A Dissertation Entitled Lighting the Dark Molecular Gas and a Bok
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A Dissertation entitled Lighting the dark molecular gas and a Bok globule by Aditya G. Togi Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics Dr. John-David T. Smith, Committee Chair Dr. Adolf Witt, Committee Member Dr. Lee Armus, Committee Member Dr. Rupali Chandar, Committee Member Dr. Sanjay Khare, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo May 2016 Copyright 2016, Aditya G. Togi 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 Lighting the dark molecular gas and a Bok globule by Aditya G. Togi Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics The University of Toledo May 2016 Stars are the building blocks of galaxies. The gas present in galaxies is the pri- mary fuel for star formation. Galaxy evolution depends on the amount of gas present in the interstellar medium (ISM). Stars are born mainly from molecular gas in the GMCs. Robust knowledge of the molecular hydrogen (H2) gas distribution is neces- sary to understand star formation in galaxies. Since H2 is not readily observable in the cold interstellar medium (ISM), the molecular gas content has traditionally been inferred using indirect tracers like carbon-monoxide (CO), dust emission, gamma ray interactions, and star formation efficiency. Physical processes resulting in enhance- ment and reduction of these indirect tracers can result in misleading estimates of molecular gas masses. My dissertation work is based on devising a new temperature power law distribution model for H2, a direct tracer, to calculate the total molecular gas mass in galaxies. The model parameters are estimated using mid infrared (MIR) H2 rotational line fluxes obtained from IRS-Spitzer (Infrared Spectrograph- Spitzer) instrument and the model is extrapolated to a suitable lower temperature to recover the total molecular gas mass. The power law model is able to recover the dark molec- ular gas, undetected by CO, in galaxies at metallicity as low as one-tenth of our Milky Way value. I have applied the power law model in U/LIRGs and shocks of Stephan’s Quintet to understand molecular gas properties, where shocks play an important role in exciting H2. Comparing the molecular gas content derived through our power law iii model can be useful in studying the application of our model in mergers. The pa- rameters derived by our model is useful in understanding variation in molecular gas properties in shock regions of Stephan’s Quintet. Low mass stars are formed in small isolated dense cores known as Bok globules. Multiple star formation events are seen in a Bok globule. In my thesis I also studied a Bok globule, B207, and determined the physical properties and future evolutionary stage of the cloud. My thesis spans studying ISM properties in galaxies from kpc to sub-pc scales. Using the power law model in the coming era of James Webb Space Telescope (JWST) with the high sensitivity MIR Instrument (MIRI) spectrograph we will be able to understand the properties of molecular gas at low and high redshifts. iv This work is dedicated to my determination to pursue astronomy from past 25 years Acknowledgments There are number of people that deserve special thanks as they have played a spe- cial role in successful completion of this thesis. Foremost, I would like to thank my advisor, Dr. J. D. Smith for offering me to join his group and continue in pursuing my research on galaxy ISM. His knowledge, insight, patience, encouragement, and desire to help have been valuable stimulation for my scientific development. I want to express my gratitude to Dr. Adolf Witt who has imparted the scientific wisdom and amazed me with his scientific working style. I am very much thankful for their advise on writing papers and my scientific afternoon talks with Dr. Witt on astronomy to politics to philosophy will long be cherished. I want to express my thanks to Dr. Lee Armus, who has been an inexhaustible source of scientific and technical wisdom and is a wonderful person to work with. I am thankful to him for all the support he provided during my IPAC Visiting Graduate Student Fellowship program at Caltech. I am thankful to my other committee members Dr. Rupali Chandar and Dr. Sanjay Khare for accepting to be in my Ph.D. committee. I am grateful to to Dr. Rick Irving for all the help and took out time in resolving my computer issues. Finally, I would like to thank all graduate students at the Uni- versity of Toledo. Whether talking fundamental physics, helping with code, or editing my writing, their help was fundamental to both my work and especially tolerating my insanity throughout. vi I have always cherished the company of my friends Amogh, Hemant, Nikhil, Prab- hakar, Rajesh, Santosh Kumar, Shaiju, Sushilkumar, Venkatesh, Vidhi Mishra, Vish- wanath, and an endless list of people who always heard and supported me during times of distress. Finally, I would like to thank my family members for their constant support and motivation throughout my academic journey. Thank you all for having been with me in this wonderful journey. vii Contents Abstract iii Acknowledgments vi Contents viii List of Tables xiii List of Figures xv List of Abbreviations xviii 1 The Interstellar Medium (ISM) 1 1.1 Introduction................................ 