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A Case Study of the Galactic H Ii Region M17 and Environs: Implications for the Galactic Star Formation Rate

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A Case Study of the Galactic H Ii Region M17 and Environs: Implications for the Galactic Star Formation Rate A Case Study of the Galactic H ii Region M17 and Environs: Implications for the Galactic Star Formation Rate by Matthew Samuel Povich A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Astronomy) at the University of Wisconsin – Madison 2009 ii Abstract Determinations of star formation rates (SFRs) in the Milky Way and other galaxies are fundamentally based on diffuse emission tracers of ionized gas, such as optical/near-infrared recombination lines, far- infrared continuum, and thermal radio continuum, that are sensitive only to massive OB stars. OB stars dominate the ionization of H II regions, yet they make up <1% of young stellar populations. SFRs therefore depend upon large extrapolations over the stellar initial mass function (IMF). The primary goal of this Thesis is to obtain a detailed census of the young stellar population associated with a bright Galactic H II region and to compare the resulting star formation history with global SFR tracers. The main SFR tracer considered is infrared continuum, since it can be used to derive SFRs in both the Galactic and extragalactic cases. I focus this study on M17, one of the nearest giant H II regions to the Sun (d =2.1kpc),fortwo reasons: (1) M17 is bright enough to serve as an analog of observable extragalactic star formation regions, and (2) M17 is associated with a giant molecular cloud complex, ∼100 pc in extent. The M17 complex is a significant star-forming structure on the Galactic scale, with a complicated star formation history. This study is multiwavelength in nature, but it is based upon broadband mid-infrared images from the Spitzer/GLIMPSE survey and complementary infrared Galactic plane surveys. I begin by measuring the global spectral energy distribution from the integrated infrared–radio emission of the M17 H II region and photodissociation region (PDR). The morphology of the infrared emission is consistent with a wind- blown bubble model for the H II region, with a large, central cavity surrounded by ionized gas. I report the discovery of 6 stellar-wind bow shocks driven by O stars in M17 and in (or near) the even larger H II region RCW 49. The bow shocks graphically illustrate the important role played by stellar winds in shaping the structure and dynamics of H II regions and lead to the identification of 4 new candidate O stars. A dramatic, new view of the M17 environs is provided by the high-resolution, wide-area coverage of GLIMPSE. I report the discovery that the M17 H II region itself is located on the rim of a 20-pc diameter bubble outlining the PDR of a ∼5MyroldHII region. This configuration raises the tantalizing possibility that the formation of M17 ionizing cluster was triggered by the expansion of the bubble. Several dozen intermediate-mass (∼>3M) young stellar objects (YSOs) are distributed around the rim of the bubble, indicating even more recent triggered star formation. I estimate the star formation rate in the extended M17 environment from the YSO mass function (YMF), and compare this to the star formation rate in −1 the central ionizing cluster, SFRX ∼ 0.01 M yr , derived from the observed X-ray luminosity function iii (XLF) of the young stellar population. The XLF and the YMF both show great promise for establishing new calibrations of star formation rates the Galaxy, since both trace the intermediate-mass range of the IMF rather than the extreme high-mass tail. I conclude with comparisons between SFRX and the SFRs predicted for M17 by widely-used calibra- tions based on IR and radio continuum emission. The results imply that current SFR determinations based on ionized gas tracers are uncertain by factors of ∼2–3, due chiefly to large uncertainties in the masses and ionizing photon production rates of O stars and the need to correct for the high binary fractions observed among massive stars. iv To Kerean — The scope of space and time is vast beyond comprehension, and yet, against all odds, we found each other. and To Jim — I will “keep looking up,” and when I do, I will be thinking of you. v Acknowledgments It seems to me that the process of working toward a Ph.D. needs to be its own reward; if the end goal is all that is desired, the experience of several years of post-graduate study may devolve into an odyssey of frustration and suffering. My education has become my career, and my dissertation now seems more like a conjunction than an exclamation point. My task would have been made much more difficult, and my journey far less pleasant, if not for the people who have influenced my life and my path during these past five years. I have been extremely fortunate to work with an advisor, Ed