11,015 research outputs found
Constrained realizations and minimum variance reconstruction of non-Gaussian random fields
With appropriate modifications, the Hoffman--Ribak algorithm that constructs
constrained realizations of Gaussian random fields having the correct ensemble
properties can also be used to construct constrained realizations of those
non-Gaussian random fields that are obtained by transformations of an
underlying Gaussian field. For example, constrained realizations of lognormal,
generalized Rayleigh, and chi-squared fields having degrees of freedom
constructed this way will have the correct ensemble properties. The lognormal
field is considered in detail. For reconstructing Gaussian random fields,
constrained realization techniques are similar to reconstructions obtained
using minimum variance techniques. A comparison of this constrained realization
approach with minimum variance, Wiener filter reconstruction techniques, in the
context of lognormal random fields, is also included. The resulting
prescriptions for constructing constrained realizations as well as minimum
variance reconstructions of lognormal random fields are useful for
reconstructing masked regions in galaxy catalogues on smaller scales than
previously possible, for assessing the statistical significance of small-scale
features in the microwave background radiation, and for generating certain
non-Gaussian initial conditions for -body simulations.Comment: 12 pages, gzipped postscript, MNRAS, in pres
On estimating redshift and luminosity distributions in photometric redshift surveys
The luminosity functions of galaxies and quasars provide invaluable
information about galaxy and quasar formation. Estimating the luminosity
function from magnitude limited samples is relatively straightforward, provided
that the distances to the objects in the sample are known accurately;
techniques for doing this have been available for about thirty years. However,
distances are usually known accurately for only a small subset of the sample.
This is true of the objects in the Sloan Digital Sky Survey, and will be
increasingly true of the next generation of deep multi-color photometric
surveys. Estimating the luminosity function when distances are only known
approximately (e.g., photometric redshifts are available, but spectroscopic
redshifts are not) is more difficult. I describe two algorithms which can
handle this complication: one is a generalization of the V_max algorithm, and
the other is a maximum likelihood approach. Because these methods account for
uncertainties in the distance estimate, they impact a broader range of studies.
For example, they are useful for studying the abundances of galaxies which are
sufficiently nearby that the contribution of peculiar velocity to the
spectroscopic redshift is not negligible, so only a noisy estimate of the true
distance is available. In this respect, peculiar velocities and photometric
redshift errors have similar effects. The methods developed here are also
useful for estimating the stellar luminosity function in samples where accurate
parallax distances are not available.Comment: 9 pages, 6 figures, submitted to MNRA
Black Mayors in Non-Majority Black (Medium Sized) Cities: Universalizing the Interests of Blacks
The nature of political representation of Black constituents\u27 interests from their elected Black representatives is changing in the twentyfirst century. Increasingly, African Americans are being elected to political offices where the majority of their constituents are not African American. Previous research on this question tended to characterize Black politicians\u27 efforts to represent their Black constituents\u27 interests in two frames: deracialized or racialized (McCormick and Jones 1993; Cruse 1990). However, the advent of the twenty-first century has exhausted the utility ofthat polarization. Black politicians no longer find explicit racial appeals appropriate for their electoral goals, given the changing demographic environment, and greater acceptance of African American politicians in highprofile positions of power. Black politicians also increasingly find that a lack of attention to racial disparities facing constituents within their political boundaries does not effectively address why certain groups like Blacks are disproportionately and negatively affected than others, across a range of issues. Rather than continue to make efforts to represent Black interests within those two frames, Black politicians have begun to universalize the interests of Blacks
An excursion set model for the distribution of dark matter and dark matter haloes
A model of the gravitationally evolved dark matter distribution, in the
Eulerian space, is developed. It is a simple extension of the excursion set
model that is commonly used to estimate the mass function of collapsed dark
matter haloes. In addition to describing the evolution of the dark matter
itself, the model allows one to describe the evolution of the Eulerian space
distribution of the haloes. It can also be used to describe density profiles,
on scales larger than the virial radius, of these haloes, and to quantify the
way in which matter flows in and out of Eulerian cells. When the initial
Lagrangian space distribution is white noise Gaussian, the model suggests that
the Inverse Gaussian distribution should provide a reasonably good
approximation to the evolved Eulerian density field, in agreement with
numerical simulations. Application of this model to clustering from more
general Gaussian initial conditions is discussed at the end.Comment: 15 pages, 5 figures, submitted to MNRAS Sept. 199
Black Lives Matter: Imagining and Realizing an Equitable Black Future
The idea for this special Ethnic Studies Review edition, Black Lives Matter: Imagining and Realizing an Equitable Black Future, germinated prior to the election of the 45th president of the United States. However, what this series of articles and commentaries contribute to the movement for Black lives is even more critically important
Substructure in dark matter halos: Towards a model of the abundance and spatial distribution of subclumps
I develop a model for the abundance and spatial distribution of dark matter
subclumps. The model shows that subclumps of massive parent halos formed at
earlier times than subclumps of the same mass in lower mass parents;
equivalently, halos in dense regions at a given time formed earlier than halos
of the same mass in less dense regions. This may provide the basis for
interpreting recent observations which indicate that the stellar populations of
the most massive elliptical galaxies are also the oldest.Comment: 5 pages, 2 figures, submitted to MNRA
Stochasticity in halo formation and the excursion set approach
The simplest stochastic halo formation models assume that the traceless part
of the shear field acts to increase the initial overdensity (or decrease the
underdensity) that a protohalo (or protovoid) must have if it is to form by the
present time. Equivalently, it is the difference between the overdensity and
(the square root of the) shear that must be larger than a threshold value. To
estimate the effect this has on halo abundances using the excursion set
approach, we must solve for the first crossing distribution of a barrier of
constant height by the random walks associated with the difference, which is
now (even for Gaussian initial conditions) a non-Gaussian variate. The
correlation properties of such non-Gaussian walks are inherited from those of
the density and the shear, and, since they are independent processes, the
solution is in fact remarkably simple. We show that this provides an easy way
to understand why earlier heuristic arguments about the nature of the solution
worked so well. In addition to modelling halos and voids, this potentially
simplifies models of the abundance and spatial distribution of filaments and
sheets in the cosmic web.Comment: 5 pages, 1 figure. Matches published versio
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