Searches for Dark Matter Self-Annihilation Signals from Dwarf Spheroidal Galaxies and the Fornax Galaxy Cluster with Imaging Air Cherenkov Telescopes
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Searches for dark matter self-annihilation signals from dwarf spheroidal galaxies and the Fornax galaxy cluster with imaging air Cherenkov telescopes Dissertation zur Erlangung des Doktorgrades des Fachbereichs Physik der Universität Hamburg vorgelegt von Björn Helmut Bastian Opitz aus Warburg Hamburg 2014 Gutachter der Dissertation: Prof. Dr. Dieter Horns JProf. Dr. Christian Sander Gutachter der Disputation: Prof. Dr. Dieter Horns Prof. Dr. Jan Conrad Datum der Disputation: 17. Juni 2014 Vorsitzender des Prüfungsausschusses: Dr. Georg Steinbrück Vorsitzende des Promotionsausschusses: Prof. Dr. Daniela Pfannkuche Leiter des Fachbereichs Physik: Prof. Dr. Peter Hauschildt Dekan der MIN-Fakultät: Prof. Dr. Heinrich Graener Abstract Many astronomical observations indicate that dark matter pervades the universe and dominates the formation and dynamics of cosmic structures. Weakly inter- acting massive particles (WIMPs) with masses in the GeV to TeV range form a popular class of dark matter candidates. WIMP self-annihilation may lead to the production of γ-rays in the very high energy regime above 100 GeV, which is observable with imaging air Cherenkov telescopes (IACTs). For this thesis, observations of dwarf spheroidal galaxies (dSph) and the For- nax galaxy cluster with the Cherenkov telescope systems H.E.S.S., MAGIC and VERITAS were used to search for γ-ray signals of dark matter annihilations. The work consists of two parts: First, a likelihood-based statistical technique was intro- duced to combine published results of dSph observations with the different IACTs. The technique also accounts for uncertainties on the “J factors”, which quantify the dark matter content of the dwarf galaxies. Secondly, H.E.S.S. observations of the Fornax cluster were analyzed. In this case, a collection of dark matter halo models was used for the J factor computation. In addition, possible signal enhancements from halo substructures were considered. None of the searches yielded a significant γ-ray signal. Therefore, the re- sults were used to place upper limits on the thermally averaged dark matter self-annihilation cross-section σv . Different models for the final state of the an- h i nihilation process were considered. The cross-section limits range from σv UL 19 3 1 25 3 1 h i ∼ 10− cm s− to σv UL 10− cm s− for dark matter particles masses between 100 GeV and 100hTeVi. Some∼ of the diverse model uncertainties causing this wide range of σv UL values were analyzed. h i Kurzfassung Verschiedene astronomische Beobachtungen deuten darauf hin, dass dunkle Ma- terie das Universum durchzieht und die Entstehung und Dynamik kosmischer Strukturen dominiert. Schwach wechselwirkende, massive Teilchen (WIMPs, von “weakly interacting massive particles”), deren Ruhemasse im GeV- bis TeV- Bereich liegt, sind vielversprechende Kandidaten für eine teilchenphysikalische Erklärung der dunklen Materie. Selbstvernichtungsprozesse von WIMPs können Gammastrahlung im Energiebereich oberhalb von 100 GeV erzeugen, die mit ab- bildenden Luftschauer-Tscherenkow-Teleskopen (IACTs, “imaging air Cherenkov telescopes”) beobachtet werden kann. Für diese Dissertation wurden Beobachtungen von sphäroidalen Zwerggalax- ien (dSph) und des Fornax-Galaxienhaufens mit den Tscherenkow-Teleskopen H.E.S.S., MAGIC und VERITAS verwendet, um nach Gammastrahlungs-Signalen dunkler Materie zu suchen. Die Arbeit hat zwei Teile: Erstens wurde eine auf Likelihood-Funktionen basierende Methode eingeführt, die zur Kombination bere- its publizierter dSph-Beobachtungen der verschiedenen IACTs dient. Diese statis- tische Methode kann zudem die systematischen Unsicherheiten der “J -Faktoren” miteinbeziehen, welche den DM-Inhalt der Zwerggalaxien quantifizieren. Zweit- ens wurden H.E.S.S.-Beobachtungen des Fornax-Galaxienhaufens analysiert. In diesem Falle wurden verschiedene Modelle des DM-Halos von Fornax verwen- det, um die jeweiligen J -Faktoren zu berechnen. Außerdem wurden mögliche Verstärkungen des DM-Signals durch Halo-Substrukturen betrachtet. Weder die Beobachtungen der Zwerggalaxien noch die des Galaxienhaufens lieferten ein signifikantes Gammastrahlungs-Signal. Deshalb wurden ihre Ergeb- nisse genutzt, um obere Grenzen (UL, “upper limits”) auf den thermisch gemittel- ten Selbstvernichtungs-Wirkungsquerschnitt σv der dunklen Materieteilchen zu bestimmen. Dabei wurden unterschiedlicheh i Modelle für die Endzustände der Vernichtungsprozesse benutzt. Die Grenzen auf den Wirkungsquerschnitt 19 3 1 25 3 1 liegen im Bereich von σv UL 10− cm s− bis σv UL 10− cm s− , gültig für DM-Teilchenmassenh voni 100∼ GeV bis 100 TeVh. Einigei ∼ der unterschiedlichen Modellunsicherheiten und Abhängigkeiten, die zu diesem großen Bereich von σv UL-Werten führen, wurden ebenfalls erörtert. h i Contents 1 Introduction 13 2 Theoretical background 17 2.1 Dark matter and the formation of galaxies................ 