507,902 research outputs found
Standoff Distance of Bow Shocks in Galaxy Clusters as Proxy for Mach Number
X-ray observations of merging clusters provide many examples of bow shocks
leading merging subclusters. While the Mach number of a shock can be estimated
from the observed density jump using Rankine-Hugoniot condition, it reflects
only the velocity of the shock itself and is generally not equal to the
velocity of the infalling subcluster dark matter halo or to the velocity of the
contact discontinuity separating gaseous atmospheres of the two subclusters.
Here we systematically analyze additional information that can be obtained by
measuring the standoff distance, i.e. the distance between the leading edge of
the shock and the contact discontinuity that drives this shock. The standoff
distance is influenced by a number of additional effects, e.g. (1) the
gravitational pull of the main cluster (causing acceleration/deceleration of
the infalling subcluster), (2) the density and pressure gradients of the
atmosphere in the main cluster, (3) the non-spherical shape of the subcluster,
and (4) projection effects. The first two effects tend to bias the standoff
distance in the same direction, pushing the bow shock closer to (farther away
from) the subcluster during the pre- (post-)merger stages. Particularly, in the
post-merger stage, the shock could be much farther away from the subcluster
than predicted by a model of a body moving at a constant speed in a uniform
medium. This implies that a combination of the standoff distance with
measurements of the Mach number from density/temperature jumps can provide
important information on the merger, e.g. differentiating between the pre- and
post-merger stages.Comment: 11 pages, 12 figures. Including major revision and matched to
accepted version in MNRA
Welfare Incentives and Interstate Migration: An Analysis of the Migration Decisions of Poor, Single Mothers
The purpose of this paper is to examine the role of welfare incentives in the decision to move for poor, single mothers. Using micro-level data from the Panel Study of Income Dynamics (PSID) and other sources, I develop an econometric model that estimates the influence of state welfare benefits on the interstate migration decisions of poor, single mothers, whether that be moving from states with low benefits or to states with high benefits. This study builds upon previous literature concerning interstate migration by considering new methodological approaches and theoretical models. Ultimately, the evidence suggests that while the welfare benefits offered at the current state of residence and those at potential states of residence influence the migration decisions of poor, single mothers, the effects are modest
Panel Data Tests Of PPP: A Critical Overview
This paper reviews recent developments in the analysis of non-stationary panels, focusing on empirical applications of panel unit root and cointegration tests in the context of PPP. It highlights various drawbacks of existing methods. First, unit root tests suffer from severe size distortions in the presence of negative moving average errors. Second, the common demeaning procedure to correct for the bias resulting from homogeneous cross-sectional dependence is not effective; more worryingly, it introduces cross-correlation when it is not already present. Third, standard corrections for the case of heterogeneous cross-sectional dependence do not generally produce consistent estimators. Fourth, if there is between-group correlation in the innovations, the SURE estimator is affected by similar problems to FGLS methods, and does not necessarily outperform OLS. Finally, cointegration between different groups in the panel could also be a source of size distortions. We offer some empirical guidelines to deal with these problems, but conclude that panel methods are unlikely to solve the PPP puzzl
Gender Bias in Power Relationships: Evidence from Police Traffic Stops
[Excerpt] We test for the existence of gender bias in power relationships. Specifically, we examine whether police officers are less likely to issue traffic tickets to men or to women during traffic stops. Whereas the conventional wisdom, which we document with surveys, is that women are less likely to receive tickets, our analysis shows otherwise. Examination of a pooled sample of traffic stops from five locations reveals no gender bias, but does show significant regional variation in the likelihood of citations. Analysis by location shows that women are more likely to receive citations in three of the five locations. Men are more likely to receive citations in the other two locations. To our knowledge, this study is the first to test for gender bias in traffic stops, and clearly refutes the conventional wisdom that police are more lenient towards women
Regularizing made-to-measure particle models of galaxies
Made-to-measure methods such as the parallel code NMAGIC are powerful tools
to build galaxy models reproducing observational data. They work by adapting
the particle weights in an N-body system until the target observables are well
matched. Here we introduce a moving prior regularization (MPR) method for such
particle models. It is based on determining from the particles a distribution
of priors in phase-space, which are updated in parallel with the weight
adaptation. This method allows one to construct smooth models from noisy data
without erasing global phase-space gradients. We first apply MPR to a spherical
system for which the distribution function can in theory be uniquely recovered
from idealized data. We show that NMAGIC with MPR indeed converges to the true
solution with very good accuracy, independent of the initial particle model.
Compared to the standard weight entropy regularization, biases in the
anisotropy structure are removed and local fluctuations in the intrinsic
distribution function are reduced. We then investigate how the uncertainties in
the inferred dynamical structure increase with less complete and noisier
kinematic data, and how the dependence on the initial particle model also
increases. Finally, we apply the MPR technique to the two
intermediate-luminosity elliptical galaxies NGC 4697 and NGC 3379, obtaining
smoother dynamical models in luminous and dark matter potentials.Comment: 16 pages, 15 figures, 2 tables. Accepted for publication in MNRA
Dynamic models of residential segregation: brief review, analytical resolution and study of the introduction of coordination
In his 1971's Dynamic Models of Segregation paper, the economist Thomas C.
Schelling showed that a small preference for one's neighbors to be of the same
color could lead to total segregation, even if total segregation does not
correspond to individual preferences and to a residential configuration
maximizing the collective utility.
The present work is aimed at deepening the understanding of the properties of
dynamic models of segregation based on Schelling's hypotheses. Its main
contributions are (i) to offer a comprehensive and up-to-date review of this
family of models; (ii) to provide an analytical solution to the most general
form of this model under rather general assumptions; to the best of our
knowledge, such a solution did not exist so far; (iii) to analyse the effect of
two devices aimed at decreasing segregation in such a model.Comment: 52 pages, 21 figures, working pape
Why has productivity growth stagnated in most Latin-American countries since the neo-liberal reforms?
Latin Americaâs economic performance since the beginning of neo-liberal reforms has been poor; this not only contrasts with its own performance pre-1980, but also with what has happened in Asia since 1980. I shall argue that the weakness of the regionâs new paradigm is rooted as much in its intrinsic flaws as in the particular way it has been implemented. Latin Americaâs economic reforms were undertaken primarily as a result of the perceived economic weaknesses of the region â i.e., there was an attitude of âthrowing in the towelâ vis-Ă -vis the previous state-led import substituting industrialisation strategy, because most politicians and economists interpreted the 1982 debt crisis as conclusive evidence that it had led the region into a cul-de-sac. As Hirschman has argued, policymaking has a strong component of âpath-dependencyâ; as a result, people often stick with policies after they have achieved their aims, and those policies have become counterproductive. This leads to such frustration and disappointment with existing policies and institutions that is not uncommon to experience a ârebound effectâ. An extreme example of this phenomenon is post-1982 Latin America, where the core of the discourse of the economic reforms that followed ended up simply emphasising the need to reverse as many aspects of the previous development (and political) strategies as possible. This helps to explain the peculiar set of priorities, the rigidity and the messianic attitude with which the reforms were implemented in Latin America, as well as their poor outcome. Something very different happened in Asia, where economic reforms were often intended (rightly or wrongly) as a more targeted and pragmatic mechanism to overcome specific economic and financial constraints. Instead of implementing reforms as a mechanism to reverse existing industrialisation strategies, in Asia they were put into practice in order to continue and strengthen ambitious processes of industrialisation
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