Class I Methanol Masers Toward External Galaxies
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CLASS I METHANOL MASERS TOWARD EXTERNAL GALAXIES by Tiege P. McCarthy, B.Sc. Hons Submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy School of Natural Sciences University of Tasmania January, 2020 This thesis may be made available for loan and limited copying in accor- dance with the Copyright Act 1968 Signed: Tiege P. McCarthy Date: 08/01/2020 ABSTRACT The role extragalactic class I methanol masers play in their host galaxies is currently not well understood. With only a few known examples of these masers to date, inves- tigating their nature is very difficult. However, determining the physical conditions and environments responsible for these masers will provide a useful tool for the study of external galaxies. In this thesis we present over a hundred hours of Australia Tele- scope Compact Array (ATCA) observations aimed at expanding our available sample size, increasing our understanding of the regions in which these masers form,and making comparison with class I methanol emission towards Galactic regions. We present the first detection of 36.2-GHz class I methanol maser emission toward NGC 4945, which is the second confirmed detection of this masing transition and the brightest class I maser region ever detected. This emission is offset from the nucleus of the galaxy and is likely associated with the interface region between the bar and south- eastern spiral arm. Additionally, we report the first detection of the 84.5-GHz class I maser line toward NGC 253, making NGC 253 the only external galaxy displaying 3 separate masing transitions of methanol. Upper limits on the 36.2- class I and 37.7- GHz class II maser lines are also reported for 12 other galaxies that were searched as part of this project. Alongside observations of the class I methanol lines, we also observed various thermal and masing molecular lines towards these sources. This includes the imaging of 7- mm HC3N, CS and SiO lines towards NGC 4945, which had not previously been observed at high resolution before, along with some3-mm lines, such as HNCO, towards NGC 253. v Long termmonitoring of the 36.2-GHz transition from NGC 4945 and NGC 253 was also performed. This monitoring produced detections of new maser regions in both NGC 4945 and NGC 253 and provided the first evidence for variability from these extragalactic class I masers, with the luminosity of the new maser region in NGC 4945 increasing by approximately 300 percent compared to past observations. We determine this is likely the result of excitation changes in the maser environments, however, further monitoring is required to accurately quantify this variability. We also present interferometric imaging of the 95.2-GHz class I methanol masers towards 32 6.7-GHz class II maser sources along with detailed comparison between the class I (36.2-, 44.1- and 95.2-GHz) and class II (6.7- and 12.2-GHz) lines in these high- mass star formation regions. Flux-density ratios for matched (spatially associated) 44.1- and 95.2-GHz class I masers shows that the 3:1 ratio previously reported in the literature holds true even at parsec scales. Investigation of our statistically complete sample indicates that 95.2-GHz class I methanol masers do not preferentially favour more evolved sources. Our observations reinforce that when using class I methanol masers to investigate shock fronts and star-formation region dynamics, observation of multiple class I transitions is necessary in order to form acomplete picture. ACKNOWLEDGEMENTS The three and a half years of my candidature has produced many people I would like to thank. First I would like to thank my partner Sophia for supporting me, dealing with my stress and loving me through the past 8 years of my university career. Without you I would not likely be doing what I am today, and for that, I am incredibly grateful. Next I would like to thank myprimary supervisor Simon Ellingsen. Thank you for still giving me another shot despite the disaster that was my honours project. Thank you for being available all the time, for spur of the moment questioning or requests for help. Without your constant support and expertise there is no way I would have finished, let alone enjoyed my candidature as much as I did. I would also like to thank Maxim Voronkov, my co-supervisor, for despite being in- credibly busy, always taking the time to respond to myemails, review/comment on my papers, and explain maser theory in great detail. Also, thank you for helping show metheropesofATCA observing as a fresh Ph.D student in Narrabri. Also, thank you to Shari Breen, who ever since our trip to Narrabri in 2017 has become my unofficial co-supervisor. Thank you for noob checking my CABB schedule files in the early days, teaching