Dissertation Doctor of Natural Sciences
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Dissertation 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 Put forward by Kirsten Schnülle, Dipl.-Phys. born in: Bremen Oral examination: July 27, 2017 Studying the Radiative Response of Circumnuclear Dust of AGNs Referees: Prof. Dr. Hans-Walter Rix Prof. Dr. Jochen Heidt v Abstract Radius measurements of dust tori around the central engine of luminous active galaxies open up the possibility of probing cosmological models out to redshifts beyond where su- pernovae can be used. Yet, the value of dust tori as standard candles is constrained by the substantial intrinsic scatter in the size-luminosity relation found for samples of AGNs. Indicated by single objects, a probable cause of this scatter is a non-trivial variation in the dust location with the luminosity of the central engine, due to sublimation events and subse- quent long-term reformation of the hot dust surface radiating at near-infrared wavelengths. In this work, I developed a refined dust reverberation model allowing to measure torus sizes and additional observables characterizing the temperature state, stability, and distri- bution of the innermost, hot dust in AGNs. Optical to near-infrared photometric data were observed for 24 type 1 AGNs. Of these, I have analyzed the Seyfert 1 galaxies NGC 4151, Ark 120, NGC 5548, and NGC 3227. The derived inner torus radii fit very well into the established size-luminosity relation. While three objects are well-described by a standard model without sublimation, the data of one object could be fit satisfactorily only after allowing for dust sublimation events in the model. vii Zusammenfassung Radiusmessungen von Staubtori um leuchtkräftige aktive Galaxienkerne bieten die Mög- lichkeit, kosmologische Modelle jenseits von Rotverschiebungen zu testen, bei denen Su- pernovae als Standardkerze dienen können. Die Nutzbarkeit von Staub-Tori als Standard- kerzen ist jedoch stark eingeschränkt durch die wesentliche intrinsische Streuung in der Radius-Leuchtkraft-Relation für AGN-Samples. Wie Daten einzelner Objekte nahelegen, ist eine wahrscheinliche Ursache dieser Streuung eine nicht-triviale Änderung des Torusradius mit der Leuchtkraft der Akkretionsscheibe, hervorgerufen durch Sublimationsereignisse und anschließende Neubildung der heißen Stauboberfläche, welche im nah-infraroten Wel- lenlängenbereich emittiert. In dieser Arbeit habe ich ein erweitertes “dust reverberation”-Modell entwickelt, welches die Möglichkeit bietet, Torusradien und weitere Observablen zu messen, die den Temperatur- zustand, die Stabilität und die Verteilung des innersten heißen Staubes in AGN charakter- isieren. Optische bis nah-infrarote photometrische Daten wurden für 24 AGN des Typs 1 aufgenom- men, wovon ich die Seyfert 1-Galaxien NGC 4151, Ark 120, NGC 5548 und NGC 3227 mit der hier entwickelten Methode analysiert habe. Die abgeleiteten inneren Torusradien passen sehr gut in die etablierte Radius-Leuchtkraft-Relation. Während drei Objekte durch ein Standardmodell ohne Staubsublimation beschrieben werden können, konnten die Daten eines Objekts erst zufriedenstellend abgebildet werden, nachdem ich Sublimationsereignisse im Modell zugelassen habe. CONTENTS ix Contents 1 Introduction 1 1.1 Thiswork........................................ .. 1 1.2 ActiveGalacticNuclei ............................ ...... 2 1.2.1 TheAGNphenomenon ............................. 2 1.2.2 Unificationmodels.............................. .. 3 1.2.3 Astrophysical importance of AGNs and AGN tori . ...... 5 1.3 DustinAGNs...................................... .. 7 1.3.1 Observational evidence for the dusty torus . ........ 8 1.3.2 Fromsmoothtoclumpytorusmodels . ... 9 1.3.3 Howtomeasurethedustradius . .. 13 1.3.4 The radius-luminosity relation . ...... 17 1.3.5 Wheredoesthedustcomefrom? . .. 21 2 Multi-wavelength hot dust reverberation 23 2.1 Single-epochapproach............................ ...... 23 2.2 Multi-epochapproach ............................. ..... 24 2.2.1 Standard model without dust sublimation . ...... 24 2.2.2 Modelincludingdustsublimation . ..... 26 2.3 Optical and near-infrared photometry of variable sources ............ 30 2.3.1 TheOmega2000andGRONDinstruments. .. 30 2.3.2 Datareduction ................................. 34 2.3.3 GALFIT....................................... 34 2.3.4 ISIS......................................... 36 2.4 Interpolation of the input AD signal . ......... 37 2.5 Model fitting using a Markov Chain Monte Carlo algorithm . .......... 39 2.5.1 Differential Evolution Markov Chain . ....... 39 2.5.2 Gelmanconvergencediagnostics . ..... 40 2.5.3 Implementation of the algorithm . ..... 