98 research outputs found
Understanding Galaxy Formation and Evolution
The old dream of integrating into one the study of micro and macrocosmos is
now a reality. Cosmology, astrophysics, and particle physics intersect in a
scenario (but still not a theory) of cosmic structure formation and evolution
called Lambda Cold Dark Matter (LCDM) model. This scenario emerged mainly to
explain the origin of galaxies. In these lecture notes, I first present a
review of the main galaxy properties, highlighting the questions that any
theory of galaxy formation should explain. Then, the cosmological framework and
the main aspects of primordial perturbation generation and evolution are
pedagogically detached. Next, I focus on the ``dark side'' of galaxy formation,
presenting a review on LCDM halo assembling and properties, and on the main
candidates for non-baryonic dark matter. It is shown how the nature of
elemental particles can influence on the features of galaxies and their
systems. Finally, the complex processes of baryon dissipation inside the
non-linearly evolving CDM halos, formation of disks and spheroids, and
transformation of gas into stars are briefly described, remarking on the
possibility of a few driving factors and parameters able to explain the main
body of galaxy properties. A summary and a discussion of some of the issues and
open problems of the LCDM paradigm are given in the final part of these notes.Comment: 50 pages, 10 low-resolution figures (for normal-resolution, DOWNLOAD
THE PAPER (PDF, 1.9 Mb) FROM http://www.astroscu.unam.mx/~avila/avila.pdf).
Lectures given at the IV Mexican School of Astrophysics, July 18-25, 2005
(submitted to the Editors on March 15, 2006
Toward an internally consistent astronomical distance scale
Accurate astronomical distance determination is crucial for all fields in
astrophysics, from Galactic to cosmological scales. Despite, or perhaps because
of, significant efforts to determine accurate distances, using a wide range of
methods, tracers, and techniques, an internally consistent astronomical
distance framework has not yet been established. We review current efforts to
homogenize the Local Group's distance framework, with particular emphasis on
the potential of RR Lyrae stars as distance indicators, and attempt to extend
this in an internally consistent manner to cosmological distances. Calibration
based on Type Ia supernovae and distance determinations based on gravitational
lensing represent particularly promising approaches. We provide a positive
outlook to improvements to the status quo expected from future surveys,
missions, and facilities. Astronomical distance determination has clearly
reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press
(chapter 8 of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
On the spin distributions of CDM haloes
We used merger trees realizations, predicted by the extended Press-Schechter
theory, in order to study the growth of angular momentum of dark matter haloes.
Our results showed that: 1) The spin parameter resulting from the
above method, is an increasing function of the present day mass of the halo.
The mean value of varies from 0.0343 to 0.0484 for haloes with
present day masses in the range of to
. 2)The distribution of is close to
a log-normal, but, as it is already found in the results of N-body simulations,
the match is not satisfactory at the tails of the distribution. A new
analytical formula that approximates the results much more satisfactorily is
presented. 3) The distribution of the values of depends only weakly
on the redshift. 4) The spin parameter of an halo depends on the number of
recent major mergers. Specifically the spin parameter is an increasing function
of this number.Comment: 10 pages, 8 figure
Testing dark energy beyond the cosmological constant barrier
Although well motivated from theoretical arguments, the cosmological constant
\emph{barrier}, i.e., the imposition that the equation-of-state parameter of
dark energy () is , seems to introduce
bias in the parameter determination from statistical analyses of observational
data. In this regard, \emph{phantom} dark energy or \emph{superquintessence}
has been proposed in which the usual imposition is relaxed.
Here, we study possible observational limits to the \emph{phantom} behavior of
the dark energy from recent distance estimates of galaxy clusters obtained from
interferometric measurements of the Sunyaev-Zel'dovich effect/X-ray
observations, Type Ia supernova data and CMB measurements. We find that there
is much \emph{observationally} acceptable parameter space beyond the
\emph{barrier}, thus opening the possibility of existence of more exotic forms
of energy in the Universe.Comment: 5 pages, 5 figures, to appear in Phys. Rev.
A method for comparing multiple imputation techniques: A case study on the U.S. national COVID cohort collaborative
Healthcare datasets obtained from Electronic Health Records have proven to be extremely useful for assessing associations between patientsâ predictors and outcomes of interest. However, these datasets often suffer from missing values in a high proportion of cases, whose removal may introduce severe bias. Several multiple imputation algorithms have been proposed to attempt to recover the missing information under an assumed missingness mechanism. Each algorithm presents strengths and weaknesses, and there is currently no consensus on which multiple imputation algorithm works best in a given scenario. Furthermore, the selection of each algorithm's parameters and data-related modeling choices are also both crucial and challenging. In this paper we propose a novel framework to numerically evaluate strategies for handling missing data in the context of statistical analysis, with a particular focus on multiple imputation techniques. We demonstrate the feasibility of our approach on a large cohort of type-2 diabetes patients provided by the National COVID Cohort Collaborative (N3C) Enclave, where we explored the influence of various patient characteristics on outcomes related to COVID-19. Our analysis included classic multiple imputation techniques as well as simple complete-case Inverse Probability Weighted models. Extensive experiments show that our approach can effectively highlight the most promising and performant missing-data handling strategy for our case study. Moreover, our methodology allowed a better understanding of the behavior of the different models and of how it changed as we modified their parameters. Our method is general and can be applied to different research fields and on datasets containing heterogeneous types
Deliberação online em consultas pĂșblicas? O caso da assembleia legislativa de Minas Gerais
Este artigo busca analisar uma consulta pĂșblica online sobre reforma polĂtica promovida em 2011 pela Assembleia Legislativa de Minas Gerais. Embasado pela teoria deliberacionista, o estudo de 752 posts guiou-se pela discussĂŁo de seis tĂłpicos: (1) inclusividade; (2) provimento de razĂ”es; (3) reciprocidade; (4) respeito mĂștuo; (5) orientação para o bem comum; e (6) articulação entre arenas. Os resultados indicam: (1) uma sobrerepresentação de participantes masculinos e oriundos da RegiĂŁo Metropolitana de Belo Horizonte, embora nĂŁo tenha havido concentração de posts em alguns comentadores; (2) o predomĂnio de posts on topic e que apresentam justificativas para as posiçÔes advogadas; (3) Ăndices muito baixos de reciprocidade entre posts e de uso de marcadores de interatividade; (4) graus muito elevados de respeito; (5) diferentes formas de ligação das justificativas a ideias de bem comum; (6) o baixo impacto decisĂłrio e a conexĂŁo com outras arenas da esfera pĂșblica, destacando-se a relação com uma matĂ©ria publicada em jornal de grande circulação. Os achados sĂŁo ambivalentes para os defensores da deliberação, indicando baixa probabilidade de deliberação interna ao fĂłrum, mas algum potencial deliberativo quando se adotam lentes mais amplas. Os cruzamentos entre variĂĄveis sugerem que as pessoas tendem a prover mais justificativas em discussĂ”es mais controversas, embora tendam a dialogar menos nesses casos. Assim, a hipĂłtese da preferĂȘncia por conversas entre like-minded encontra respaldo nos dados analisados
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