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A Dissertation Entitled Star Cluster Populations in the Spiral Galaxy A Dissertation entitled Star Cluster Populations in the Spiral Galaxy M101 by Lesley A. Simanton 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. John-David Smith, Committee Member Dr. Steven Federman, Committee Member Dr. Bo Gao, Committee Member Dr. Bradley Whitmore, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo August 2015 Copyright 2015, Lesley A. Simanton 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 Star Cluster Populations in the Spiral Galaxy M101 by Lesley A. Simanton Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics The University of Toledo August 2015 Most stars form in groups and clusters. Stars clusters range in age from very young (< 3 Myr, embedded in gas clouds) to some of the most ancient objects in the universe (> 13 Gyr), providing clues to the formation and evolution of their host galaxies. Our knowledge of the diversity of star cluster populations has expanded over the last few decades, especially by being able to examine star clusters outside of the Milky Way (MW). In this dissertation, we continue this expansion of extragalactic star cluster studies by examining the star cluster system of the nearby spiral galaxy M101. We utilize photometry from Hubble Space T elescope images to assess luminosity, color, size, and spatial distributions of old star clusters, and spectroscopy from the Gemini- North telescope to determine ages, metallicities, and velocities of a subset of both young and old clusters in M101. We find that the range of cluster luminosities, ages, and metallicities in M101 is nearly continuous. We discover a population of fairly massive, old disk clusters, and conclude that the disk of M101 may have had a higher rate of cluster formation in the past than in the MW, and that it may be better suited to cluster survival. We find evidence that some clusters in M101 have intermediate ages of several Gyr, whereas the MW has few such clusters. Our analysis of the velocities of young clusters suggests that they rotate with the HI gas disk, while the old globular clusters appear to be in the halo. iii This work is dedicated to my all of family and friends, especially to my parents, Robert and Joyce Simanton, and my great aunt, Donna Simanton, who made my education possible; to my sister and lifelong friend, Wendy Holland; and to my very supportive and motivating fianc´e,Jim Coogan. Acknowledgments I would like to thank my advisor, Rupali Chandar, for all of her support, encourage- ment, and contributions to this work. She inspired my interest and excitement in star clusters and led me through many challenges and difficulties throughout my research. I would also like to thank collaborators Brad Whitmore and Bryan Miller for their contributions to this work and for answering my questions promptly and with genuine interest in my education and discoveries. I would like to thank my other committee members, Steve Federman, J. D. Smith, and Bo Gao for taking the time to give crucial input and criticisms to help shape the results and interpretation presented here. Finally, I would like to thank the graduate students at the University of Toledo De- partment of Physics and Astronomy who have furthered my understanding of physics and astronomy by helping me find the answers to questions, engaging in discussions, sharing papers, and empathizing in the sometimes frustrating day-to-day necessities of conducting scientific research. v Contents Abstract iii Acknowledgments v Contents vi List of Tables ix List of Figures x List of Abbreviations xii List of Symbols xiii 1 Introduction 1 1.1 What are Star Clusters, and What Can They Tell Us about Galaxies? 1 1.2 The Current Picture of Galaxy Formation and Evolution: How do spiral galaxies fit in? . 3 1.3 Models for Studying Extragalactic Star Clusters . 5 2 Two Populations of Old Star Clusters in the Spiral Galaxy M101 Based on HST/ACS Observations 8 2.1 Background . 8 2.2 Observations, Cluster Selection, and Completeness . 11 2.2.1 Observations . 11 vi 2.2.2 Cluster Selection . 14 2.2.3 Completeness . 29 2.3 Results and Analysis . 34 2.3.1 Cluster Luminosity Distribution . 34 2.3.2 Cluster Colors and Luminosities . 38 2.3.3 Sizes . 44 2.3.4 Spatial Distribution . 49 2.4 Discussion . 54 2.4.1 Are There Two Populations of Old Clusters in M101? . 