16,004 research outputs found
Quenching of Star Formation
In the last decade we have seen an enormous increase in the size and quality
of spectroscopic galaxy surveys, both at low and high redshift. New statistical
techniques to analyse large portions of galaxy spectra are now finding favour
over traditional index based methods. Here we will review a new robust and
iterative Principal Component Analysis (PCA) algorithm, which solves several
common issues with classic PCA. Application to the 4000AA break region of
galaxies in the VIMOS VLT Deep Survey (VVDS) and Sloan Digital Sky Survey
(SDSS) gives new high signal-to-noise ratio spectral indices easily
interpretable in terms of recent star formation history. In particular, we
identify a sample of post-starburst galaxies at z~0.7 and z~0.07. We quantify
for the first time the importance of post-starburst galaxies, consistent with
being descendants of gas-rich major mergers, for building the red sequence.
Finally, we present a comparison with new low and high redshift "mock
spectroscopic surveys" derived from a Millennium Run semi-analytic model.Comment: 7 pages, 3 figures. Conference proceedings in "Classification and
Discovery in Large Astronomical Surveys", 2008, C.A.L. Bailer-Jones (ed.
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Hydrological variations in central China over the past millennium and their links to the Tropic Pacific and North Atlantic Oceans
Variations of precipitation, aka the Meiyu rain, in East Asian summer monsoon (EASM) domain during the last millennium could help enlighten the hydrological response to future global warming. Here we present a precisely dated and highly resolved stalagmite δ18O record from the Yongxing Cave, central China. Our new record, combined with a previously published one from the same cave, indicates that the Meiyu rain has changed dramatically in association with the global temperature change. In particular, our record shows that the Meiyu rain has been weakened during the Medieval Climate Anomaly (MCA), but intensified during the Little Ice Age (LIA). During the Current Warm Period (CWP), our record indicates a similar weakening of the Meiyu rain. Furthermore, during the MCA and CWP, our records show that the atmospheric precipitation is similarly wet in northern China and similarly dry in central China, but relatively wet during the CWP in southern China. This spatial discrepancy indicates a complicated localized response of the regional precipitation to the anthropogenic forcing. The weakened (intensified) Meiyu rain during the MCA (LIA) matches well with the warm (cold) phases of Northern Hemisphere surface air temperature. This Meiyu rain pattern also corresponds well with the climatic conditions over the Tropical Indo-Pacific warm pool. On the other hand, our record shows a strong association with the North Atlantic climate as well. The reduced (increased) Meiyu rain correlates well with positive (negative) phases of North Atlantic Oscillation. In addition, our record links well with the strong (weak) Atlantic meridional overturning circulation during the MCA (LIA) period. All above-mentioned localized correspondences and remote teleconnections on decadal to centennial timescales indicate that the Meiyu rain is coupled closely with oceanic processes in the Tropical Pacific and North Atlantic Oceans during the MCA and LIA
Fast Hamiltonian sampling for large scale structure inference
In this work we present a new and efficient Bayesian method for nonlinear
three dimensional large scale structure inference. We employ a Hamiltonian
Monte Carlo (HMC) sampler to obtain samples from a multivariate highly
non-Gaussian lognormal Poissonian density posterior given a set of
observations. The HMC allows us to take into account the nonlinear relations
between the observations and the underlying density field which we seek to
recover. As the HMC provides a sampled representation of the density posterior
any desired statistical summary, such as the mean, mode or variance, can be
calculated from the set of samples. Further, it permits us to seamlessly
propagate non-Gaussian uncertainty information to any final quantity inferred
from the set of samples. The developed method is extensively tested in a
variety of test scenarios, taking into account a highly structured survey
geometry and selection effects. Tests with a mock galaxy catalog based on the
millennium run show that the method is able to recover the filamentary
structure of the nonlinear density field. The results further demonstrate the
feasibility of non-Gaussian sampling in high dimensional spaces, as required
for precision nonlinear large scale structure inference. The HMC is a flexible
and efficient method, which permits for simple extension and incorporation of
additional observational constraints. Thus, the method presented here provides
an efficient and flexible basis for future high precision large scale structure
inference.Comment: 14 pages, 7 figure
Combining Semi-analytic Models with Simulations of Galaxy Clusters: the Need for Heating from Active Galactic Nuclei
We present hydrodynamical N-body simulations of clusters of galaxies with
feedback taken from semi-analytic models of galaxy formation. The advantage of
this technique is that the source of feedback in our simulations is a
population of galaxies that closely resembles that found in the real universe.
We demonstrate that, to achieve the high entropy levels found in clusters,
active galactic nuclei must inject a large fraction of their energy into the
intergalactic/intracluster media throughout the growth period of the central
black hole. These simulations reinforce the argument of Bower et al., who
arrived at the same conclusion on the basis of purely semi-analytic reasoning.Comment: 25 pages and 10 colour figures. Accepted by Ap
-means clustering of extremes
The -means clustering algorithm and its variant, the spherical -means
clustering, are among the most important and popular methods in unsupervised
learning and pattern detection. In this paper, we explore how the spherical
-means algorithm can be applied in the analysis of only the extremal
observations from a data set. By making use of multivariate extreme value
analysis we show how it can be adopted to find "prototypes" of extremal
dependence and we derive a consistency result for our suggested estimator. In
the special case of max-linear models we show furthermore that our procedure
provides an alternative way of statistical inference for this class of models.
Finally, we provide data examples which show that our method is able to find
relevant patterns in extremal observations and allows us to classify extremal
events
Corporate Censorship
A chapter in a book about censorship in the global contemporary art world edited by Roisin Kennedy, University College Dublin, and Rhiann Coulter, Trinity College Dublin, (to be) published by IB Tauris in 2018, this book was the product of a panel at the Association of Art Historians annual conference. The chapter focuses solely on London so as to avoid potentially misleading and generalised statements about the censorship and contemporary art more globally. London serves as a case study of the specific pressures under which artists practice under neoliberalism and the often-unconscious internalisation of neoliberal values by contemporary artists, including socially-engaged practitioners. The article defines censorship, distinguishing it from the ordinary operations of the art world to include and exclude, and also distinguishing it from the ordinary operations of markets, which reduce diversity in order to rationalise. Both of these have been confused with instances of censorship.Peer reviewe
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