57,807 research outputs found
Monte Carlo Markov Chain parameter estimation in semi-analytic models of galaxy formation
We present a statistical exploration of the parameter space of the De Lucia and Blaizot version of the Munich semi-analytic (SA) model built upon the Millennium dark matter simulation. This is achieved by applying a Monte Carlo Markov Chain method to constrain the six free parameters that define the stellar and black hole mass functions at redshift zero. The model is tested against three different observational data sets, including the galaxy K-band luminosity function, B - V colours and the black hole-bulge mass relation, separately and combined, to obtain mean values, confidence limits and likelihood contours for the best-fitting model. Using each observational data set independently, we discuss how the SA model parameters affect each galaxy property and find that there are strong correlations between them. We analyse to what extent these are simply reflections of the observational constraints, or whether they can lead to improved understandings of the physics of galaxy formation.
When all the observations are combined, we find reasonable agreement between the majority of the previously published parameter values and our confidence limits. However, the need to suppress dwarf galaxy formation requires the strength of the supernova feedback to be significantly higher in our best-fitting solution than in previous work.
To balance this, we require the feedback to become ineffective in haloes of lower mass than before, so as to permit the formation of sufficient high-luminosity galaxies: unfortunately, this leads to an excess of galaxies around L*. Although the best fit is formally consistent with the data, there is no region of parameter space that reproduces the shape of galaxy luminosity function across the whole magnitude range.
For our best fit, we present the model predictions for the bJ-band luminosity and stellar mass functions. We find a systematic disagreement between the observed mass function and the predictions from the K-band constraint, which we explain in light of recent works that suggest uncertainties of up to 0.3 dex in the mass determination from stellar population synthesis models.
We discuss modifications to the SA model that might simultaneously improve the fit to the observed mass function and reduce the reliance on excessive supernova feedback in small haloes
Constraints on Neutrino Mass and Light Degrees of Freedom in Extended Cosmological Parameter Spaces
From a combination of probes including the cosmic microwave background
(WMAP7+SPT), Hubble constant (HST), baryon acoustic oscillations (SDSS+2dFGRS),
and supernova distances (Union2), we have explored the extent to which the
constraints on the effective number of neutrinos and sum of neutrino masses are
affected by our ignorance of other cosmological parameters, including the
curvature of the universe, running of the spectral index, primordial helium
abundance, evolving late-time dark energy, and early dark energy. In a combined
analysis of the effective number of neutrinos and sum of neutrino masses, we
find mild (2.2 sigma) preference for additional light degrees of freedom.
However, the effective number of neutrinos is consistent with the canonical
expectation of 3 massive neutrinos and no extra relativistic species to within
1 sigma when allowing for evolving dark energy and relaxing the strong
inflation prior on the curvature and running. The agreement improves with the
possibility of an early dark energy component, itself constrained to be less
than 5% of the critical density (95% CL) in our expanded parameter space. In
extensions of the standard cosmological model, the derived amplitude of linear
matter fluctuations sigma_8 is found to closely agree with low-redshift cluster
abundance measurements. The sum of neutrino masses is robust to assumptions of
the effective number of neutrinos, late-time dark energy, curvature, and
running at the level of 1.2 eV (95% CL). The upper bound degrades to 2.0 eV
(95% CL) when further including the early dark energy density and primordial
helium abundance as additional free parameters. Even in extended cosmological
parameter spaces, Planck alone could determine the possible existence of extra
relativistic species at 4 sigma confidence and constrain the sum of neutrino
masses to 0.2 eV (68% CL).Comment: 14 pages, 4 figures. Minor refinements, reflects version accepted for
publication in PRD (slow to get the paper published because of sickness
Optimizing Abstract Abstract Machines
The technique of abstracting abstract machines (AAM) provides a systematic
approach for deriving computable approximations of evaluators that are easily
proved sound. This article contributes a complementary step-by-step process for
subsequently going from a naive analyzer derived under the AAM approach, to an
efficient and correct implementation. The end result of the process is a two to
three order-of-magnitude improvement over the systematically derived analyzer,
making it competitive with hand-optimized implementations that compute
fundamentally less precise results.Comment: Proceedings of the International Conference on Functional Programming
2013 (ICFP 2013). Boston, Massachusetts. September, 201
The CIAO multiparadigm compiler and system: A progress report
Abstract is not available
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