mehowtocompact array, and allowing metobeapartof the StarFISH survey team. Thank you to Kelly and Karen for taking care of everything on the administration side of things. You made all travel organisation as stress free as possible and always found extra ways to throw more money at us Ph.D students. Thank you to myofficemate Andrew for putting up with my occasional ranting and engaging in regular pointless, yet enjoyable discussions. Thank you to all members vii of the regular coffee aquisition crew over the past few years (Andrew, Eloise, Ga- bor, Jonny, Katie, Lucas, Patrick and others), for the countless hours of enjoyable procrastination both at and outside of university. Finally, thank you to myfamily, for supporting menomatter what path I decided to take in life. TABLE OF CONTENTS TABLE OF CONTENTS i LIST OF TABLES iv LIST OF FIGURES v 1 ASTRONOMICAL MASERS, METHANOL, AND STARBURST GALAXIES 1 1.1 AstronomicalMasers............................ 1 1.1.1 Overview of interstellar masingspecies.............. 3 1.1.2 Megamasers and extragalactic masers.............. 5 1.2MethanolMasers.............................. 8 1.2.1 Class I and class II methanol masers............... 9 1.2.2 Methanol masers within the Milky Way ............. 10 1.2.3 Methanol maserstowardsothergalaxies............. 13 1.2.4 Comparison of methanol maser transitions from the same source 16 1.2.5 Probing the fundamentalconstantsofphysics.......... 21 1.3 The Extragalactic Targets......................... 22 1.3.1 NGC 4945 .............................. 22 1.3.2 NGC253.............................. 25 1.4 Thisproject................................. 28 2 The relationship between Class I and Class II methanol masers at high angular resolution 30 2.1Introduction................................. 30 TABLE OF CONTENTSii 2.2AuthorContributions........................... 31 3 Detection of 36.2-GHz Class I methanol maser emission toward NGC 4945 50 3.1Introduction................................. 50 3.2AuthorContributions........................... 51 4 Investigations of the class I methanol masers in NGC 4945 57 4.1Introduction................................. 57 4.2AuthorContribution............................ 58 5 Detection of 84-GHz class I methanol maser emission toward NGC 253 70 5.1Introduction................................. 70 5.2AuthorContribution............................ 71 6 Variability in extragalactic class I methanol masers 79 6.1Introduction................................. 79 6.2AuthorContribution............................ 80 7 Discussion, conclusions and future work 96 7.1 The methanol maser emission of NGC 4945 ............... 96 7.2 ThreeclassItransitionstowardNGC253................ 98 7.3 Extragalactic methanol masers...................... 100 7.3.1 NGC253.............................. 101 7.3.2 IC342................................ 101 7.3.3 NGC 6946 .............................. 102 7.3.4 Maffei2............................... 103 7.3.5 NGC 1068 .............................. 103 7.3.6 General discussion on extragalactic class I masers........ 103 7.4Futurework................................. 105 7.4.1 Searching for new extragalactic 36.2-GHz masers........ 105 7.4.2 Class I transition follow-up on known maserhosts....... 108 7.4.3 Radiative transfer modelling ................... 108 7.5 Concluding remarks............................ 111 TABLE OF CONTENTS iii A 95-GHz Maser Field Images and Spectra 113 A.1 95-GHz maser field images......................... 114 A.2 95-GHz maserspectra........................... 130 BIBLIOGRAPHY 141 LIST OF TABLES 7.1 Potential targets for a 36.2-GHz search with the JVLA. ........ 106 7.2 Potential targets for a 36.2-GHz search with the ATCA......... 107 LIST OF FIGURES 1.1 Methanol rotational energy level diagram including known class I methanol transitions.................................. 12 1.2 Original detection of 36.2-GHz methanol maser emission towards NGC 253. 14 1.3 Maser optical depth contour diagram as a function of methanol column density and H2 density at two different temperatures.......... 17 1.4 Optical depth of the four class I methanol lines vs. temperature across three different H2 densities......................... 20 1.5 Optical depth as a function of H2 density across three different tem- peratures................................... 20 1.6 Optical image of NGC 4945 taken using the 2.2-m MPG/ESO telescope in La Silla. ................................. 23 1.7 Optical image of NGC 253 as seen by the Wide Field Imager on the 2.2-m MPG/ESO telescope in La Silla. .................. 27 Chapter 1 Astronomical Masers, Methanol and Starburst Galaxies This chapter will begin with a general overview of astronomical masers, both within our Galaxy and towards extragalactic sources. Following this will be a review of the literature