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A Dissertation Entitled Stellar Populations in Nearby Merging A Dissertation entitled Stellar Populations in Nearby Merging Galaxies by Alexander J. Mulia Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics Dr. Rupali Chandar, Committee Chair Dr. Jon Bjorkman, Committee Member Dr. Michael Cushing, Committee Member Dr. Nikolas Podraza, Committee Member Dr. Bradley Whitmore, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo December 2015 Copyright 2015, Alexander J. Mulia This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of Stellar Populations in Nearby Merging Galaxies by Alexander J. Mulia Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics The University of Toledo December 2015 Galaxy mergers were common in the early universe. To better understand this critical step in galaxy evolution, we perform detailed studies of three nearby merging systems. Using images from the Hubble Space Telescope, we identify hundreds of star clusters in these systems, most of which formed as a result of a merger. By studying these clusters, we are able to constrain the properties of their host galaxies. These properties include: the timescale of the interaction, morphology of the merger’s progenitor galaxies, and the conditions in which stars and clusters formed. We find clusters in all tidal tails of our galaxy sample, even tails that were previously reported to be clusterless. Ages of clusters are similar to ages of their host tidal tails as predicted from simulations. We also find a color gradient across some tails, indicative of a gradient in ages that suggest star formation takes place primarily in the center of the tails, where gas is likely densest. In addition, cluster ages allow us to probe the star formation histories in these systems by predicting past SFRs in various regions of the galaxies using a new method involving the cluster mass function. The mergers also present an interesting environment to study star clusters themselves. We find that the formation and evolution of star clusters in mergers fits the “quasi-universal” picture of clusters seen in many other galaxies. iii This work is dedicated to two women whose support has been the foundation of my success: my fianc´ee, Ashley Nelson, and my mother, Janiece Mulia. Acknowledgments There are several people that have helped make this work the best that it could be. First, I would like to thank my advisor, Rupali Chandar, for not only passing on her knowledge, but providing me confidence and encouraging growth both inside and out of academia. To Brad Whitmore, I am grateful for his detailed and insightful comments on my papers, posters, and everything in between. Finally, I would like to thank all of my fellow graduate students at the University of Toledo. Whether talking through fundamental physics, helping with code, or editing my writing, their help was fundamental to both my work and my sanity. v Contents Abstract iii Acknowledgments v Contents vi List of Tables x List of Figures xi List of Abbreviations xiii 1 Introduction 1 1.1 The Role of Mergers in Galaxy Evolution . 1 1.1.1 ModelingtheMergers ...................... 4 1.1.2 Difficulties and Limitations of Simulations . 5 1.2 StarClusters ............................... 6 1.2.1 StarClusters:TheBasics .................... 7 1.2.2 Star Cluster Evolution: A Theorist’s Perspective . 9 1.2.3 Star Cluster Evolution: An Observer’s Perspective . 11 2 Star Clusters in the Tidal Tails of Merging Galaxies 15 2.1 Introduction................................ 15 2.2 Observations, Data Reduction, and Cluster Selection . 17 2.2.1 ObservationsandDataReduction . 17 vi 2.2.2 Cluster Selection . 21 2.3 Results................................... 29 2.3.1 ClusterDensity .......................... 29 2.3.2 Luminosity Functions (LFs) . 30 2.3.3 Colors and Ages of Clusters in Tidal Tails . 32 2.3.3.1 The Impact of Internal Reddening on Cluster Colors 35 2.3.3.2 Ages........................... 36 2.3.4 Diffuse Light in Tidal Tails . 37 2.4 Discussion................................. 42 2.4.1 Comparing Cluster Ages with Tail Ages . 42 2.4.1.1 NGC520 ........................ 44 2.4.1.2 NGC2623........................ 45 2.4.1.3 NGC3256........................ 46 2.4.1.4 The Timescale of Cluster Formation . 46 2.4.2 Comparing Cluster Ages Across Galaxies . 47 2.4.3 How are the Tidal Tail Age and Luminosity Function Related? 48 2.5 Conclusions ................................ 49 3 The Cluster Population of NGC 3256 51 3.1 Introduction................................ 51 3.2 Observations, Data Reduction, and Cluster Selection . 52 3.2.1 ObservationsandDataReduction . 52 3.2.2 Cluster Selection . 54 3.3 ClusterAgeandMassDetermination . 