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THE CLOUDY HI HALO OF THE MILKY WAY Dissertation zur Erlangung des Doktorgrades (Dr. rer. nat.) der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn vorgelegt von Leonidas Dedes aus Veroia (Griechenland) Bonn, Juni 2008 Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn 1. Referent: PD Dr. Jürgen Kerp 2. Referent: Prof. Dr. Ulrich Klein Tag der Promotion: 02.09.2008 Erscheinungsjahr 2008 Diese Dissertation ist auf dem Hochschulschriftenserver der ULB Bonn unter: http://hss.ulb.uni-bonn.de/diss_online elektronisch publiziert. Contents 1 Introduction 1 2 Method 9 2.1 Aim .................................. 9 2.2 Properties of the HI clumps..................... 9 2.3 Detection of HI clumps........................ 10 2.4 Distancedetermination . 11 2.5 MilkyWaymassmodel. 14 2.6 Measuring the observational properties of the clumps . 15 3 The spiral structure in the Milky Way 21 3.1 Introduction .............................. 21 3.2 The model of the Galactic spiral structure . 22 3.3 Results ................................ 24 3.4 Dependenceonb .......................... 29 3.5 Discussion .............................. 32 4 A search for the structure of the gaseous HI halo using the Effels- berg 100-m telescope 43 4.1 Introduction .............................. 43 4.2 Technical details and selection criteria . 43 4.3 Theclump116.20+23.55 . 45 4.4 Theclump115.00+24.00 . 49 4.5 Effelsberg sample of HI haloclumps.. 51 5 Synthesis Observations of HI clumps 61 5.1 Introduction .............................. 61 5.2 W.S.R.Tobservations . 61 5.2.1 TechnicalDetails . 61 5.2.2 Results ............................ 62 5.3 V.L.Aobservations . 66 5.3.1 TechnicalAnalysis . 66 5.3.2 Results ............................ 67 5.4 SummaryofInterferometryResults. 68 6 Arecibo Observations 71 6.1 Introduction .............................. 71 6.2 Technicaldetails .. .. .. .. .. .. .. .. .. .. .. .. .. 71 6.3 Extended emission in Arecibo and Stray Radiation correction . 72 6.4 Results ................................ 75 6.5 Clump at α=07h049m46.60′′, δ=04d32m32.006′′ .......... 81 7 Thermal equilibrium of the neutral component of the gaseous Galac- tic halo as inferred from the study of the physical properties of HI clumps 89 7.1 Introduction .............................. 89 7.2 Summary of the Observational and Physical Properties of the HI clumps. .............................. 89 7.3 Thermal equilibrium in the neutral gas . 91 7.4 HI clumps and thermal equilibrium . 93 7.5 Stability and evaporation of the HI clumps ............. 97 8 Power spectrum analysis of the Arecibo data 99 8.1 Introduction .............................. 99 8.2 Analysis................................ 99 8.3 Results ................................101 9 Discussion 107 9.1 Introduction ..............................107 9.2 Measurements and errors . 107 9.3 Distancedetermination . .109 9.4 Galacticfountain . .111 9.5 Comparison with the CNM phase:Clumps or sheets . 114 10 Conclusions 121 11 Appendix I 125 12 Appendix II:Brightness temperature ratio K of a mix of warm/cold neutral gas. 129 13 Acknowledgments 135 14 Summary 137 References 141 List of Figures 145 List of Tables 149 1 1 Introduction Neutral hydrogen is one of the most important components of the visible matter in the universe. From the time of the theoretical predictions of the 21-cm line, by the Dutch astronomer van der Hulst (1945) up to today, observations of the neutral hydrogen emission help to expand the knowledge in the field of Radio Astronomy. Almost five decades ago, Oort et al. (1958) for the first time brought to light the large scale mor- phology of our Galaxy, the Milky Way, using the HI line. Today the HI line is utilized in various fields, as a tracer to study the dark matter content of the galaxies, and as a probe of the physical conditions pervading through the interstellar medium (ISM). In the future, with the arrival of the Square Kilometer Array (SKA), studies of neutral hydrogen will give us the opportunity to understand Galaxy evolution and enable us to probe the epoch of re-ionization giving new insights in the cosmological evolution of the Universe (Carilli & Rawlings, 2004). In Galactic astronomy, the 21-cm line plays a vital role in studying a variety of astro- physical phenomena from AU scales, up to scales of hundreds of kpc. This is due to the widespread distribution of the neutral hydrogen in the Galaxy, and its transparency which allows a detection even in areas where optical observations are very difficult (e.g., regions obscured by dust). Especially the neutral hydrogen in the Milky Way disk has been studied in great detail. I.e, large scale HI observations probed thor- oughly the vertical structure of the disk, revealing an asymmetric warp reaching in height more than 4kpc above the plane at distances of 17kpc and measuring the flar- ing of the HI disk (Nakanishi & Sofue, 