1 1.2 ConstituentsoftheISM ......................... 2 1.2.1 TheISMGas ........................... 3 1.3 EvolutionofISMdust .......................... 6 1.3.1 Dust composition and size distribution . 6 1.3.2 DustprocessingintheISM . 8 1.4 Thecosmiclifecycle ........................... 9 1.5 ISMmoleculargas ............................ 11 1.5.1 Importance of H2 ......................... 11 1.5.2 Detection techniques of molecular gas . 11 1.5.3 DarkMolecularGas ....................... 13 viii 1.6 Thesisoutline ............................... 15 2 Lighting the dark molecular gas: H2 as a direct tracer 17 2.1 Introduction................................ 17 2.2 Sample................................... 20 2.2.1 MIR H2 rotationallinefluxes . 21 2.2.2 Cold molecular gas mass from CO line intensities . .. 21 2.3 Model ................................... 23 2.4 Method&Calibration .......................... 28 2.4.1 Warm H2 ............................. 28 2.4.1.1 Upper temperature, Tu ................ 29 2.4.1.2 Powerlawindex,n . 29 2.4.2 Total H2 .............................. 31 2.4.2.1 Model Sensitivity Temperature, Ts .......... 32 2.4.2.2 Model extrapolated lower temperature, Tℓ ...... 38 2.4.2.3 Mass distribution function . 40 2.5 Results, Discussions, & Applications . ... 41 2.5.1 What are the typical molecular gas temperatures in galaxies? 41 2.5.2 Estimating total MH2 ....................... 43 2.5.3 Model derived molecular gas mass in ULIRGs, LIRGs and radio galaxies .............................. 45 2.5.4 Effect of dust temperature on the warm H2 fraction . 47 2.5.5 Molecular gas in low metallicity galaxies . .. 51 2.5.5.1 Metallicity estimation . 52 2.5.5.2 Cold molecular gas from CO line emission . 52 2.5.5.3 Molecular gas from dust emission . 53 2.5.5.4 Molecular gas mass using our model . 53 ix 2.5.6 Futureprospects ......................... 57 2.6 Summary ................................. 57 3 Molecular gas properties in ISM of U/LIRGs of GOALS 75 3.1 Introduction................................ 75 3.2 Sample................................... 77 3.2.0.1 AncillarydataCO(J=1–0). 77 3.2.1 DatareductionandSpectralfitting . 78 3.3 Analysis .................................. 78 3.3.1 Disktemplate........................... 80 3.3.2 Scaling and adding the template spectrum . 80 3.3.3 PAHFIT- to recover H2 lineflux................. 82 3.3.4 H2 gasmassfrompowerlawmodel . 82 3.4 Results................................... 84 3.4.1 LIRG’sdisktemplate. 84 3.4.2 Powerlawindex.......................... 85 3.4.3 H2 mass- extrapolating power law model to 49 K . 86 3.5 Discussions ................................ 87 3.5.1 Low power law index in U/LIRGs . 87 3.5.2 Relation between LIR, PAHs and power law index . 88 3.5.3 Low αCO orhightemperature . 92 3.5.4 Tℓ fromgas-to-dustmassratioGDR . 92 3.5.5 Variation in the L850 ...................... 95 MISM 3.5.6 Caveats .............................. 97 3.6 Summary&Conclusions . .. .. 98 4 Molecular gas properties in shocks of Stephan’s Quintet 106 4.1 Introduction................................ 106 x 4.2 H2 inshocksofStephan’sQuintet . 108 4.3 Observations, Data reduction and Analysis . .... 109 4.3.1 MIR H2 emissionandspectra . 110 4.4 ExcitationdiagramFits . 111 4.4.1 Discretetemperaturefit . 111 4.4.2 Powerlawmodelfit. .. .. 129 4.5 Result ................................... 130 4.5.1 Twotemperaturefits . 130 4.5.2 H2 lineratios ........................... 132 4.5.3 Powerlawindex.......................... 133 4.5.4 Total and Warm H2 gasmass .................. 137 4.5.4.1 Total H2 gasmass ................... 137 4.5.4.2 Warm H2 fraction ................... 140 4.6 Summary ................................. 141 5 Physical properties of the cometary globule: the case of B207 143 5.1 Introduction................................ 143 5.2 Observations&ArchivalData . 144 5.2.1 Discovery Channel Telescope (DCT) observations . ... 145 5.2.2 Archival WISE and Herschel data . 147 5.3 Analysis .................................. 147 5.3.1 Surfacebrightnessprofiles . 148