Galaxy

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many are thought to have supermassive black holes at their centers. The Milky Wa

The last stages of star formation in the dwarf spheroidal galaxy Sextans

Romaine Theler

This thesis is anchored in the research of better understanding of the chemical evolution of galaxies. Many dwarf spheroidal galaxies (dSph) are orbiting the Milky Way displaying various star formation histories. The formation of these dwarf galaxies and the drivers of their evolution are still not well understood. We studied in detail one of them, Sextans, a low mass dSph composed of an old population. 90% of Sextans stars were formed 11-14 Gyr ago (Lee et al. 2009). Battaglia et al. (2011) show the presence of a radial metallicity gradient in Sextans considering observation based on intermediate resolution spectroscopy until the tidal radius. For a projected radius R < 0.8 deg stars cover the whole range of [Fe/H], while for R > 0.8 deg all stars are metal-poor [Fe/H] < -2.2 dex. Due to its distance and its low surface brightness high resolution spectra are required to obtain accurate abundances. Up to recently only 14 Sextans stars have been observed in high resolution spectroscopy (Shetrone et al. 2001; Aoki et al. 2009; Tafelmeyer et al. 2010; Honda et al. 2011) but in majority they were focused only on extremely metal-poor stars. In order to understand the chemical evolution of Sextans we needed to study a statistically significant sample of stars covering a larger metallicity range. This has been done in this work based on the largest high resolution spectroscopic sample of 87 red giant branch stars ever obtained in such low mass dwarf spheroidal galaxies. Our sample of stars located in the center of Sextans displays a large metallicity distribution ranging from -1.0 dex to -3.5 dex. We derived abundances of 10 elements : 3 alpha-elements (Mg, Ca and Ti), 5 iron-peak elements (Sc, Cr,Mn, Co and Ni) and 2 neutron-capture elements (Ba and Eu). Combining the results we had the first picture of the chemical evolution of Sextans. From [alpha/Fe] distribution reflecting the ratio between supernovae [SNe II/SNe Ia], we found that the domination of SNeIa in chemical evolution occurred around [Fe/H] = -2.0 dex. A lower knee than for Fornax and Sculptor suggests that star formation was less efficient in Sextans in agreement with their respective estimated mass. The absence of intrinsic scatter in [alpha/Fe] showed that in the center of Sextans the interstellar medium has been homogeneously enriched through star formation. Some hypothesis like tidal interactions with the Milky Way could explain this abundance homogeneity in the center of the galaxy and the metallicity gradient toward the outskirts. Abundances in iron-peak elements and neutron-capture elements give new observational constraints for a better understanding of the various nucleosynthesis sources of these elements.

EPFL2015

Development and evaluation of galaxy shape measurement algorithms for radio interferometric data

Vasileios Angelopoulos

The purpose of this Thesis is to develop, test, and characterize different models attempting to tackle the problem of measurement of galaxy shapes applied in interferometric observations. Shape measurement is a tool for estimating the underlying shear due to weak gravitational lensing by large-scale foreground matter distributions. The recent enormous progress in radio interferometric imaging during the last years, with projects such as MeerKAT, ASKAP, and in future SKA, motivates the development of advanced algorithms that utilize radio observations for this task. In most of these algorithms, the main disadvantage is that the proposed models' parameters have to be estimated in advance, assuming certain prior knowledge on the form of the objects. Our motivation is to develop and evaluate frameworks that do not require significant prior information on this form to make accurate measurements.To achieve this goal, we move in two implementation directions. In the first one, we are based on an existing approach that applies an object decomposition procedure using a dictionary of shapelet functions. Our proposal goes some steps forward trying to estimate this decomposition's key size parameter from a model built on advanced regularization. More specifically, we create an over-redundant dictionary formed as the concatenation of several groups of orthogonal shapelet basis functions with different parameters, and we use structured sparsity penalties to estimate the size parameter of the best group. As an alternative development strategy, we form an algorithm that employs multi-resolution least-squares analysis, which attempts to identify the size value that minimizes the relative residual in the fitting.The second path, instead of making the shape estimation directly on radio interferometric data, performs image reconstruction from the visibilities and measures the ellipticity of the objects in the resulting images. For this purpose, we adopt a sparsity averaging analysis algorithm that has been developed in the past and restores with high precision the intensity image from the visibilities. We also implement a similar framework that uses the CLEAN algorithm for image reconstruction, which helps compare the results between the two options and evaluate the quality of the measurements achieved.All our models are tested using a collection of simulated data that include objects of different profile types, ellipticity, size, orientation, and position in the field of view. The visibilities generation is done using either a simulated Gaussian profile coverage or a realistic SKA-like one. Additionally, we present an initial study of the same algorithms on real objects from the COSMOS survey.

EPFL2021

Development and evaluation of galaxy shape measurement algorithms for radio interferometric data

Vasileios Angelopoulos

EPFL2021

The last stages of star formation in the dwarf spheroidal galaxy Sextans

Romaine Theler

EPFL2015

The last stages of star formation in the dwarf spheroidal galaxy Sextans

Development and evaluation of galaxy shape measurement algorithms for radio interferometric data

Barred Galaxies In The Abell 901/2 Supercluster With Stages

Barred Galaxies In The Abell 901/2 Supercluster With Stages

Development and evaluation of galaxy shape measurement algorithms for radio interferometric data

The last stages of star formation in the dwarf spheroidal galaxy Sextans