lecture 10. The Milky Way s Interstellar Medium Birgitta Nordström, Copenhagen, & Gerhard Hensler, Vienna

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1 IX. The Milky Way gas distribution lecture 10. The Milky Way s Interstellar Medium Birgitta Nordström, Copenhagen, & Gerhard Hensler, Vienna 1 2 1

2 1. HI Gas HI is observed by the 21cm hyperfine-structure line. Although the transient probability is almost zero, the large number of HI atoms sums up to a significant emission. 3 Disk thickness and radial extension z r In the stationary case, forces must be in equilibrium. dφ d M z hydrostatic equilib. Self - gravitational Force : FΦ = ρ( z ) = G ρ( z ) dz dz z Scaleheight H z dp k T dρ( z ) Vertical Pressure Force : Fp = = dz m µ dz H rotational equilib. Scalelengthα r Gravitational Force : Centrifugal Force : Mr FG = G m 2 r F 2 = ω r m c 4 2

3 Spiral Structure of HIH The Sun is assumed to sit somewhere in a disk. HI is not homogeneously distributed, but clumped into HI clouds. Under the assumption of isothermality the vertical extension of the clouds is determined by the vertical gravitational potential. The distance is derived from the apparent vertical height of the clouds. Location of the Sun at R GC 8.5 kpc Disk size: R MWG,HI 20 kpc 5 The vertical HI Distribution Dickey & Lockman 1990 Most of the HI gas is concentrated into the galactic plane: 6 scaleheight H z pc. 3

4 The HI seems to consist of almost 2 layers: a thin one (H z pc), a thick one (Lockman layer: H z 1 kpc) in transient to the HII distribution (Reynolds layer) 7 The thick HI Disk A thick HI layer was found by Lockman 1984 called Lockman Layer. Its scaleheight amounts to 500 pc. 8 4

5 The Lockman Layer extends over the whole Galactic disk and is even thicker in the innermost part. 9 The radial HI surface density distribution is almost constant between 3 R/kpc 15 a thin one (H z pc), a thick one (Lockman layer: H z 1 kpc) in transient to the HII distribution (Reynolds layer) 10 5

6 More detailed observations show that: 1. HI disk is very flat to the inner galaxy and extends vertically higher outside. 2. HI filaments and arcs exist above the galactic disk, caused by dynamical effects. 3. Flaring Disk! 11 2.The HI Rotating Disk v rot v obs With the determined distances of HI density enhancements. HI is not homogeneously distributed but clumped into HI clouds. Under the assumption of isothermality the vertical extension of the clouds is determined by the vertical gravitational potential. 12 6

7 13 3. HII Regions Churchwell 2002 The r,φ distribution of HII regions in the Milky Way unravels spiral arms in the solar vicinity. 14 7

8 4. The Galactic Rotation from HII Regions For a constant rotational velocity the innermost stars pass the Sun while they outermost remain behind. 0º 90º 180º 270º 360º In the solar rest frame the stars thus show a double sine wave in their radial velocities. 15 The double wave becomes also discernible (Georgelin & Georgelin 1982) for HII regions and can be traced much farther than stars. 16 8

9 Like molecular clouds (compact) HII regions are strongly concentrated to the equatorial plane. The rotation velocity distribution of HII regions can only be determined for the solar side of the Milky Way The Galactic Rotation Curve The slope of the rotation curve depends on the Galactocentric Distance (GCD) of the Sun. Present values fixed by the IAU: R GCD = R 0 = 8.5 kpc Θ 0 = 220 km/s 18 9

10 19 The Milky Way rotation curve has the following characteristics: Steep linear increase: v(r) r rigid rotation Maximum and drop followed by a constant (or slightly increasing) v(r) M(r)/r 2 v(r) 2 /r M(r) r, but not observed Dark Matter required 20 10

11 10.6. The diffuse Hα Layer In 1974 Reynolds & collab.s found that Hα is vertically more extended and more diffuse than expected from local HII regions: Reynolds Layer. (1974) 21 Wisconsin H-Alpha Mapper WHAM surveys the ISM in the Hα line. Narrow emission lines allow to disentangle velocity structures due to the Doppler effect: v/c = λ/λ v>0: escaping v<0: approacing Results: Relative motions of ISM structures discernible 22 11

12 Emission lines of the Reynolds Layer differ from HII regions; e.g. lower [OIII]λ5007, vanishing [NI]λ5201, stronger [NII]λ6584 and [SII]λ6716. Derived temperature of T 9000 K; required energy ev, typical for the irradiation by massive stars. midplane el. density <ne > cm-3, scaleheight kpc. 23 Explanations for the Reynolds Layer : OB star radiation thru HI holes in the disk (Dove et al., 2000); ~10% LUV sufficient for ionization fraction, but emission line ratios differ ionization radiation from OB stars diffused through galactic chimneys (Miller & Cox, 1993); neutrino decay (Sciama, 1990); magnetic reconnection (Raymond 1992); turbulent mixing layer (Slavin et al., 1993); old SN remnants (Slavin 200); cooling superbubbles (Freyer & G.H., 2001): the cooling radiation of a T K hot superbubble can do it but electron temp. increases with z! 24 12