41 3 NGC 4151: A first glance analysis 43 3.1 Introduction .................................... .... 43 3.2 ThegalaxyNGC4151 ................................ .. 43 3.3 Observations .................................... .... 45 3.4 Results......................................... ... 46 3.5 Discussion ...................................... ... 48 3.6 Conclusions..................................... .... 51 x CONTENTS 4 NGC 4151: Extended analysis 53 4.1 Introduction .................................... .... 53 4.2 Observations.................................... .... 54 4.2.1 Near-infrareddata ............................. ... 54 4.2.2 Mid-infrareddata .............................. .. 56 4.2.3 Opticaldata................................... 57 4.3 Methods......................................... .. 57 4.4 Results......................................... ... 61 4.4.1 Constantpower-lawindex . ... 61 4.4.2 Further structure function models and time-variable power-law index 64 4.5 Discussion ...................................... ... 67 4.5.1 Single-blackbodymodel .. .. .. .. .. .. ... 67 4.5.2 Two-blackbodymodel............................ .. 68 4.5.3 Variabletimelag ............................... .. 70 4.6 Conclusions..................................... .... 76 5 NGC 3227 and GROND sub-sample 79 5.1 Introduction .................................... .... 79 5.2 The complete GROND AGN hot dust reverberation sample . ......... 80 5.3 NGC3227......................................... 81 5.3.1 ThegalaxyNGC3227.............................. 81 5.3.2 Observations .................................. 82 5.3.3 Methods ...................................... 87 5.3.4 Results ....................................... 88 5.3.5 Discussion.................................... 92 5.4 ThetargetsArk120andNGC5548 . .... 98 5.5 Sampleanalysis .................................. 103 5.6 Conclusions..................................... 106 6 Conclusions and outlook 109 6.1 Conclusions..................................... 109 6.2 Outlook......................................... 110 Bibliography 115 1 1 Introduction 1.1 This work In the thesis presented here, hot dust reverberation analyses are performed on optical to near-infrared (NIR) multi-epoch multi-wavelength data of type 1 active galactic nuclei (AGNs). In contrast to determining the hot dust reverberation delay by a conventional CCF analysis, a maximum likelihood approach is applied in order to be able to derive additional characteristic quantities of the hot dust, such as the hot dust temperature. Central idea is to infer to which extent the hot dust radius can sure enough be identified with the sublimation radius, as is implicitly assumed, for instance, in the radius-luminosity relation for the NIR hot dust around AGNs. For that purpose, general aim is to verify whether or not the hot dust in AGNs is found at or below sublimation temperatures. A further goal is to investigate the long-term stability of the hot dust and detect potential changes in its reverberation delay and radial distribution, in order to conclude on conditions for dust survival and formation in the radiation field of the AGN. While the first project (Chapter 3) was started off with a limited data set on a single tar- get, and a simple single-epoch approach using a non-linear least-square fitting algorithm, the data, the model and techniques were extended and refined during the course of the thesis. Specifically, for the second project (Chapter 4), the data set was substantially expanded, by using a significantly longer time series and a broader wavelength coverage. Reduction and analysis techniques were extended as well: precise photometry is performed with an image subtraction method. Further, a multi-epoch multi-wavelength Markov Chain Monte Carlo (MCMC) fit is applied, in order to recover characteristic quantities of the hot dust distribution (inspired by the works of Zu et al. (2011) and Hernitschek et al. (2014) for the BLR), and using the formalism developed by Rybicki & Press (1992). For the third project (Chapter 5), the data set was again significantly extended. To apply the methods developed in the previous analysis to a broader AGN sample, and to derive sample properties, observations were carried out on 23 additional type 1 AGNs. Three of these additional sources have been analyzed until now, as presented in Chapter 5.3. The methods from the previous analysis were modified to work on the different observational setup and different targets. Motivated by the light-curves of one target, the developed re- verberation model was substantially extended for the inclusion of dust sublimation events. 2 1 INTRODUCTION Theoretical and observational fundamentals for the context of this work are given in Chapter 1, starting from Sect. 1.2. Chapter 2 introduces