence of halo bias on
multiple halo properties using Gaussian process regression. Using a
CDM -body simulation, we carry out a comprehensive study of the
joint bias dependence on halo structure, formation history and environment. We
show that the bias is a multivariate function of halo properties that falls
into three regimes. For massive haloes, halo mass explains the majority of bias
variation. For early-forming haloes, bias depends sensitively on the recent
mass accretion history. For low-mass and late-forming haloes, bias depends more
on the structure of a halo such as its shape and spin. Our framework enables us
to convincingly prove that is a lossy proxy of
formation time for bias modelling, whereas the mass, spin, shape and formation
time variables are non-redundant with respect to each other. Combining mass and
formation time largely accounts for the mass accretion history dependence of
bias. Combining all the internal halo properties fully accounts for the density
profile dependence inside haloes, and predicts the clustering variation of
individual haloes to a level at . When an
environmental density is measured outside from the halo
centre, it outperforms and largely accounts for the bias dependence on the
internal halo structure, explaining the bias variation above a level of .Comment: MNRAS accepte
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