CosmoClub: 06-04-2017

Date: April 6th 2017

Speaker: Julie Josse (Ecole Polytechnique)

Title: Missing data imputation using principal component methods


Missing values are ubiquitous and can occur for plenty of reasons: machines that fail, survey participants who do not answer to all questions, etc. The problem of missing values is somehow exacerbated with the amount of available data: data are often multisources (several projects aim to build large repositories by compiling data from preexisting databases) and due to the wide heterogeneity of measurement methods and research objectives, these large databases often exhibit extraordinarily high number of missing values. Missing values are problematic since most statistical methods can not be applied directly on a incomplete data.
In this talk, I will present recent tools developed to handle, in a practicable way, missing values. Among them, we can note the treatment of heterogeneous data (both quantitative and categorical) as well as the possibility to go far beyond single estimations and suggest subtle ways of assessing uncertainties. I will discuss imputation methods based on (regularized) singular value decomposition that caught the attention of the community due to their ability to handle large matrices with large amount of missing entries. Then, I will show how to extend these methods to multiple imputation to get notions of confidence intervals to know which credit should be given to analyses obtained from an incomplete data set. Such multiple imputation methods also offer new ways to visualize the variability of the results due to missing values.


CosmoClub: 30-03-2017

Date: March 30th 2017

Speaker: Matthieu Simeoni (EPFL)

Title: Bluebild: a Stable, Accurate and Efficient Imager for Radio Astronomy


Radio astronomy imaging has been primarily focused on a planar approximation to a portion of the observed sphere, producing images of a fixed resolution. Historically, Fourier analysis played a pivotal role, with algorithmic modifications made to fit that paradigm. It was thought that spherical calculation was computationally impractical, and that inherent numerical instability meant only a dirty image (a very rough least-squares approximation) could be made. The computational and energy demands of instruments such as the planned Square Kilometre Array (SKA) have made a new approach imperative.
Here we present an efficient algorithm called Bluebild, that reconstructs directly on the celestial sphere, producing, for the first time, a true least-square estimate of the sky. Wide-field and flexible beamformed imaging follow naturally. It produces a continuous image description that may be stored independently of resolution, and sampled up to the fundamental telescope limit. A multi-scale sky decomposition becomes an intrinsic part of the process, and algorithmic linearity permits uncertainty assessment across the chain. The algorithm is fast, far simpler and more intuitive than previous methods. We show sky images produced are more accurate, and can be analysed in much more depth. Results with real LOFAR data will be presented.


CosmoClub: 27-03-2017

Date: March 27th 2017

Speaker: Francois Meyer (University of Colorado, Boulder/INRIA)

Title: Detecting Structural  Changes in Dynamic Community Networks


The study of time-varying (dynamic) networks (graphs) is of fundamental importance for computer network analytics. Several methods have been proposed to detect the effect of significant structural changes in a time series of graphs.
The main contribution of this work is a detailed analysis of a dynamic community graph. This model is formed by adding new vertices, and randomly attaching them to the existing nodes. The goal of the work is to detect the time at which the graph dynamics switches from a normal evolution -- where balanced communities grow at the same rate -- to an abnormal behavior -- where communities start merging.
In order to circumvent the problem of identifying the communities, we use a metric to quantify structural changes as a function of time.  The detection of anomalies becomes one of testing the hypothesis that the graph is undergoing a significant structural change.
This is work in collaboration with Peter Wills.


CosmoClub: 17-02-2017

Date: February 17th 2017

Speaker: Vivien Scottez (IAP)

Title: Clustering-based Redshift estimation


The clustering of galaxies has emerged as a powerful way to estimate the redshift distribution of a sample. I will briefly review how we can get access to redshift information from the crosscorrelation function. Then, using the MICE2 simulation I will present how we can get individual redshifts for each galaxy as well as the corresponding accuracy.


CosmoClub: 02-02-2017

Date: February 2nd 2017

Speaker: Virginie Ollier (ENS Cachan/Supélec)

Title: Robust Calibration of Radio Interferometers in Non-Gaussian Environment [slides]


The development of new phased array systems in radio astronomy, as the low frequency array (LOFAR) and the square kilometre array (SKA), formed of a large number of small and flexible elementary antennas, has led to significant challenges. Among them, calibration is a crucial step in order to provide meaningful high dynamic range images and is commonly performed under the assumption of Gaussianity of the noise.
Nevertheless,  observations in the context of radio astronomy are known to be affected by the presence of outliers which are due to several causes, e.g., weak non-calibrator sources or man made radio frequency interferences. In order to take into account the outlier effects, the noise  can be assumed to follow a spherically invariant random distribution.
Based on this modeling, a robust calibration algorithm is exposed in this presentation. More precisely, this new scheme is based on the design of an iterative relaxed concentrated maximum likelihood estimation procedure and allows to obtain closed-form expressions for the unknown parameters with a reasonable computational cost.


CosmoClub: 01-12-2016

Date: December 1st 2017

Speaker: Emille Ishida (Université Clermont-Auvergne)

Title: The Cosmostatistics Initiative (COIN) - reshaping scientific interdisciplinary collaborations [slides]


The Cosmostatistics Initiative (COIN) is an international working group built under the auspices of the International Astrostatistics Association (IAA). Its goal is to propose an alternative approach to scientific collaboration while contributing to the establishment of Astrostatistics as a discipline on its own. In this talk  I will describe the motivation and the logistic behind the COIN collaboration, and its Residence Programs, and provide a couple of examples on how interdisciplinary collaboration can shed light on long standing astronomical problems.