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High-Velocity Clouds Around the Andromeda Galaxy and the Milky Way

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The Relics of Structure Formation High-Velocity Clouds around the Andromeda Galaxy and the Milky Way Dissertation zur Erlangung des Doktorgrades (Dr. rer. nat.) der Mathematisch-Naturwissenschaftlichen Fakult¨at der Rheinischen Friedrich-Wilhelms-Universit¨at Bonn vorgelegt von Tobias Westmeier aus Korbach Bonn, Februar 2007 Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakult¨at der Rheinischen Friedrich-Wilhelms-Universit¨at Bonn. Erstgutachter und Betreuer: PD Dr. J¨urgen Kerp Zweitgutachter: Prof. Dr. Klaas S. de Boer Fachnaher Gutachter: Prof. Dr. Klaus Desch Fachangrenzender Gutachter: Prof. Dr. Andreas Bott Externe Gutachterin: Dr. Mary E. Putman Tag der Promotion: 6. Juni 2007 “Our belief in any particular natural law cannot have a safer basis than our unsuccessful critical attempts to refute it.” (Sir Karl R. Popper, “Conjectures and Refutations: The Growth of Scientific Knowledge”, 1969) Part of this work is based on observations with the 100-m radio telescope of the MPIfR (Max-Planck-Institut f¨ur Radioastronomie) at Effelsberg. The Westerbork Synthesis Radio Telescope is operated by the ASTRON (Netherlands Foundation for Research in Astronomy) with support from the Netherlands Foundation for Scientific Research (NWO). The National Radio Astronomy Observatory is a facility of the National Science Founda- tion operated under cooperative agreement by Associated Universities, Inc. Contents Abstract – Kurzfassung 11 1 Introduction 15 1.1 The discovery and definition of HVCs . 15 1.2 The distribution of HVCs across the sky . ..... 16 1.3 ThephysicalparametersofHVCs. 20 1.3.1 Radialvelocities ............................. 20 1.3.2 Distances ................................. 21 1.3.3 Metalabundances ............................ 23 1.3.4 Multi-phasestructure . 24 1.3.5 Interaction with the ambient medium . 25 1.4 HVC detections at other wavelengths . ..... 26 1.4.1 MoleculargasinHVCs . 26 1.4.2 Thermal dust emission from HVCs . 27 1.4.3 Hα emissionfromHVCs......................... 27 1.4.4 X-rayemissionfromHVCs . 28 1.5 TheoriginofHVCs................................ 29 1.5.1 Hypotheses about the origin of HVCs . 29 1.5.2 HVCs as primordial dark-matter halos? . 31 1.5.3 Compacthigh-velocityclouds . 32 1.6 Summary ..................................... 35 2 Effelsberg H I survey for HVCs around M 31 37 2.1 TheAndromedaGalaxy ............................. 37 2.1.1 Historicstudies.............................. 37 2.1.2 Modernstudies.............................. 39 2.1.3 TheHVCpopulationofM31 . 41 2.2 Observations, data reduction, and analysis . ......... 42 2.2.1 Observations ............................... 42 2.2.2 Datareduction .............................. 45 2.2.3 Dataanalysis ............................... 47 2.2.4 Completenessofthedata . 47 2.3 Search for the CHVC population of M 31 . 51 5 6 Contents 2.3.1 DiscoveryofCHVCcandidates . 53 2.3.2 Consequences of our non-detection . 54 2.4 ThedetectedpopulationofHVCs. 58 2.4.1 Generalproperties ............................ 58 2.4.2 Discussionofindividualobjects . 65 2.5 Comparison with models and theoretical predictions . .......... 67 2.5.1 The Local Group population model of de Heij et al. (2002a) . 67 2.5.2 Comparison with the satellite galaxies of M 31 . ..... 75 2.5.3 Comparison with CDM simulations . 78 2.6 Summaryandconclusions . 80 3 Follow-up synthesis observations of M 31 HVCs 83 3.1 Introduction.................................... 83 3.2 Observations and data reduction . 85 3.2.1 Sampleselection ............................. 85 3.2.2 Observations ............................... 85 3.2.3 Datareduction .............................. 86 3.3 Results....................................... 88 3.3.1 GeneralpropertiesoftheHVCs. 88 3.3.2 DescriptionofindividualHVCs . 90 3.4 The internal structure of the HVCs . 93 3.4.1 Multi-phasestructure . 93 3.4.2 Dynamicalmasses ............................ 94 3.4.3 Virialmasses ............................... 95 3.5 TheoriginoftheHVCs ............................. 96 3.5.1 Evidencefortidalinteraction . 96 3.5.2 Evidence for dark-matter-dominated clouds . 98 3.5.3 Comparison with the satellite galaxies of M 31 . 100 3.6 Summaryandconclusions . 102 4 Low column density gas in the Galactic halo 115 4.1 Introduction.................................... 115 4.2 Observations ...................................117 4.2.1 OpticalspectroscopywithUVES . 117 4.2.2 H I observationswithEffelsberg. .118 4.2.3 H I observations with the VLA . 118 4.3 Resultsanddiscussion . 119 4.4 Summaryandconclusions . 122 5 Summary and outlook 125 5.1 ThesearchforHVCsaroundM31 . 125 5.2 Optical absorption line studies of HVCs around M 31 . ........128 List of common abbreviations 131 Bibliography 133 List of Figures 1.1 The25-mradiotelescopeatDwingeloo. 16 1.2 H I Column density map and spectrum of HVC 357.8+12.4 159. 