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Dissertation in Astronomy Submitted to the Combined Faculties of The Dissertation in Astronomy submitted to the Combined Faculties of the Natural Sciences and Mathematics of the Ruperto-Carola-University of Heidelberg,Germany, for the degree of Doctor of Natural Sciences presented by M.Sc.Kasper Borello Schmidt born in Nyborg,Denmark Oral examination:July 9th, 2012 Frontiers of Galaxy Evolution –Time-Domain Observations and 3D Spectroscopy – Referees:Prof.Dr.Hans-Walter Rix Prof.Dr.Joachim Wambsganß Abstract Understanding the formation and evolution of galaxies through cosmic time has been a central focus of astrophysics in the last decades: how did the interplay between dark matter structure for- mation, star formation, galaxy merging, and active galactic nuclei (AGN) give rise to the observed galaxy properties at different redshifts? This thesis presents innovative observational approaches to two aspects of this problem: finding and studying AGN through their variability, and making a first systematic census of galaxy mergers at z > 1 through three-dimensional spectroscopy. First we present a new and simple technique for selecting extensive, complete, and pure quasar samples via their intrinsic variability, parameterizing the single-band light-curve structure func- tion through a power-law to identify quasars among other variable and non-variable sources. Using extensive multi-epoch observations from SDSS Stripe 82 containing ∼60 epochs taken over ∼8 years, we demonstrate the power of this approach. The presented algorithm identifies quasars with a completeness and purity above 90% at all redshifts. Even for Pan-STARRS 1 mock data of only 6 epochs over 3 years, variability is still an encouragingly efficient quasar classifier. Data on intrinsic quasar variability enable a wide range of astrophysical science. We quantify the color variability, confirming and greatly fleshing out previous claims, that quasars become bluer as they brighten. We find a strong redshift dependence of this blueing, which we can attribute to emission lines contributing to the SDSS bands at different redshifts. We find that the color variations of single quasars are much more pronounced than the ranges in color seen in time-averaged ensembles of quasars. This indicates, that the observed color variations cannot be explained by changes in the mean steady state AGN accretion rate, but must arise from accretion disk ‘hotspots’ or similar phenomena. In the second, distinct part of the thesis, we present the first large sample of morphologically selected galaxy mergers with three-dimensional spectroscopy at z ∼ 1:5. With individual masses and star formation rates derived from multi-band photometry, we created emission line maps from the slitless grism spectroscopy of 3D-HST as proxy for star formation maps, providing a com- prehensive empirical picture of where star formation takes place in galaxy mergers at the epoch, where the cosmic star formation and merger rates peaked. We find that a broad range of star formation morphologies occur at all redshifts, irrespective of star formation rate and total stellar mass, in these mergers. An initial illustrative comparison to a set of cosmological simulations shows, that simulated mergers with similar mass and gas content show star formation in both merger components far more often than for the observed 3D-HST mergers. This suggests that mergers at z ∼ 1:5 most commonly happen between galaxies of distinctly different gas fractions. Zusammenfassung Das Verstandnis¨ der Entstehung und Entwicklung von Galaxien durch die kosmischen Epochen ist seit Jahrzehnten ein zentraler Fokus der Astrophysik: Wie fuhrte¨ das Wechselspiel zwischen der Strukturentstehung der dunklen Materie, Sternentstehung, Galaxienverschmelzung und ak- tive Galaxiekerne (AGN) zu den beobachteten Eigenschaften der Galaxien bei unterschiedlichen Rotverschiebungen? Diese Dissertation prasentiert¨ neue Beobachtungsherangehensweisen an zwei Aspekte dieses Problems: Das Auffinden und Untersuchen von AGN durch deren Vari- abilitat¨ und die Erstellung der ersten systematischen Untersuchung von Galaxienverschmelzun- gen bei z > 1 durch dreidimensionale Spektroskopie. Zuerst prasentieren¨ wir eine neue und einfache Technik um große, komplette, und reine Stich- proben an Quasaren durch deren intrinsische Variabilitat¨ zu bestimmen. Wir parametrisieren die Strukturfunktion deren Einzelbandlichtkurven durch ein Potenzgesetz um Quasare von an- deren variablen und nicht-variablen Quellen zu unterscheiden. Mit Hilfe von Multi-Epochen- Beobachtungen des SDSS Stripe 82, welche ∼60 Epochen uber¨ ∼8 Jahre umfassen, konnen¨ wir die Effizient dieser Suchweise zeigen. Der vorgelegte Algorithmus identifiziert Quasare mit