54 2.4.2 What Are the Faint, Red Clusters in M101? . 55 2.5 Conclusions . 59 3 Gemini/GMOS Spectra of Old and Young Star Clusters in M101: Ages and Metallicities 62 3.1 Background . 62 3.2 Observations . 63 3.2.1 Cluster Candidate Selection with HST . 63 3.2.2 Gemini/GMOS Observations . 66 3.3 Results and Analysis . 74 3.3.1 Initial Age Categorization . 74 3.3.2 Measuring Ages and Metallicities Using BaSTI Models . 81 3.3.3 Ages and Metallicities . 88 3.3.4 Spatial-Metallicity Distribution . 91 3.4 Discussion . 92 3.4.1 Are There Young to Intermediate Age GCs in M101? . 92 3.4.2 Metallicities of the YMCs and GCs . 93 3.5 Conclusions . 94 vii 4 Gemini/GMOS Spectra of Old and Young Star Clusters in M101: Kinematics 96 4.1 Background . 96 4.2 Results and Analysis . 98 4.2.1 Velocity Measurements . 98 4.2.2 Velocity Distance Comparison . 101 4.2.3 Age Velocity Comparison . 106 4.2.4 Rotational Velocity Calculation and Comparison . 107 4.3 Discussion . 110 4.3.1 The Structure of M101 . 110 4.4 Conclusions . 111 5 Conclusions and Future Work 112 5.1 Searching for Faint, Old Star Clusters in Other Spiral Galaxies . 113 viii List of Tables 2.1 M101 Old Star Cluster Catalog . 15 2.2 Coefficients for Surface Density Fits . 53 3.1 Spatial and Photometric Properties of M101 Clusters with Spectra . 64 3.2 Spectroscopic Properties of M101 Cluster Spectra . 72 4.1 Kinematics of M101 Clusters with Spectra . 99 4.2 Coefficients for vcluster vs. Rsemi-minor Fits . 105 4.3 Rotational Velocity and Velocity Dispersion Comparisons . 109 ix List of Figures 1-1 Examples of MW Star Clusters . 6 1-2 Examples of LMC and M101 Star Clusters . 7 2-1 HST ACS/WFC Fields Covering M101 . 13 2-2 Radial Profiles of Stars vs. Clusters . 30 2-3 HST BV I Color Images of Typical Red M101 Star Clusters . 31 2-4 Fraction of Selected Artificial Clusters vs. mv . 32 2-5 Fraction of Selected Artificial Clusters vs. rgc . 33 2-6 Magnitude and Luminosity Distributions for Red M101 Clusters and MW GCs ...................................... 37 2-7 B − V vs. V − I for Red Clusters and YMCs in M101 . 41 2-8 B − I Color Histograms of Red M101 Clusters . 42 2-9 B − V Color Magnitude Diagram for Red M101 Clusters, MW GCs, LMC Intermediate Age Clusters, and MW Old Open Clusters . 43 2-10 reff Distribution for Red M101 Clusters and MW GCs . 46 2-11 MV vs. reff for Red M101 Clusters and MW GCs . 47 2-12 MV vs. reff for Red M101 Clusters and Artificial Clusters . 48 2-13 Positions of Red M101 Clusters . 50 2-14 rgc Distribution for Red M101 Clusters . 51 2-15 Surface Density Distributions of Red M101 Clusters and MW GCs . 52 3-1 Gemini-North Images of GMOS Mask 1 . 67 x 3-2 Gemini-North Images of GMOS Mask 2 . 68 3-3 HST BV I Color Images of M101 YMCs with Spectra . 69 3-4 HST BV I Color Images of M101 GCs with Spectra . 70 3-5 GMOS Spectra of Mask 1 YMCs . 75 3-6 GMOS Spectra of Mask 1 YMCs Continued . 76 3-7 GMOS Spectra of Mask 2 YMCs . 77 3-8 GMOS Spectra of Mask 1 GCs . 78 3-9 GMOS Spectra of Mask 1 GCs Continued . 79 3-10 GMOS Spectra of Mask 2 GCs . 80 3-11 Lick/IDS Index Defined Regions . 84 3-12 Lick/IDS Index Defined Regions Continued . 85 3-13 Index-index Model Grids From BaSTI Synthetic Spectra for Old Ages . 86 3-14 Index-index Model Grids From BaSTI Synthetic Spectra for Young Ages 87 3-15 Example of Rejected vs. Accepted Lick/IDS Indices for Fitting Ages and Metallicities . 89 3-16 [Fe/H] vs. log Age for M101 YMCs and GCs with Spectra . 90 3-17 [Fe/H] vs. rgc for M101 YMCs and GCs with Spectra . 91 4-1 Example Correlation Functions for an M101 YMC and GC . 102 4-2 Positions of the M101 YMCs and GCs with Spectra . 103 4-3 HI Gas Map with Line-of-Sight Velocity Contours . 104 4-4 vcluster vs. Rsemi-minor for the M101 YMCs and GCs with Spectra . 105 4-5 vcluster − vdisk vs. log Age for M101 YMCs and GCs with Spectra . 106 4-6 vrot vs. rgc for the M101 YMCs and HI Gas . 108 5-1 HST /WFC3 Field in NGC 6946 . 115 xi List of Abbreviations MW . Milky Way GC . globular cluster YMC . young massive cluster LMC . Large Magellanic Cloud SSP . simple stellar population LF . luminosity function HST ..................... Hubble Space T elescope WFPC2 . Wide Field Planetary Camera 2 ACS . Advanced Camera for Surveys CMD . color magnitude diagram WFC . Wide Field Channel PSF . point spread function S/N . signal-to-noise ratio FWHM .
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