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Observing and Simulating Galaxy Evolution OBSERVING AND SIMULATING GALAXY EVOLUTION - from X-ray to millimeter wavelengths Dissertation submitted for the degree of PHILOSOPHIÆ DOCTOR to the PhD School of The Faculty of Science, University of Copenhagen on April 10 2015, by Karen Pardos Olsen Supervisors: Sune Toft and Thomas Greve Cover art: Nut, goddess of the sky in ancient Egyptian religion. Contrary to most other religions, the sky was feminine and her brother, Geb, personified the Earth. At dusk, Nut would swallow the sun god, Ra, who would pass through her belly during the night and be reborn at dawn. Sometimes she was depicted as a cow, but most of the time as a star-covered nude woman arching over, and protecting, the Earth. OBSERVING AND SIMULATING GALAXY EVOLUTION - from X-ray to millimeter wavelengths ABSTRACT It remains a quest for modern astronomy to answer what main mechanisms set the star forma- tion rate (SFR) of galaxies. Massive galaxies present a good starting point for such a quest due to their relatively easy detection at every redshift. Since stars form out of cold and dense gas, a comprehensive model for galaxy evolution should explain any observed connection between SFR and the amount and properties of the molecular gas of the interstellar medium (ISM). In proposed models of that kind, an active galactic nucleus (AGN) phase is often invoked as the cause for the decrease or cease of star formation. This thesis consists of models and observa- tions of gas and AGNs in massive galaxies at z ∼ 2, and how they may affect the overall SFR and the subsequent evolutionary trajectory of massive galaxies to z = 0. For an improved understanding of how observed gas emission lines link to the underly- ing ISM physics, a new code is presented here; SImulator of GAlaxy Millimeter/submillimeter Emission (SÍGAME). By post-processing the outputs of cosmological simulations of galaxy for- mation with sub-grid physics recipes, SÍGAME divides the ISM into different gas phases and derives the density and temperature structure of these, with locally resolved radiation fields. In the first study, SÍGAME is combined with the radiative transfer code LIME to model the spectral line energy distribution (SLED) of CO. A CO SLED close to that of the Milky Way is found for normal star-forming massive galaxies at z ∼ 2, but 50% smaller αCO factors, with the latter decreasing towards the center of each model galaxy. In a second study, SÍGAME is adapted to model the fine-structure line of singly ionized carbon, [CII] at 158 µm, the most powerful emission line of neutral ISM. Applying SÍGAME to the same type of galaxies, most [CII] emission can be traced back to the molecular part of their ISM. The observed L[CII]-SFR relation at z > 0:5 is reproduced and a similar relation is established on kpc scales for the first time theoretically. A third study uncovers the presence of AGNs among massive galaxies at z ∼ 2, and sheds light on the AGN-host co-evolution by connecting the fraction and luminosity of AGNs with galaxy properties. By analyzing a large survey in X-ray, AGNs of high and low X-ray luminos- ity are extracted among massive galaxies at z ∼ 2 via AGN classification methods, and stacking techniques of non-detections, in X-ray. Consequently, it is found that about every fifth massive galaxy, quenched or not, contain an X-ray luminous AGN. Interestingly, an even higher frac- tion of low-luminosity AGNs reside in the X-ray undetected galaxies, and preferentially in the quenched ones, lending support to the importance of AGNs in impeding star formation during galaxy evolution. iii CONTENTS Abstract iii Contents iv 1 Background 1 1.0.1 Setting the scale . 1 1.0.2 The cosmic web of galaxies . 2 1.1 ‘Normal star-forming galaxies’ . 2 1.1.1 The turning point at z ∼ 2 ............................ 