17 − 1.3 All-sky H I column density map of high-velocity clouds. 18 1.4 All-sky radial velocity map of high-velocity clouds. .......... 19 1.5 Position-velocity diagrams of a model HVC population. ......... 21 1.6 H I spectrum of HVC 050 68 193 showing a two-component line profile. 24 − − 1.7 Examples for head-tail and horseshoe morphology. ........ 25 1.8 Sketch of possible formation scenarios for HVCs. ......... 29 1.9 Cumulative number of dark-matter halos as a function of circular velocity. 31 1.10 H I column density map of HVC 289+33+251. 33 1.11 All-sky map showing the distribution and velocities of CHVCs. 34 2.1 The Andromeda Galaxy at different wavelengths. ...... 38 2.2 The100-mradiotelescopeatEffelsberg. 40 2.3 LDS map showing the outline of our Effelsberg survey of M 31. ....... 42 2.4 Two example spectra from the Effelsberg H I blind survey of M 31. 44 2.5 Flow chart of the spectral baseline correction procedure............ 45 2.6 Antenna pattern of the Effelsberg telescope at λ =21cm. .......... 46 2.7 The geometry of the field mapped around M 31. 49 2.8 Spatial completeness of the M 31 survey as a function of projected distance. 50 2.9 Fraction of detected spectral lines as a function of signal-to-noise ratio. 51 2.10 H I column density maps of our blind survey of M 31. 52 2.11 Map of the positions of HVC/CHVC candidates. ...... 54 2.12 Example spectra of two CHVC candidates. ..... 55 2.13 Locations of the 17 HVCs detected in the Effelsberg survey.......... 58 2.14 Two position-velocity diagrams across M 31 and its HVCs. .......... 59 2.15 Histograms of the observational parameters of the 17 detected HVCs. 61 2.16 H I spectra of HVCs number 1–9 around M 31. 62 2.17 H I spectra of HVCs number 10–17 around M 31. 63 2.18 Comparison of H I and virial masses of the HVCs. 64 2.19 H I columndensitymapofDavies’Cloud. 65 2.20 The extra-planar gas near the eastern and south-eastern edge of M 31. 66 2.21 Radial distribution and mass spectrum of the HVCs aroundM31. ..... 72 7 8 List of Figures 2.22 Distribution of HVCs as a function of projected distance and H I mass. 73 2.23 KS probability for different radial scale lengths. ........... 74 2.24 KS probability for different radial scale lengths, combining the HVC complex. 75 2.25 Comparison of HVCs and satellite galaxies around M 31. .......... 77 2.26 H I mass vs. projected distance of HVCs and satellite galaxies around M 31. 78 2.27 Distribution of dark-matter satellites around their parent halo. 79 3.1 The Westerbork Synthesis Radio Telescope. ....... 84 3.2 The uv coverage and continuum map of one of our observations. 85 3.3 Overview of the 9 fields observed around M 31 with the WSRT. ...... 88 3.4 Densities, velocities, and velocity gradients of M 31 HVCs 1–12. 90 3.5 H I column densities and radial velocities of M 31 HVCs 2–12. ..... 92 3.6 Two position-velocity diagrams across M 31 HVCs 1–12. ........ 93 3.7 High-resolution H I spectra of M 31 HVC 1 and M 31 HVC 7. 94 3.8 Comparison of RGB stars and H I gasaroundM31. 97 3.9 Comparison of HVCs and satellite galaxies around M 31. .........100 3.10 H I mass vs. projected distance of HVCs and satellite galaxies around M 31. 101 3.11 MapsandprofilesofM31HVC1. .104 3.12 MapsandprofilesofM31HVC2. .105 3.13 MapsandprofilesofM31HVC3. .106 3.14 MapsandprofilesofM31HVC4. .107 3.15 MapsandprofilesofM31HVC6. .108 3.16 MapsandprofilesofM31HVC13.. 109 3.17 MapsandprofilesofM31HVC14.. 110 3.18 MapsandprofilesofM31HVC15.. 111 3.19 MapsandprofilesofM31HVC16.. 112 3.20 High-resolution maps of M 31 HVCs 1 and 2. 113 3.21 High-resolution maps of M 31 HVCs 3 and 5. 114 4.1 H I, Ca II, and Na I spectra in the direction of PKS 1448 232. 116 − 4.2 The uv coverage and continuum map of our VLA observations. 119 4.3 H I map of the HVCs observed in the direction of PKS 1448 232. 120 − 4.4 H I spectra of the HVCs observed in the direction of PKS 1448 232. 121 − 5.1 Comparison of M 31 in neutral hydrogen and RGB stars. .......127 5.2 Ca II absorption in the direction of two quasars . 129 List of Tables 2.1 Observational parameters of the Effelsberg H I blind survey of M 31. 43 2.2 List of the 17 discrete HVCs around M 31 detected in our survey. 60 2.3 ListofM31satellitegalaxies. ..... 76 3.1 Physical parameters of the high-velocity clouds found near M 31. 87 3.2 Determination of the dynamical masses of M 31 HVCs 1, 2, 6, 8, and 10. 95 4.1 Summary of the UVES Ca II and Na I observations of PKS 1448 232. 122 − 4.2 Summary of the VLA H I observations in the direction of PKS 1448 232. 122 − 9 10 List of Tables Abstract – Kurzfassung English More than 40 years after their discovery by Muller et al. (1963) the origin of high-velocity clouds (HVCs) is still in dispute, the main reason being the lack of reliable distance measurements for most HVCs around the Milky Way. Consequently, a large number of different hypotheses about the origin of HVCs have been proposed over the past decades.
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