13 7. Molecular Gas φ,z distribution Dame et al Dame et al CO distribution reveals a very flat band (Hz 80 pc) within the innermost galactic region ( Molecular Ring ) with some fraying (mostly to the north: clouds in the solar vicinity), widens in the outermost MWG; Coincidence of CO structures with extinction features; 26 Total molecular mass derived from CO: MH M 13

14 The Molecular Ring In the Milky Way a Molecular Ring exists between 2-4 and 6-7 kpc. 27 Molecular Gas r,φ distribution Molecular Cloud distribution obtrudes an arm-like structure

15 CO velocity distribution Myers et al CO relative velocity Dame et al

16 31 Dame et al Characteristics of the CO velocity distribution (Doppler-shifted CO line at λ = 2.6 mm with respect to the Local Standard of Rest ): Individual arms, Long ranges of zero relative velocities, Inner molecular ring, An innermost expanding ring, 32 A fast rotating central disk. 16

17 8. Dust distribution The COBE NIR total dust distribution still includes the zodiacal light. If subtracted the Milky Way plane with fringes and arcs appears and extragalactic sources are visible. 33 Dark Clouds examples: Coalsack BHR 71 Barnard68 (A V 20 m ) Horsehead Nebula 34 17

18 Interstellar Extinction Extinction by dark clouds reduce stellar brightness mv by AV distance module must be corrected by AV : mv MV = 5 log d 5 + AV < AV> 1 m kpc -1 EB-V correlates better with column density of molecular gas than of pure atomic gas dust is combined with mol.clouds 50% of the IS metals are bound in dust! Savage & Mathis (1979) ARAA Cloud location determined by Wolf diagrams Neckel & Klare 1960 A V r 36 18

19 Applying Wolf s diagrams (see Part I) one can derive the distribution of dust concentrations in the solar vicinity. Neckel & Klare The non-thermal Radio Emission Radio observations of the Milky Way at 408 MHz (= 73 cm) represents the non-thermal hot interstellar gas. Characteristics visible: Distinct single sources and spots, Thin and thick inner disk, North-polar spur, Fringes and diffuse filaments, 38 Extragalactic sources 19

20 φ,z distribution Different sources of nonthermal radio emission can be identified. 39 Radio observations of the Milky Way at 408 MHz (= 73 cm) represents the non-thermal hot interstellar gas. It allows to see through the whole galaxy and to construct a 3-d distribution. The edge-on view is almost symmetrical and shows the thick disk structure

21 r,φ distribution The 408 MHz Radio structure of the Milky Way shows clear spiral structures Hot X-ray Gas red: soft ( kev): are easily absorbed by HI, i.e. emission locally or from layers above the galactic disk light blue: middle ( kev): structures of average distances 42 blue: hard ( kev): penetrates through the HI disk; bright spots 21

22 11. The Local Galactic Environment The Sun sits closely behind the Sagittarius arm. 43 The Local Cavity 2003 The determination of the interstellar Na D1 and D2 absorption features in stellar spectra allows to conclude about the neutral interstellar gas. In the solar vicinity: The Sun is sitting in a neutral gas 44 cavity of pc extension. 22

23 Equivalent widths contours of the NaI absorption unravels: (left) an hole devoided of HI in the galactic plane The Local Cavity (right) a cylinder almost perpendicular to the galactic plane (inclination: ~20º to NGP) The Local Chimney Lallement et al The Local Bubble The local cavity must be filled with another gas phase: hot gas = The Local Bubble The hot X-ray gas distribution can be derived from X-ray shadowing by cool clouds and from EUV absorption lines of highly ionized metals to near objects. density: ρ cm -3 temperature: T 10 6 K 46 23

24 Cloud Shadowing Technique Measure: On-cloud intensity Off-cloud intensity Cloud distance dl N(H) IX, off = If + I IX, cl = If + Ib exp{ κhi+ H 2 ds} I f = τ X dl I b dl b N HI+H2 cloud ds + dust XMM-Newton shadowing of B68 47 X-ray emission shows two facts: 1. a roundish bright hard X-ray distribut., 2. an extended shell: North Polar Spur Result: another X- ray bubble beyond the Local Bubble: Loop I Loop I 48 24

25 The hard-to-soft X-ray gas shows a neutral gas absorpt. sheet that detaches the Local Bubble from another soft X-ray bubble, Loop I 49 The Local Cloud Sun is embedded in gas that is denser and cooler than the Local Bubble and only partly ionized, the Local Cloud, also called Local Fluff : n e cm -3, T 6000 K 50 25