17 2.1.1 Early observational evidence................... 17 2.1.2 ΛCDM and hierarchical structure formation......... 19 2.1.3 Properties of simulated dark matter haloes.......... 25 2.1.4 Observational constraints on dark matter haloes...... 28 2.1.5 Small-scale problems of cold dark matter........... 29 2.1.6 Alternative theories and models................. 31 2.2 Dark matter particle candidates....................... 33 2.2.1 The standard model of particle physics............. 33 2.2.2 The “WIMP miracle”........................ 36 2.2.3 Supersymmetry............................ 39 2.2.4 Other models of particle CDM.................. 41 2.3 Methods of WIMP detection........................ 42 2.3.1 γ-rays from DM annihilations.................. 44 3 Imaging Air Cherenkov Telescopes 49 3.1 VHE γ-rays and their detection...................... 50 3.1.1 Background suppression and estimation............ 52 3.2 The High Energy Stereoscopic System (H.E.S.S.)........... 54 3.3 H.E.S.S. data analysis............................. 55 3.3.1 Background estimation with the “template” method.... 57 3.3.2 The effective gamma-ray detection area............ 58 3.3.3 The “model” analysis........................ 59 3.4 MAGIC and VERITAS............................ 60 4 Combined likelihood analysis of dark matter searches 63 4.1 The likelihood stacking technique..................... 64 4.1.1 The likelihood function and its use............... 64 4.1.2 The likelihood function of IACT dark matter searches.. 66 4.1.3 The combination of likelihood functions........... 67 5 6 CONTENTS 4.1.4 Computational implementation................. 68 4.2 Observational parameters and data.................... 69 4.2.1 J distributions............................. 69 4.2.2 Effective detection areas...................... 71 4.2.3 γ-ray spectra from DM annihilations.............. 72 4.3 Results....................................... 74 4.3.1 Cross-check with published data................. 74 4.3.2 Single-observation likelihood limits............... 76 4.3.3 Combined likelihood limits.................... 82 4.3.4 The combination of likelihoods: A closer look....... 85 4.4 Summary and conclusions.......................... 89 5 A search for dark matter in the Fornax cluster with H.E.S.S. 93 5.1 Dark matter in the Fornax galaxy cluster................ 94 5.1.1 Mass profiles of Fornax and NGC 1399............ 95 5.1.2 Flux enhancement from DM halo substructure....... 99 5.2 H.E.S.S. observations and data analysis.................. 102 5.3 Interpretation of the results......................... 106 5.3.1 Upper limits on the integrated γ-ray flux from Fornax.. 107 5.3.2 Upper limits on the dark matter annihilation cross-section 108 5.3.3 The Sommerfeld effect and wino dark matter........ 115 5.4 Summary and conclusions.......................... 116 6 Summary and outlook 119 6.1 Results and conclusions............................ 119 6.2 Future directions................................ 120 6.2.1 Improved constraints on the small-scale properties of dark matter haloes............................. 121 6.2.2 Further observations and refined analyses with existing telescopes................................ 121 6.2.3 Better observatories......................... 122 6.2.4 Combination with complementary results from other ex- periments................................ 123 6.3 Final remarks.................................. 124 Appendices 125 A Bayes’ theorem, probabilities and likelihood functions 127 A.1 Bayes’ theorem................................. 127 A.2 Parameter transformations.......................... 128 A.3 J factor PDFs.................................. 129 B The “log-normal” J distributions 131 C Combined likelihood analysis: Additional figures 135 CONTENTS 7 Bibliography 141 Acknowledgements 159 8 CONTENTS List of Figures 2.1 Rotation curve of NGC 6503........................ 19 2.2 Cosmological densities............................ 22 2.3 Concordance cosmology........................... 23 2.4 Thermodynamic history of the universe................. 25 2.5 Standard Model particles........................... 34 2.6 DM freeze-out and relic density...................... 38 2.7 Higgs mass correction diagrams...................... 40 2.8 DM processes and detection......................... 43 2.9 γ-ray spectrum ingredients.......................... 47 2.10 γ-ray flux from DM annihilations..................... 48 3.1 VHE γ-ray sources............................... 50 3.2 γ-ray air shower, Cherenkov light detection.............. 51 3.3 Simulated air shower camera images, Hillas parameters....... 53 3.4 The H.E.S.S. array of Cherenkov telescopes............... 54