4 1.1.2 The case of ‘massive galaxies’ . 4 1.2 The Interstellar Medium (ISM) . 5 1.2.1 Chemical composition . 6 1.2.2 Distribution and density . 7 1.2.3 Thermal state . 8 1.3 The Active Galactic Nucleus (AGN) . 10 1.3.1 Observing an AGN in X-ray . 11 1.4 The ‘spectral fingerprint’ of a galaxy . 12 1.5 An exciting time for observations . 14 1.5.1 Radio Telescopes . 14 1.5.2 X-ray Telescopes . 15 1.6 Galaxy simulations . 16 1.6.1 ‘Zoom-in’ simulations . 17 1.6.2 The callibration to z = 0 ............................. 17 1.7 References . 18 2 Introduction to this thesis 20 2.1 Evolution of massive galaxies across cosmic time . 20 2.1.1 High-redshift massive galaxy populations . 20 2.1.2 Observing galaxies at z > 4 via their gas emission lines . 22 2.1.3 The red and dead . 22 2.1.4 Quenching star formation . 23 2.1.5 Connecting the dots . 24 2.2 This thesis . 26 2.2.1 All eyes on the gas . 26 2.2.2 Quick summary of my projects . 27 2.3 References . 29 IMODELING THE ISM OF z ∼ 2 MASSIVE GALAXIES WITH SÍGAME 33 3 Observing and modeling CO emission lines in galaxies 35 3.1 Why CO? . 35 3.2 Observations of CO line emission . 36 3.2.1 The XCO factor . 36 3.3 Modeling of CO emission lines . 37 4 CO emission from simulated massive z = 2 main sequence galaxies (Paper I) 40 4.1 Aim of this project . 40 4.2 Cosmological Simulations . 41 4.2.1 SPH simulations . 41 4.2.2 The model galaxies . 41 4.3 Modeling the ISM with SÍGAME ............................. 44 4.3.1 Methodology overview . 44 4.3.2 The Warm and Cold Neutral Medium . 45 4.3.3 HI to H2 conversion . 47 4.3.4 Structure of the molecular gas . 49 4.3.5 Radiative transfer of CO lines . 55 4.3.6 Combining the GMC line profiles in a galaxy . 56 4.4 Simulating massive z = 2 main sequence galaxies . 57 4.4.1 Total molecular gas content and H2 surface density maps . 57 4.4.2 CO line emission maps and resolved excitation conditions . 59 4.4.3 The CO-to-H2 conversion factor . 60 4.4.4 Global CO line luminosities and spectral line energy distributions . 61 4.5 Comparison with other models . 63 4.6 Conclusions . 66 4.7 References . 69 5 Observing and modeling [CII] emission lines 72 5.1 Why [CII]?......................................... 72 5.2 Observations of [CII] emission in galaxies . 72 5.2.1 The [CII] deficit . 73 5.2.2 Contributing gas phases to [CII] emission . 74 5.2.3 [CII] as a star formation rate tracer . 74 5.3 Modeling [CII] emission . 75 6 Simulating [CII] line emission at z = 2 (Paper II) 76 6.1 Aim of this project . 76 6.2 Methodology overview . 76 6.3 SPH Simulations . 78 6.3.1 SPH simulations of z = 2 main sequence galaxies . 79 6.4 Modeling the ISM . 79 6.4.1 Giant Molecular Clouds . 81 6.4.2 The diffuse, ionised gas . 85 6.5 The [CII] line emission . 85 6.5.1 Integration of the [CII] line emission . 86 6.5.2 [CII] emission from molecular regions . 88 6.5.3 [CII] emission from PDRs . 88 6.5.4 [CII] emission from diffuse, ionised regions . 89 6.6 Results and discussion . 89 6.6.1 Radial distributions of L[CII] ........................... 89 6.6.2 The L[CII]-SFR relation . 92 6.6.3 Resolved Σ[CII] − ΣSFR relation . 93 6.7 The role of metallicity . 94 6.8 Caveats . 95 6.9 Conclusion . 96 7 Outlook 102 7.1 Improvements on SÍGAME ................................102 7.1.1 Dust temperatures with radiative transfer . 102 7.1.2 Asymmetric GMCs . 102 7.1.3 Heating by X-rays and turbulent dissipation . 103 7.2 Going to higher redshift . 103 7.2.1 The evolution of XCO with redshift . 103 7.2.2 The full callibration to normal galaxies at z ∼ 2 . 103 7.2.3 Galaxies during the epoch of re-ionization . 103 7.3 References . 105 II THE AGN-GALAXY CO-EVOLUTION AT z ∼ 2 109 8 How to detect an AGN 111 8.1 X-ray emission from an AGN . 112 8.1.1 Previous observations . 114 9 On the prevalence of AGN at z ∼ 2 (Paper III) 115 9.1 Aim of this project . ..
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