26 and the Local Bubble, a stellar ring Any conclusion about the origin of both features tends to: 1. Highly energetic event 2. Production of hot gas Most probable: Superbubble with triggered star formation (inclined to the galactic disk like W4) 55 Berghöfer & Breitschwerdt The Pleiades moving group B1 has an age of almost Myrs; 2. Its kinematics let B1 pass the local ISM within 100 pc (10-20 Myrs ago): ~ 20 supernova explosions! 56 26

27 The Motion of the Sun The Sun moves with respect to the Local Cloud (He 0 experiment) by: v =(26.3±0.4) km/s l ecl, =(74.5±0.5)º b ecl, =(-5.2±0.2)º 57 A gross Picture of the solar Environment 58 27

28 The Sun s proper Motion The Sun moves with respect to the Local Standard of Rest (LSR): with v 16.5 km/s in direction of l 53º, b 25º The Gaseous Halo Gas expands out of the Galactic disk. Questions: bound with fall-back? unbound? 60 28

29 Hot Halo Gas Hot gas exists in the Galactic halo. Observations: soft diffuse X-ray gas, shadowing of HI clouds, Rotation 61 Measure to NSs, absorption features to extragalactic sources, OVI in the Galactic halo; low density: n 0,OVI cm -3, large scaleheight: H z,ovi 5.5 kpc

30 Hot Galactic Halo Gas (OIV with FUSE), outflowing and infalling hot Complexes Galactic Chimneys and HI Shells > 600 pc D 6.5 kpc McClure-Griffiths et al. (2003) 64 30

31 HII-Region W4 Galactic HI W4 is a young massive stellar association; Stellar winds and explosions have produced a hot gas cavity within the HI that expands; The Superbubbles expands out of the 65 galactic HI disk Cumulative Stellar Winds + Explosions in stellar Associations lead to Superbubbles of hot Gas Superbubbles expand out of the gaseous disk and form hot gas halo 66 Schematic view of W4 31

32 Cometary clouds in W4 obtrude to be deformed by the evironmental hot gas outflow The Disk-Halo Connection Local Bubble Aquila Shell W4 Superbubbles expand out of the Galactic disk; they are visible in Xrays and by HI shells; by this, they transport hot gas into the halo

33 Probing the structure of chimneys in or Milky Way, absorption along different sight lines are utilized. Highly, intermediate, and low ionized gas, and neutral H coexist in the halo. 69 Chimneys are supported by the vertically opening of magnetic field. The synchroton radiation in the Radio range traces magnetic fields. 70 The outflow of hot gas is supported by the diffusion of Cosmic Rays. 33

34 15. The γ-ray Sky γ-ray brightness represents the direction of Cosmic Rays. Components: 1. diffuse γ-ray background 2. CRs confined to the galactic plane: flat central region discrete galactic sources (supernonae) + extragalactic sources 16. Cosmic Rays Discovered 1912 by Victor Hess (Nobel Prize 1936): Höhenstrahlung, ionsation in earth s atmosphere decreases up tp 1.5 km but increases again above upwards. Element abundances solar: 89% H, 10% He, 1% metals; but overabundant in Li, Be, B! High-energy charged particles: electrons, positrons, protons, and atoms nuclei: close to light speed, mainly originating from extra-solar sources, but also used for particles produced in energetic solar events, hitting the Earth from all directions, but entereing thru the magnetic field

35 ν GeV γ-rays Visible CMB Radio Flux Energy (ev) / / / / / / / TeV sources! / / cosmic / rays / / / / / / / MeV = p+ at 0.43 c 10 GeV = p+ at c Process of acceleration at highest energies is still unsolved. Highest energy CR measured to date >1020 ev (= Ekin of a baseball at 100 mph) CR energy distribut. of particles almost the same: knee at 1015eV, power law above. Cosmic Ray Energies total energy distribution of CR 74 35

36 Energy distributions of CR particles show almost same power law; Above ev ( knee ) steep decrease; N(>E=1GeV) = k(e + 1) -a, with a ~1.6, k ~ 5000 m -2 srd -2 sec -2. total energy distribution of CR 75 Galactic and Extragalactic Cosmic Rays -2.6 Knee New component with hard spectrum? Ankle 76 36

37 >>> energy in extra-galactic cosmic rays: ~ 3x10-19 erg/cm 3 or ~ erg/yr per (Mpc) 3 for years 3x10 39 erg/s per galaxy 3x10 44 erg/s per active galaxy 2x10 52 erg per gamma ray burst >>> energy in cosmic rays ~ equal to the energy in light! 77 1 TeV = 1.6 erg Origins of CRs Supernova explosions release high-energy particles and compress the surrounding ISM. Particles are accelerated by the shock-compressed magnetic fields. Radio emission of Cas A 78 37

38 Supernova Beam Dump RX J Gas Infall see sect.. XIV! HI clouds are detectable at high b in the Galactic halo

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