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Modeling and Interpretation of the Ultraviolet Spectral Energy Distributions of Primeval Galaxies

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Modeling and Interpretation of the Ultraviolet Spectral Energy Distributions of Primeval Galaxies Ecole´ Doctorale d'Astronomie et Astrophysique d'^Ile-de-France UNIVERSITE´ PARIS VI - PIERRE & MARIE CURIE DOCTORATE THESIS to obtain the title of Doctor of the University of Pierre & Marie Curie in Astrophysics Presented by Alba Vidal Garc´ıa Modeling and interpretation of the ultraviolet spectral energy distributions of primeval galaxies Thesis Advisor: St´ephane Charlot prepared at Institut d'Astrophysique de Paris, CNRS (UMR 7095), Universit´ePierre & Marie Curie (Paris VI) with financial support from the European Research Council grant `ERC NEOGAL' Composition of the jury Reviewers: Alessandro Bressan - SISSA, Trieste, Italy Rosa Gonzalez´ Delgado - IAA (CSIC), Granada, Spain Advisor: St´ephane Charlot - IAP, Paris, France President: Patrick Boisse´ - IAP, Paris, France Examinators: Jeremy Blaizot - CRAL, Observatoire de Lyon, France Vianney Lebouteiller - CEA, Saclay, France Dedicatoria v Contents Abstract vii R´esum´e ix 1 Introduction 3 1.1 Historical context . .4 1.2 Early epochs of the Universe . .5 1.3 Galaxytypes ......................................6 1.4 Components of a Galaxy . .8 1.4.1 Classification of stars . .9 1.4.2 The ISM: components and phases . .9 1.4.3 Physical processes in the ISM . 12 1.5 Chemical content of a galaxy . 17 1.6 Galaxy spectral energy distributions . 17 1.7 Future observing facilities . 19 1.8 Outline ......................................... 20 2 Modeling spectral energy distributions of galaxies 23 2.1 Stellar emission . 24 2.1.1 Stellar population synthesis codes . 24 2.1.2 Evolutionary tracks . 25 2.1.3 IMF . 29 2.1.4 Stellar spectral libraries . 30 2.2 Absorption and emission in the ISM . 31 2.2.1 Photoionization code: CLOUDY ........................ 31 2.2.2 General spectrum synthesis code: SYNSPEC ................. 33 2.2.3 Summary of the modeling . 33 2.3 The star formation history of galaxies . 34 3 Calibration of the SSP models in the ultraviolet 37 3.1 Introduction . 38 3.2 Ultraviolet signatures of young stellar populations . 40 3.2.1 Stellar population synthesis modelling . 40 3.2.2 Ultraviolet spectral indices . 41 3.2.3 Dependence of index strength on age, metallicity and integrated stellar mass 41 3.2.4 Associated broadband magnitudes . 46 3.3 Interpretation of ultraviolet star-cluster spectroscopy . 46 3.3.1 Observational sample . 46 3.3.2 Model library . 48 3.3.3 Age, metallicity and stellar-mass estimates . 49 vi Contents 4 Influence of the ISM on ultraviolet spectra of star-forming galaxies 57 4.1 ISMmodelling ..................................... 58 4.1.1 Approach . 58 4.1.2 Examples of model spectra . 64 4.2 Ultraviolet tracers of stars and the ISM . 67 4.2.1 Features tracing young stars . 67 4.2.2 Features tracing nebular emission . 71 4.2.3 Features tracing interstellar absorption . 72 4.2.4 Important composite features . 72 5 On statistical tools for galaxy SED-fitting 79 5.1 Standard methodology . 80 5.2 In this work: BEAGLE.................................. 81 6 Results from galaxy SED-fitting with BEAGLE 85 6.1 Lyα and C iii] emission in z = 7 − 9 galaxies: accelerated reionization around luminous star-forming systems? . 86 6.2 The three most metal-poor nearby galaxies . 90 6.3 10 nearby galaxies with hard optical spectra . 95 7 Conclusions and perspectives 99 Appendices 103 A Ultraviolet stellar absorption-line indices 105 B Correction of ultraviolet index strengths for Galactic absorption 109 C P-Cygni profiles 113 Bibliography 115 vii Abstract In this work, I combine state-of-the-art models for the production of stellar radiation and its transfer through the interstellar medium (ISM) to investigate ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in star-forming galaxies. I start by assessing the reliability of the stellar population synthesis modelling by fitting absorption-line indices in the ISM-free ultraviolet spectra of 10 Large-Magellanic-Cloud clusters. In doing so, I find that neglecting stochastic sampling of the stellar initial mass function in these young (∼ 10{100 Myr), low-mass clusters affects negligibly ultraviolet-based age and metallicity estimates but can lead to significant overestimates of stellar mass. Then, I proceed and develop a simple approach, based on an idealized description of the main features of the ISM, to compute in a physically consistent way the combined influence of nebular emission and interstellar absorption on ultraviolet spectra of star-forming galaxies. My model accounts for the transfer of radiation through the ionized interiors and outer neutral envelopes of short-lived stellar birth clouds, as well as for radiative transfer through a diffuse intercloud medium. I use this approach to explore the entangled signatures of stars, the