56 3.3.1 SEDFitting............................ 58 3.3.2 Calibrating the Hα Filter .................... 59 3.4 Results................................... 62 vii 3.4.1 Luminosity Function . 62 3.4.2 Age Distribution . 63 3.4.3 MassFunction........................... 65 3.4.3.1 Uncertainties on β ................... 67 3.4.3.2 IsThereanUpperMassCutoff? . 68 3.5 How Does Distance Affect the Observed Cluster Distributions? . .. 69 3.6 HowEfficientlyDoesNGC3256FormClusters? . 72 3.6.1 CMF/SFRStatistic........................ 74 3.6.2 TheΓStatistic .......................... 76 3.7 SummaryandConclusions ........................ 78 4 The Cluster Populations of NGC 520 and NGC 2623 81 4.1 Introduction................................ 81 4.2 ObservationsandClusterSelection . 82 4.3 Results................................... 85 4.3.1 Luminosity Functions . 85 4.3.2 Color Distributions . 87 4.3.2.1 Ages........................... 89 4.4 Constraining Star Formation Rates Using the CMF/SFR Statistic . 89 4.4.1 Approximating the Cluster Mass Function . 90 4.4.2 SFRs in the Main Bodies of Mergers . 91 4.4.3 SFRs in Tidal Tails of Mergers . 92 4.4.4 Uncertainties ........................... 93 4.5 Discussion................................. 95 4.5.1 LFsAcrosstheGalaxySample . 95 4.5.2 Do NGC 520 and NGC 2623 Lack Recent Star Formation? . 96 4.5.2.1 GasDynamics ..................... 96 viii 4.5.2.2 Other Considerations . 97 4.5.3 Timescales of Star Formation in NGC 520 . 99 4.5.4 Conditions of Star Formation in NGC 520 . 99 4.6 Conclusions ................................ 100 5 Conclusions and Future Work 102 5.1 Cluster Evolution in Merging Galaxies . 102 5.2 UsingClustersasaTool ......................... 103 5.3 FutureWork................................ 104 References 106 ix List of Tables 2.1 Galaxyproperties............................... 19 2.2 Selectioncuts ................................. 23 2.3 Propertiesofstarclusters .......................... 28 2.4 Diffuselightmeasurements. .. .. 41 3.1 Estimates of cluster formation efficiency in NGC 3256 . 79 x List of Figures 1-1 HubbleExtremeDeepField ......................... 2 1-2 ImageoftheAntennae ............................ 3 1-3 SimulatedevolutionofNGC2623 . 5 1-4 HST imageofthemainbodyoftheAntennae . 7 1-5 Examplesofaglobularandopencluster . 8 1-6 Sample mass-age diagram for a synthetic population of clusters ..... 12 1-7 Examples of YMC age distributions and mass functions . .. 13 2-1 Fieldofviewforgalaxysample ....................... 17 2-2 ACS/WFC F 814W imagesofthegalaxysample. 18 2-3 Thumbnailsofstarclusters ......................... 24 2-4 NGC520starclustercandidates . 25 2-5 NGC2623starclustercandidates . 26 2-6 Piewedgestarclustercandidates . 26 2-7 NGC3256starclustercandidates . 27 2-8 Luminosity functions of tidal tail star clusters . ... 31 2-9 Color-color diagrams of tidal tail star clusters . ... 33 2-10 Spatial distribution of young and old NGC 520 clusters . ... 34 2-11 Spatial distribution of young and old NGC 3256 clusters . ... 34 2-12 LocationofhaloclustersinNGC520andNGC3256 . 37 2-13 B − I colormapofgalaxysample. .. .. 38 2-14 Diffuse light color-color diagram for galaxy sample . .. 39 xi 2-15 Color-color and density diagrams for tidal tail clusters . ..... 43 3-1 BVIHα imageofNGC3256......................... 53 3-2 Concentration index and mV distributionsforNGC3256 . 55 3-3 Color-colordiagramofNGC3256clusters . 57 3-4 Hα line emission map of NGC 3256 with cluster locations and ages . 61 3-5 LuminosityfunctionsofNGC3256clusters. 62 3-6 Mass-agerelationforNGC3256clusters . 64 3-7 AgedistributionsforNGC3256clusters . 65 3-8 MassfunctionsforNGC3256clusters. 66 3-9 Schechter functions fit to NGC 3256 mass functions . .... 69 3-10 Source detections in the Antennae from original and degradedimages . 70 3-11 Luminosity functions for Antennae cluster candidates . ...... 71 3-12 Color-color diagrams for Antennae cluster candidates . ...... 73 3-13CMF/SFRstatisticforNGC3256. 76 4-1 HRCcoverageofNGC520andNGC2623 . 83 4-2 Concentration index and mV distributionforNGC520 . 84 4-3 Cluster candidate locations in NGC 520 . 85 4-4 Cluster candidate locations in NGC 2623 . 86 4-5 Luminosity function of NGC 520 and NGC 2623 cluster candidates ... 86 4-6 Color-color diagrams of NGC 520 and NGC 2623 cluster candidates... 88 4-7 Color-color and density diagram for clusters in the secondary nucleus of NGC520 ................................... 91 4-8 CMF/SFR statistic for clusters in the secondary nucleus of NGC 520 .. 92 4-9 CMF/SFRstatisticforpiewedgeclusters . 93 4-10 CMF/SFR statistic for clusters in the southern tidal tail of NGC520 .. 94 4-11 CMF/SFR
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