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The Pennsylvania State University The Graduate School Eberly College of Science ILLUMINATING THE STAR CLUSTERS AND DWARF GALAXIES BY MULTI-SCALE BARYONIC SIMULATIONS A Dissertation in Astronomy & Astrophysics by Moupiya Maji © 2018 Moupiya Maji Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2018 The dissertation of Moupiya Maji was reviewed and approved∗ by the following: Yuexing Li Associate Professor of Astronomy & Astrophysics Dissertation Advisor, Chair of Committee Robin Ciardullo Professor of Astronomy & Astrophysics Donghui Jeong Assistant Professor of Astronomy & Astrophysics Jane Charlton Professor of Astronomy & Astrophysics Sarah Shandera Assistant Professor of Physics Donald Schneider Professor of Astronomy & Astrophysics Chair of Graduate Program ∗Signatures are on file in the Graduate School. ii Abstract Over the past decade, advances in computational architecture have made it possible for the first time to investigate some of the fundamental questions around the birth and the growth of the building blocks of the universe; star clusters and galaxies. In these stellar and star-forming systems, baryonic physics play an important role in determining their formation and evolution. Therefore, in my research I have explored star-forming systems using high resolution baryonic cosmological simulations and explored the origin of star clusters, anisotropic spatial distribution of satellite galaxies and the effect of reionization on the evolution of dwarf galaxies. Observations of globular clusters show that they have universal lognormal mass 5 functions with a characteristic peak at 2 × 10 M , although the origin of this peaked distribution is unclear. Here I have investigated the formation and evolution of star clusters (SCs) in interacting galaxies using high-resolution hydrodynamical simulations performed with two different codes. I have found that massive star clusters in the 5.5 7.5 range of ∼ 10 − 10 M form preferentially in extremely high pressure gas clouds in highly-shocked regions produced by galaxy interactions. These findings provide the first simulation confirmation of the analytical theory of high pressure induced cluster formation. Furthermore, these massive star clusters have quasi-lognormal initial mass 6 functions with a peak around ∼ 10 M . The number of clusters declines with time due to destructive processes, but the shape and the peak of the mass functions do not change significantly during the course of galaxy collisions. These results suggest that gas-rich galaxy mergers provide a favorable environment for the formation of globular clusters, and that the lognormal mass functions and the unique peak may originate from the extreme high-pressure conditions of the birth clouds and may survive the dynamical evolution. Observations of classical Milky Way satellites suggest that they are aligned in a plane inclined to the Galactic stellar disk, a phenomenon which later became known as the “disk of satellites”(DoS). However, N-body simulations of galaxies predict an isotropic distribution of subhalos around the host galaxy and this discrepancy has been strongly criticized as a failure of ΛCDM. In this thesis, I have explored this highly debated topic by reanalyzing the observations and exploring the satellite distributions iii in high-resolution baryonic simulations. In particular, I have demonstrated that a small sample size can artificially produce a highly anisotropic spatial distribution and a strong clustering of the angular momenta of the satellites and have shown that the current Milky way DoS is transient. Furthermore, I have analyzed two cosmological simulations using the same initial conditions of a Milky-Way-sized galaxy, an N-body run with dark matter only, and a hydrodynamic one with both baryonic and dark matter, and found that the hydrodynamic simulation produces a more anisotropic distribution of satellites than the N-body one. These results suggest that an anisotropic distribution of satellites in galaxies can originate from baryonic processes in the hierarchical structure formation model, but the claimed highly flattened, coherently rotating DoS of the Milky Way may be biased by the small- number selection effect. Finally, I have investigated the distribution and kinematics of satellites around a large sample of few thousand host galaxies in the Illustris simulation and found that the DoS become more isotropic with increasing number of satellites and no clear coherent rotation is found in most (∼ 90%) of the satellite systems. Furthermore, their overall evolution indicates that the DoS may be part of large scale filamentary structure. These findings can help resolve the contradictory claims of DoS in galaxies and show that baryonic processes