ionized and the neutral ISM in ultraviolet spectra of star-forming galaxies. I find that, aside from a few notable exceptions, most standard ultraviolet indices defined in the spectra of ISM-free stellar populations are prone to significant contamination by the ISM, which increases with metallicity. I also identify several nebular-emission and interstellar-absorption features, which stand out as particularly clean tracers of the different phases of the ISM. ix R´esum´e Je combine de nouveaux mod´eles de production de radiations stellaires et de transport radiatif ´a travers le milieu interstellaire (MI). Cela permet de caract´eriser les ´etoiles ainsi que le MI neutre et ionis´edans des galaxies formant des ´etoiles (GFE), via des raies ultraviolettes dans leur spectre. J'´evalue la fiabilit´edes mod`eles stellaires en ajustant dans l'ultraviolet les indices d'absorption mesur´es dans les spectres stellaires de 10 amas d'´etoiles dans le Grand Nuage de Magellan. Je montre que n´egliger l'´echantillonnage stochastique de la fonction de masse initiale stellaire de ces amas jeunes et peu massifs a une faible influence dans l'estimation d'^ageet de m´etallicit´e, mais peut entra^ıner une surestimation significative des estimations de leur masse. Ensuite, je d´eveloppe une approche bas´eesur une description ´epur´eedes principales caract´eristiques du MI, afin de calculer de mani`ere auto-coh´erente l'influence combin´eede l'´emission et de l'absorption de ce milieu dans le spectre ultraviolet des GFE. Ce mod`ele tient compte du transport radiatif aussi bien `atravers les couches int´erieures ionis´ees, qu'`atravers les couches ext´erieures neutres des nuages de formation d'´etoiles ainsi que le milieu diffus entre ces nuages. J'utilise cette approche pour ´etudier la signature enchev^etr´ee des ´etoiles, du milieu neutre et du milieu ionis´edans les spectres ultraviolets des GFE. J'obtiens que la plupart des indices stellaires dans l'ultraviolet sont susceptibles de pr´esenter une contamination par le MI qui augmente avec la m´etallicit´e.Enfin, j'identifie des raies d'´emission et d'absorption interstellaires pouvant discriminer efficacement les diff´errentes phases du MI. Chapter 0. R´esum´e 1 dam 3 Chapter 1 Introduction In this thesis, I present a new approach to model in a physically consistent way the competing effects of stellar absorption, nebular emission and interstellar absorption in ultraviolet spectra of star-forming galaxies. I achieve this by combining state-of-the-art models for the production of stellar radiation and its transfer through the ISM to investigate ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in star-forming galaxies. The goal of this chapter is to summarize the main physical processes controlling the different components of galaxies, placing this in the general framework of what we know so far about the formation and evolution of galaxies. Contents 1.1 Historical context . .4 1.2 Early epochs of the Universe . .5 1.3 Galaxy types . .6 1.4 Components of a Galaxy . .8 1.4.1 Classification of stars . .9 1.4.2 The ISM: components and phases . .9 1.4.3 Physical processes in the ISM . 12 1.5 Chemical content of a galaxy . 17 1.6 Galaxy spectral energy distributions . 17 1.7 Future observing facilities . 19 1.8 Outline . 20 4 1.1. Historical context 1.1 Historical context At the beginning of the 20th century, the knowledge that we had of the Universe was very different from the one we have today. The Milky Way was thought to be the only galaxy in the Universe, at that epoch known as Island universes. This idea was taken to debate the 26th of April 1920, in the renowned Great debate, and the main question to answer was whether these distant Island universes or nebulae, such as Andromeda, were part of the Milky Way or if they were independent galaxies. Harlow Shapley defended the point of view that spiral nebulae are part of the Milky Way with a size argument; he claimed that if Andromeda were not part of the Milky Way, the distance between the two would be of 108 light years, a distance greater than what astronomers at that epoch thought was the size of the Universe. In contrast, Heber Doust Curtis defended the nature of Andromeda as an independent nebula for various reasons. His main arguments were that the nova1 rate in Andromeda was found to be higher than that in the Milky Way and also that velocity measurements performed by Vesto Slipher on several of these nebulae suggested that they are not gravitationally bound to the Milky Way. A few years later, in 1925, Edwin Hubble established that these nebulae are too distant to be part of the Milky Way, suggesting that the Universe extends further than what was thought before, and that those diffuse objects have sizes similar to that of the Milky Way. Hubble was able to measure the distance to these objects through observations of Cepheid stars, whose Period-Luminosity relation2 had been discovered by Henrietta Swan Leavitt.
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