The atomic gas in the Milky Way

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1 The atomic gas in the Milky Way Peter M.W. Kalberla Argelander-Institut für Astronomie Bonn This project has been funded with support from the European Commission. This publication reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein

2 The Milky Way is a prime astronomical target Advantages: Only in our Galaxy the targets are close enough for detail studies Our Galaxy may be taken as an example Disadvantages: We are located within Complicated geometry Distance ambiguities 2

3 Origin of the Milky Way Eratosthenes tells us that the Milky Way was the result of a trick played by Zeus on his wife Hera so that she would suckle his illegitimate son Heracles and hence make him immortal. Hermes laid the infant Heracles at Hera s breast while she was asleep, but when she woke and realized who the baby was perhaps by the strength with which he sucked she pushed him away and her milk squirted across the sky to form the Milky Way. Tintoretto: The Origin of the Milky Way Photo The National Gallery, London.) 3

4 EMU in the Sky Composed of dark clouds of dust in the Milky Way, the Emu rises above an Aboriginal rock-engraving in Kuring-Gai Chase National Park, Sydney, Australia. 4

5 What is more important - dust or stars? 5

6 Origin of the universe 6

7 Big bang nucleosynthesis After the big bang: Hydrogen (74% mass fraction) und Helium, some Helium 3, Lithium and Deuterium All heavy elements were produced in Stars and supernovae e.g Crab, alias M1, Taurus A, NGC

8 Tycho, 11. November

9 Element abundance, Solar & Cosmic Mass number (protons + neutrons) 9

10 Molecules and dust Radio: Emission Optical: Absorption 10

11 Molecular gas Molecular clouds can be characterized as: Diffuse molecular Clouds T K n 100 cm -3 Dark clouds T K n cm -3 Molecular clouds contain a significant fraction of dust. Star formation 11

12 The cycle of matter The phases of the ISM are in constant, continual motion. Stars form within the densest regions of it, areas called molecular clouds. These stars replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. 12

13 13

14 Star formation: NGC

15 Ionized gas HII regions are located close to young massive stellar objects Temperature: T K Density n cm -3 15

16 Hot ionized gas Hot gas (Plasma) T 10 6 K n cm -3 from supernova 16

17 HI gas everywhere two phases Narrow lines: Cold Interstellar clouds T 80 K n 1 cm -3 Broad lines: Warm neutral gas T 6000 K n cm -3 17

18 Line radiation: line width is a thermometer Maxwell-Boltzmann distribution Inelastic scattering 18

19 Cold HI gas Dust (Planck APOD) 19

20 HI self-absorption Cold gas in front of an extended region with strong background emission. Temperature: 30K Fraunhofer lines in analogy 20

21 Cold gas - also molecules: CO 21

22 Optical depth Diffuse HI: dominant in emission, low optical depth Clumps: dominant in absorption, moderate to high optical depth Dickey & Lockman, ARAA,

23 Clumps may have diameters of a few pc VLA WSRT Dedes & Kalberla,

24 Two phases caused by thermal condensations Thermal instabilities driven by turbulence may convert warm into cold gas Such cold clouds are most probably not stable and may change shape in years Clouds may be as small as a few tens of AU 24

25 Basic question: shape of the Milky Way NGC 891 M51 The whirlpool Galaxy 25

26 The Milky Way: the ideal laboratory Accurate all sky data are available LAB (Leiden/Argentine/Bonn) survey on a 0.5 O grid GASS data (Parkes) for δ < 0 O with 14 beam EBHIS (Effelsberg) data for δ > -5 O with 9 beam (2012) All data available for public use The Milky Way allows a high spatial resolution analysis But: velocity - distance relation versus line of sight blending Unfortunately, we are positioned within the disk and need a rotation curve to interpret the data (necessity of a mass model) 26

27 Global properties of the HI disk 27

28 slicing the cake (warp, flaring and surface density) We need to convert observations to physical meaningful quantities: n(r,z)

29 LAB: Volume densities n(r,z,az), az = 110 o Warp - flaring radial density distribution 29

30 HI in the Galactic plane at R = 15 kpc extra-planar emission outside the disk at a 3-σ level 30

31 The Milky Way warp fit 3 modes 31

32 Flaring of the HI scale-height: data and model Exponential fit 32

33 Radial HI distribution.. surface densities Phase-transition kink Transition to olecular gas 33

34 Radial HI distribution.. volume densities 34

35 Milky Way global properties Total diameter: 70 kpc or Light-years gas is twice as extended as stellar component Total baryonic mass Solar masses 13% of this is gas, predominantly HI 5 to 10 times more dark matter needed to explain rotation curve and shape of the Galaxy 35

36 Milky Way, details in HI Data show rich wealth of structures, only partly understood A highly dynamical picture Interactions: Star formation Supernova explosion Gas accretion Cannibalism 36

37 From disk to halo 37

38 Supernova explosions shape the HI disk 38

39 Turbulence in the local ISM 39

40 Magellan and gas accreted by the Milky Way 40

41 The HVC sky, velocities 41

42 High velocity clouds Gas accreted by the Milky Way 1-2 solar masses per year Fuel needed for star formation 42

43 LAB survey in Aitoff projection Movie running from -400 to 400 km/s (approaching to receding gas) anti-center in the middle 43

44 LAB survey at v = -32 km/s 30 beam 44

45 GASS (Parkes 64-m) - 15 beam 45

46 GASS clean data (-0.12 to 50 K, log scale) 46

47 RFI 47

48 All sky Galactic HI surveys at AIfA LAB 36, all sky (2005) GASS 15, δ < 1 o (2010) EBHIS 9, δ > -5 o (2012) PI: J. Kerp ASKAP/APERTIF 1-3 (2013.) 48

8.1 Radio Emission from Solar System objects

8.1 Radio Emission from Solar System objects 8.1.1 Moon and Terrestrial planets At visible wavelengths all the emission seen from these objects is due to light reflected from the sun. However at radio