einer Vollstandigkeit¨ und Reinheit uber¨ 90% bei allen Rotverschiebungen. Sogar fur¨ simulierten daten fur¨ den Pan-STARRS 1 Survey, mit nur 6 Epochen uber¨ 3 Jahre, ist die Variabilitat¨ ein ermutigend effizienter Klassifikator fur¨ Quasare. Daten zur intrinsischen Variabilitat¨ der Quasare ermoglichen¨ einen weiten Bereich von astrophysikalischer Anwendungen. Wir quantifizieren die Farb-Variabilitat¨ und bestatigen¨ und untermauern unsere vorherigen Hinweise, daß Quasare mit zunehmender Helligkeit blauer werden. Wir finden eine starke Rotverschiebungs-Abhangigkeit¨ dieser Farb-Variabilitaet. Dies konnen¨ wir Emissionslinien zuschreiben, die zu den SDDS Bandern¨ bei verschiedenen Rotverschiebungen beitragen. Wir finden, daß die Farbvariationen von Einzel- quasaren viel ausgepragter¨ sind als die Farbbereiche in zeitlich gemittelten Quasarensembles. Die zeigt, daß die beobachteten Farbvariationen nicht durch Anderungen¨ in der stationaren¨ Durch- nittsakkretionsrate der AGN erklarbar¨ sind, sondern von ‘hotspots’ oder ahnlichen¨ Phanomenen¨ der Akkretionsscheiben herruhren¨ mussen.¨ In dem zweiten eigenstandigen¨ Teil der Dissertation prasentieren¨ wir die erste große, morphol- ogisch selektierte Auswahl von Galaxienverschmelzungen mit dreidimensionaler Spektroskopie bei z ∼ 1:5. Mit individuellen Massen und Sternentstehungsraten, die von Multiband-Photometrie abgeleitet werden, erzeugen wir Karten der Emissionslinien von der spalt-losen Grismspek- troskopie von 3D-HST stellvertretend fur¨ Sternentstehungskarten. Diese stellen ein umfassendes, empirisches Bild bereit, das zeigt wo Sternentstehung in Galaxienverschmelzungen in dieser Epoche, in der kosmische Sternentstehung und Verschmelzungsraten einen Hohepunkt¨ erreichen, passiert. Die Untersuchung hat gezeigt, dass ein weiter Bereich von Sternentstehungsmorpholo- gien bei allen Rotverschiebungen passiert, unabhangig¨ von der Sternentstehungsrate und totalen stellaren Masse in diesen Verschmelzungen. Ein illustrativer Anfangsvergleich von einem Satz von kosmischen Simulationen weist daruaf hin, daß Verschmelzungen bei z ∼ 1:5 am haufigsten¨ zwischen Galaxien mit deutlich unterschiedlichen Gasanteilen passieren. Contents 1 Introduction 1 1.1 Our Place in the Universe 2 1.2 The Standard Big Bang Model 3 1.3 Structure Formation &(Λ)CDM Building Blocks 5 1.4 Building a Galaxy 10 1.5 Galaxies’Active (Quasar)Phase 12 1.5.1 Quasars – AGN at Its Best 16 1.6 Outline of this Thesis 17 2 Selecting Quasars via Variability 19 2.1 – Prologue – 20 2.2 Igniting Galaxies –The Quasar (AGN) Engine 22 2.3 Variability Characterization of Sources and the Structure Function 27 2.3.1 Color Selection 27 2.3.2 Source variability: power-law structure functions 28 2.4 SDSS Stripe 82: a Testbed for Variability Studies 31 ix x Contents 2.4.1 Spectroscopically confirmed quasars in Stripe 82 32 2.4.2 Stellar locus ‘contaminants’ in Stripe 82 33 2.4.3 UV excess (UVX) objects 33 2.4.4 Non-UV excess (nUVX) objects 34 2.4.5 Quasar Candidate Color Selection without u-band Data 34 2.4.6 Eliminating light curve ‘outliers’ in Stripe 82 35 2.4.7 Down-sampling Stripe 82 light curves to the Pan-STARRS 1 cadence 35 2.5 Power-Law Structure Functions for Sources in Stripe 82 38 2.6 Results 39 2.6.1 The A–γ Distribution 40 2.6.2 Completeness and Purity 42 2.6.2.1 Quasars in the UVX catalog 44 2.6.2.2 Quasars in the nUVX catalog 45 2.6.2.3 Quasars in the griz Color Box 47 2.7 Discussion 50 2.8 Conclusions 52 2.9 Outlook 54 2.9.1 Probabilistic Combination of Selection Methods 54 2.9.2 Selecting Lensed Quasars with Variability 55 2.10 – Epilogue – 58 3 The Quasar Color Variability 59 3.1 – Prologue – 60 3.2 Probing the Physics of Quasars 62 3.2.1 The (Rest-Frame Optical)Quasar Spectrum 62 Contents xi 3.2.2 Eddington Ratios and Black Hole Masses 64 3.2.3 Emission Line Ratio Diagnostic –The BPT Diagram 65 3.2.4 The Variability of Quasars 67 3.2.5 Reverberation Mapping -Measuring Sizes of BLRs 67 3.2.6 Spectral Hardness –Brighter Means Blue 69 3.3 SDSS Stripe 82 Data 70 3.4 Fitting Color Variability in Magnitude Space 70 3.5 Results 73 3.5.1 Color Variability in gr 73 3.5.2 Reproducing the Color Variability Redshift Dependence with Simple Variability Model 75 3.5.3 Color Variability in ui 77 3.6 Discussion 79 3.6.1 Color Variability as a Function of Eddington Luminosity and Black Hole Mass 79 3.6.2 The Color Variability as a Function of the Light Curve Variability Characteristics 80 3.6.3 Color Variability and Changes in the Mean Accretion Rate 85 3.6.3.1 Color Variability of Individual Quasars vs. the Color Distribution of Quasar Ensembles 85 3.6.3.2 Color Variability vs.Accretion Disk Models 86 3.7 Conclusion 89 3.8 Outlook 91 3.8.1 Modeling of Spectral Quasar Variability
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