203 research outputs found
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
Bayesian Cosmic Web Reconstruction: BARCODE for Clusters
We describe the Bayesian BARCODE formalism that has been designed towards the
reconstruction of the Cosmic Web in a given volume on the basis of the sampled
galaxy cluster distribution. Based on the realization that the massive compact
clusters are responsible for the major share of the large scale tidal force
field shaping the anisotropic and in particular filamentary features in the
Cosmic Web. Given the nonlinearity of the constraints imposed by the cluster
configurations, we resort to a state-of-the-art constrained reconstruction
technique to find a proper statistically sampled realization of the original
initial density and velocity field in the same cosmic region. Ultimately, the
subsequent gravitational evolution of these initial conditions towards the
implied Cosmic Web configuration can be followed on the basis of a proper
analytical model or an N-body computer simulation. The BARCODE formalism
includes an implicit treatment for redshift space distortions. This enables a
direct reconstruction on the basis of observational data, without the need for
a correction of redshift space artifacts. In this contribution we provide a
general overview of the the Cosmic Web connection with clusters and a
description of the Bayesian BARCODE formalism. We conclude with a presentation
of its successful workings with respect to test runs based on a simulated large
scale matter distribution, in physical space as well as in redshift space.Comment: 18 pages, 8 figures, Proceedings of IAU Symposium 308 "The Zeldovich
Universe: Genesis and Growth of the Cosmic Web", 23-28 June 2014, Tallinn,
Estoni
Statistical strong lensing. I. Constraints on the inner structure of galaxies from samples of a thousand lenses
Context: The number of known strong gravitational lenses is expected to grow
substantially in the next few years. The statistical combination of large
samples of lenses has the potential of providing strong constraints on the
inner structure of galaxies. Aims: We investigate to what extent we can
calibrate stellar mass measurements and constrain the average dark matter
density profile of galaxies by statistically combining strong lensing data from
thousands of lenses. Methods: We generate mock samples of axisymmetric lenses.
We assume that, for each lens, we have measurements of two image positions of a
strongly lensed background source, as well as magnification information from
full surface brightness modelling, and a stellar population synthesis-based
estimate of the lens stellar mass. We then fit models describing the
distribution of the stellar population synthesis mismatch parameter
(the ratio between the true stellar mass and the stellar
population synthesis-based estimate) and dark matter density profile of the
population of lenses to an ensemble of 1000 mock lenses. Results: The average
, projected dark matter mass and dark matter density slope can be
obtained with great precision and accuracy, compared with current constraints.
A flexible model and the knowledge of the lens detection efficiency as a
function of image configuration are required in order to avoid a biased
inference. Conclusions: Statistical strong lensing inferences from upcoming
surveys have the potential to calibrate stellar mass measurements and to
constrain the inner dark matter density profile of massive galaxies.Comment: Published on Astronomy & Astrophysics. A 2-minute summary video can
be found at this link: https://youtu.be/En0-uJobqE
New method for initial density reconstruction
A theoretically interesting and practically important question in cosmology is the reconstruction of the initial density distribution provided a late-time density field. This is a long-standing question with a revived interest recently, especially in the context of optimally extracting the baryonic acoustic oscillation (BAO) signals from observed galaxy distributions. We present a new efficient method to carry out this reconstruction, which is based on numerical solutions to the nonlinear partial differential equation that governs the mapping between the initial Lagrangian and final Eulerian coordinates of particles in evolved density fields. This is motivated by numerical simulations of the quartic Galileon gravity model, which has similar equations that can be solved effectively by multigrid Gauss-Seidel relaxation. The method is based on mass conservation, and does not assume any specific cosmological model. Our test shows that it has a performance comparable to that of state-of-the-art algorithms that were very recently put forward in the literature, with the reconstructed density field over ∼80% (50%) correlated with the initial condition at k ≲ 0.6 h=Mpc (1.0 h=Mpc). With an example, we demonstrate that this method can significantly improve the accuracy of BAO reconstruction
Evolution of galactic planes of satellites in the eagle simulation
We study the formation of planes of dwarf galaxies around Milky Way (MW)-mass haloes in the EAGLE galaxy formation simulation. We focus on satellite systems similar to the one in the MW: spatially thin or with a large fraction of members orbiting in the same plane. To characterize the latter, we introduce a robust method to identify the subsets of satellites that have the most coplanar orbits. Out of the 11 MW classical dwarf satellites, 8 have highly clustered orbital planes whose poles are contained within a 22° opening angle centred around (l, b) = (182°, −2°). This configuration stands out when compared to both isotropic and typical ΛCDM satellite distributions. Purely flattened satellite systems are short-lived chance associations and persist for less than 1Gyr. In contrast, satellite subsets that share roughly the same orbital plane are longer lived, with half of the MW-like systems being at least 4Gyr old. On average, satellite systems were flatter in the past, with a minimum in their minor-to-major axes ratio about 9Gyr ago, which is the typical infall time of the classical satellites. MW-like satellite distributions have on average always been flatter than the overall population of satellites in MW-mass haloes and, in particular, they correspond to systems with a high degree of anisotropic accretion of satellites. We also show that torques induced by the aspherical mass distribution of the host halo channel some satellite orbits into the host’s equatorial plane, enhancing the fraction of satellites with coplanar orbits. In fact, the orbital poles of coplanar satellites are tightly aligned with the minor axis of the host halo
Weak lensing by voids in weak lensing maps
Cosmic voids are an important probe of large-scale structure that can constrain cosmological parameters and test cosmological models. We present a new paradigm for void studies: void detection in weak lensing convergence maps. This approach identifies objects that relate directly to our theoretical understanding of voids as underdensities in the total matter field and presents several advantages compared to the customary method of finding voids in the galaxy distribution. We exemplify this approach by identifying voids using the weak lensing peaks as tracers of the large-scale structure. We find self-similarity in the void abundance across a range of peak signal-to-noise selection thresholds. The voids obtained via this approach give a tangential shear signal up to ∼40 times larger than voids identified in the galaxy distribution
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