may be the key to solve the so-called small scale ΛCDM problems. Additionally, I have also explored the effects of reionization on the star formation histories of dwarfs galaxies, using a cosmological hydrodynamic simulation of Milky Way and its satellite galaxies. I have found that most dwarfs are extremely old systems and star formation is quenched earlier in lower mass galaxies. During reionization, most of the lower mass dwarfs are destroyed while the remaining massive dwarfs become severely baryon deficient. The dwarf galaxies play a very important role in shaping the path of cosmic history, especially in terms of reionization. Observing and studying the ultrafaint dwarfs hold the key to understanding the physics of early universe in great depth. iv Table of Contents List of Figures viii List of Tables xvii List of Symbols xviii Acknowledgments xxi Chapter 1 Introduction 1 1.1 The mysteries above . 1 1.2 Star clusters . 2 1.2.1 Open Clusters . 2 1.2.2 Globular clusters . 4 1.2.3 Young massive star clusters . 7 1.3 Puzzles of star clusters : Origin of the GCLF . 9 1.4 Dwarf galaxies . 10 1.4.1 Observations . 11 1.4.2 Properties . 12 1.4.3 Star formation history . 13 1.4.4 Theory . 14 1.5 Puzzles of dwarf galaxies : Disk of Satellites . 15 1.6 Thesis Outline . 16 Chapter 2 The formation and evolution of star clusters in interacting galaxies 17 2.1 Introduction . 17 2.2 Method . 19 2.2.1 Hydrodynamic Codes . 20 2.2.2 Galaxy Model . 21 2.2.3 Star Cluster Identification . 22 v 2.3 Formation of Star Clusters . 23 2.3.1 Starbursts in Interacting Galaxies . 23 2.3.2 Initial Cluster Mass Functions . 28 2.3.3 Physical Conditions of Cluster Formation and Origin of Log- normal Cluster Mass Functions . 30 2.4 Evolution of Massive Star Clusters . 35 2.5 Discussion . 39 2.6 Conclusions . 41 Chapter 3 Is there a Disk of Satellites around the Milky Way? 43 3.1 Introduction . 43 3.2 Methods . 44 3.3 Results . 46 3.3.1 DoS properties with different methods and sample sizes . 46 3.3.1.1 Structural properties . 46 3.3.1.2 Kinematic properties . 48 3.3.2 Dynamical evolution of satellites . 50 3.3.3 Evolution of DoS isotropy in simulations . 52 3.4 Conclusions . 55 3.5 Acknowledgments . 55 Chapter 4 The nature of Disk of Satellites around Milky Way-like galaxies 57 4.1 Introduction . 57 4.2 Methods . 60 4.2.1 Plane Identification Methods . 60 4.2.1.1 Principal Component Analysis (PCA) . 60 4.2.1.2 Tensor of Inertia method . 63 4.3 Abundance and Spatial Distribution of Satellites at z=0 . 64 4.3.1 Effects of Baryons . 64 4.3.2 Effects of Sample Size and Plane Identification Method . 66 4.4 Kinematic Properties of Satellites at z=0 . 69 4.5 Evolution of Satellites . 74 4.5.1 Evolution of Spatial Distribution . 74 4.5.2 Evolution of the Kinematics . 76 4.6 Discussions . 77 4.7 Summary . 81 vi Chapter 5 Disks of Satellites around Galaxies in Illustris Simulation 83 5.1 Introduction . 83 5.2 The simulation . 86 5.3 Satellite Systems in Illustris . 86 5.3.1 Abundance . 86 5.3.2 Spatial distribution of satellite systems . 87 5.3.3 Kinematic properties of satellite systems . 90 5.3.4 Evolution of the satellite systems . 92 5.4 Discussions . 94 5.5 Conclusions . 96 Chapter 6 Evolution of dwarf galaxies 98 6.1 Introduction . 98 6.2 Method . 99 6.3 Results I: Present day properties of dwarf galaxies . 100 6.3.1 Abundance of Dwarfs . 100 6.3.2 Baryon fraction of dwarfs . 100 6.4 Results II: Star Formation History in dwarfs . 103 6.4.1 Star formation quenching in dwarfs . 105 6.4.2 Age of Dwarfs . 107 6.5 Results III: Evolution of the Dwarfs . 107 6.5.1 Evolution of the mass function . 108 6.5.2 Evolution of the baryon fraction . 110 6.5.3 Effect of Reionization . 111 6.6 Discussions . 114 6.7 Summary . 116 Bibliography 118 vii List of Figures 1.1 Open Cluster M25. First observed in 1745, this open cluster lies in the Sagittarius constellation, about ∼ 600 parsecs away from Milky Way and is about 6 parsecs in diameter. It has an estimated age of 90 Myr, the young stars of this cluster is shown as blue points here. Image Credit : Image Jean-Charles Cuillandre (CFHT) and Giovanni Anselmi (Coelum Astronomia), Hawaiian Starlight . 3 1.2 Globular cluster M13. First observed in 1714, this cluster lies in the Hercules constellation, about 6.8 Kpc from MW, and is about ∼ 45 5 pc in diameter. The estimated mass of this GC is 6 × 10 M and it is ∼ 11.65 Gyrs old. Image credit : ESA/Hubble and NASA . 4 1.3 Metallicity distribution of 137 MW globular clusters. The bimodal distribution are fitted with two gaussian curves. Figure credit : Zinn 1985 (original), Harris 1996. 5 1.4 Luminosity function of globular clusters around Virgo giant elliptical galaxies. The data points represent the GCs in four galaxies : NGC 4365, 4472, 4486 and 4649 and the line is the best fit gaussian curve.