2004; Levine et al., 2006a; Kalberla et al., 2007). More recent studies detected a distant spiral arm in the outer part of the galaxy (McClure-Griffiths et al., 2004) and found evidence for disruption in the HI disk in the form of giant bubbles and super-shells (McClure-Griffiths et al., 2006). On smaller scales, regarding the structure of the ISM in Milky Way disk, it is known for some time that the diffuse neutral hydrogen resides in two phases: the warm neutral medium phase with a temperature T < 8000K and a cold dense neutral phase with T ∼ 80K as- sociated with discrete small HI structures, detected in absorption line studies (Heiles, 1997). Furthermore, high above the Milky Way disk, another class of astronomical objects has been detected in the 21-cm line in 1963, the high-velocity-clouds (HVC), a pop- ulation of HI clouds incompatible with Galactic rotation having a deviation velocity υ > 90kms−1(Wakker & van Woerden, 1991). An estimate regarding their distance gives a lower limit of 5kpc (z>3.5kpc) for Complex C (Wakker et al., 1999) and for Complex A a distance d=4kpc, implying a height above the plane between 2.5kpc and 7kpc (van Woerden et al., 1998). Above the Milky Way disk, moving with high velocities, lays a population of old, low metalicity stars, Globular Clusters and remnants of the accretion from Dwarf compan- ions of the Milky Way. This layer of stars surrounding the Milky Way disk forms what 2 1 Introduction is known as the Halo. Similarly in the case of dark matter it is proposed that it forms a dark matter halo surrounding the disk and dominating the gravitational potential of the Galaxy. The question then arises, if there exists a stellar and dark matter Halo of the Milky Way Galaxy, is it possible that there exists also a similar gaseous layer surrounding the Galactic disk? If this is true, what is the origin of such a layer? Is it the result of an interaction between the Galaxy and the intergalactic medium? Is it the result from an accretion? Is it the product of other physical processes in the Galaxy like supernova explosion? The answer to the first question is positive. Even from the early fifties, it was Spitzer (1956) who proposed that there exists a semi-static hot gaseous layer which surrounds the Milky Way disk. The vertical extent of the Galactic corona would be approxi- 6 8 mately 8 kpc with a temperature reaching 10 K and a total mass of 10 M⊙. The existence of such a layer was proposed mainly to explain the stability of high latitude clouds but also to facilitate the equilibrium of spiral arms in the presence of strong magnetic fields. Interestingly the idea of a Galactic corona was partially inspired from the claim of Pickelner (1953) for the existence of a homogeneous substratum of rel- atively cool gas, with a density of 0.1 cm−3 supported from turbulent motion with velocity 70 kms−1. The first observational confirmation of the hot galactic corona hypothesis was given by Münch & Zirin (1961). Using absorption lines of distant stars outside the Milky Way disk, evidence was found for gas clouds at heights z ∼ 1 kpc above the Galactic disk. From these clouds they found that the acceptible temperature for the Galactic corona is the one postulated by Spitzer of 106 K. Shapiro & Field (1976) proposed the mechanism of the “Galactic fountain” in or- der to explain the presence of coronal gas and to introduce a process to cool the gas through convection and radiation. According to the mechanism, hot gas, produced by supernovae explosions in the disk, is buoyant. It streams upwards with a veloc- ity perpendicular to the disk smaller or equal than the speed of sound. The outcome of this motion may have the following results: a) if the cooling is inefficient, the hot gas bubble reaches great heights above the disk, forming and becoming part of the Galactic corona of the disk, in hydrostatic equilibrium b) or if the cooling mechanism is efficient, at height zc the hot bubble cools rapidly and undergoes a phase transition forming cool dense HI clouds. These clouds are not buoyant and so beyond this point they follow a ballistic motion under the influence of gravity which will bring them again at height zc. It was the latter result of this mechanism that prompted Bregman (1980) to associate the galactic fountain mechanism with the formation of the HVC. A few years later there was observational evidence that beyond the hot coronal gas surrounding the disk, there possible exists neutral gas extending high above the Milky Way disk. Lockman (1984) detected the 21-cm emission from these patches of neutral 3 gas located in the inner Galaxy at heights greater than |z|>1000 pc. He proposed that the neutral gas in this region is following Galactic rotation and that this gas may form a layer that can not be solely thermally supported nor can it be considered as a single component isothermal layer.
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