Churchwell, who is more than a great scientist; he is a first-rate human being in all respects. I thank Ed for his friendship and his tutelage, his gentle encouragement and steady supply of ideas, and his refraining from vanishing following his retirement a few years back. I have also benefited immensely from my interactions with my unofficial “lieutenant advisor,” Barbara Whitney, who typifies the smart, savvy, and grounded astronomer I have always admired. I am grateful to all of my coauthors on the papers that constitute Chapters 2, 3, and 4, the backbone of this thesis, and especially Marilyn Meade, Brian Babler, Thomas Robitaille, and Robert Benjamin—they will know why they have been singled out here. I thank Debra Shepherd and Christer Watson for bringing me on board papers they lead and then allowing me free reign to develop and test new analysis techniques that became central to this Thesis. Conversations with Leisa Townsley and Jay Gallagher were helpful not just for their scientific content, but for keeping me excited about my own science as well. John Raymond, my undergraduate mentor, was instrumental in setting me on my professional trajectory and equipping me to succeed as a graduate student. Kenny Wood also made an enormous imprint on my early experiences in astronomical research, and as Madison’s chief cheerleader, Kenny strongly influenced my choice of graduate school. It turns out to have been an excellent choice. IacknowledgetheSpitzer telescope and instrument teams, and the staff at the Spitzer Science Center, for producing and operating one of the greatest observatories in the known universe, without which this thesis certainly would not have been possible. I am also grateful to the National Science Foundation for the award of an Astronomy & Astrophysics Postdoctoral Fellowship, the vehicle for the next stage of my professional development. The graduate students in the Astronomy Department are a remarkably intelligent, gifted, active, funny, and tight-knit group of people. I have greatly enjoyed spending time in their company, and someday vi I might even forgive them for electing me Student Czar. I give special thanks to my fellow inmates of Sterling 5533, who put up with much nonsense from me over the years: Aaron Geller, Laura Trouille, Ryan Keenan, Tommy Nelson, and Ella Braden. I will also always carry around fond memories those nights spent sitting at the City Bar (or occasionally in my living room) playing euchre with Kyle Westfall, Nathaniel Doane, and Aaron. I dedicate this Thesis to two of the most important people in my life: my wife, Kerean Grant Povich, and my late, dear friend James Clark. I am forever grateful to Kerean for agreeing to spend three years in the frozen tundra of Wisconsin for the love of a humble graduate student—if not for Kerean, this Thesis almost certainly would have been longer in the making. She is, quite simply, the most amazing woman I have ever met. I believe Jim thought so too. I had known Jim all of my life and hardly ever had I seen him make a new friend as quickly as he adopted Kerean. The two of them should enjoy sharing the dedication page. Jim was the world’s most gentle soul, always quick to laugh, devoted friend, avid stargazer. He and I taught ourselves the constellations together using H. A. Rey’s book, a red-filtered flashlight, and the dark skies of Downeast Maine. I mention last the two people who have given me their unconditional love and support from the very beginning, my parents, Judith and Michael Povich. The knowledge that they were always behind me has given me the confidence to pursue my dreams and to choose the more difficult, risky path, when it mattered most. vii Contents 1 Introduction 1 1.1 H II RegionsandStarFormationRates............................. 2 1.1.1 Extragalactic Star Formation Parameterizations . 3 1.1.2 TheMilkyWayAsaStar-FormingGalaxy....................... 7 1.2TheGalacticPlaneintheAgeofIRSurveys.......................... 10 1.2.1 IR Surveys Before Spitzer ................................. 11 1.2.2 The Spitzer GalacticPlaneSurveys............................ 11 1.3ThesisGoalsandOutline..................................... 13 References................................................. 15 2 A Multiwavelength Study of M17: The Spectral Energy Distribution and PAH Emission Morphology of a Massive Star-Formation Region 18 Abstract.................................................. 19 2.1Introduction............................................. 20 2.2TheData.............................................. 22 2.2.1 GLIMPSEand2MASS.................................. 22 2.2.2 MSX ............................................. 23 2.2.3 IRAS ............................................ 23 2.2.4 MAGPISRadioContinuum................................ 24 2.2.5 Spitzer/IRSSpectra.................................... 24 2.3SEDandEnergyBalance....................................
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