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Spiral Galaxies LECTURE 1 2.1 2027-2 School on Astrophysical Turbulence and Dynamos 20 - 30 April 2009 Galaxies and the Universe A. Shukurov University of Newcastle U.K. The Abdus Salam International Centre for Theoretical Physics School on Astrophysical Turbulence and Dynamos Galaxies and the Universe Anvar Shukurov School of Mathematics and Statistics, Newcastle University, U.K. 1. Introduction: basic facts and parameters 2. Spiral galaxies LECTURE 1 2.1. Rotation 2.2. Hydrostatic equilibrium of the gas layer 2.3. Interstellar gas and its multi-phase structure 2.4. Interstellar turbulence LECTURE 2 3. Elliptical galaxies 4. Galaxy clusters 5. Observations of astrophysical velocity and magnetic fields 5.1. The Doppler shift of spectral lines and velocity measurements 5.2. Detection and measurement of astrophysical magnetic fields (a) Zeeman splitting of spectral lines (b) Faraday rotation of polarised emission (c) Synchrotron emission LECTURE 3 (d) Light polarization by interstellar dust 6. Galactic and extragalactic magnetic fields from observations 6.1. Spiral galaxies 6.2. Galaxy clusters 6.3. Briefly on accretion discs Warning % cgs units will be used, 1 G = 10-4 T, 1G = 10-6 G = 0.1 nT. % G 6.67108 cm3 g1 s2; k 1.381016 erg K1; m 24 7 12 p 1.7 10 g, 1 year 3 10 s; 1 eV 1.6 10 erg. % 1 parsec 3 1018 cm 3.26 light years, 1 kpc = 103 pc; c 31010 cm3 cm s1 (1 parsec distance: Earth orbit’s parallax = 1 second of arc). % Solar mass, radius and luminosity: 33 10 33 1 M 210 g; R 710 cm; L 410 erg s . % Notation: HI = neutral hydrogen, HII = p+ = ionized hydrogen, CIV = triply ionized carbon, etc. Further reading % R. G. Tayler, Galaxies: Structures and Evolution, Cambridge Univ. Press, 1993 (a high-level scientific popular book on galactic astrophysics, entertaining and informative, very well written). % J.E Dyson and D.A Williams, The Physics of the Interstellar Medium, Second Edition (The Graduate Series in Astronomy), IOP, 1997 (a good textbook on interstellar medium, presents appropriate equations and their solutions together with clear qualitative arguments and explanations and order of magnitude estimates). % A. Shukurov & Dmitry Sokoloff, Astrophysical dynamos. In: Ph. Cardin, L.F. Cugliandolo, editors, Les Houches, Session LXXXVIII, 2007, Dynamos, Amsterdam: Elsevier, 2008, pp. 251- 299 (lecture notes from a Summer School, covering significant part of the material presented in the ICTP lectures; a reprint can be found on the ICTP School website). 1. Introduction: basic facts and parameters Galaxies: % Stars, % interstellar gas, % cosmic rays (relativistic charged particles, mainly protons and electrons), % dust Clusters of galaxies the largest gravitationally bound objects in the Universe: % collections of galaxies (elliptical near the centre, spirals at the outskirts), % intracluster gas These lectures: focus on astrophysical plasmas, especially on HD & MHD aspects Galaxies: Hubble’s morphological classification (the pitchfork diagram) The structure of a spiral galaxy Stellar bulge, quasi- spherical, similar to an Spheroidal stellar halo, elliptical galaxy is Sa old stars and consisting of younger stars in Sc Dark matter halo, only felt via gravity Rotating stellar disc, young stars; light absorbed by dust NGC891, edge-on (i = 89 ), Sb Spiral arms M51, face-on (i = 22 ), Sc Galaxy characteristics are uniquely related to classification E0-E7 S0 Sa Sb Sc Irr Nuclear "All Bulge" Bulge & Disk Large Small None Bulge No disk Spiral Arms None None Tight/smooth Open/clumpy Occasional traces Gas Almost none Almost none 1% 25% 510% 1050% Young Stars, Dominate None None Traces Lots HII Regions appearance All old Mostly(?) young Stars Old Some young ( 1010 yr) (but some very old) Spectral GK GK GK FK AF AF Type Color Red Red Blue 8 13 12 9 8 11 Mass (M) 10 10 (More) 10 10 (Less) 10 10 Luminosity 1061011 (More) 1011108 (Less) 1081011 (L) http://cass.ucsd.edu/public/tutorial/Galaxies.html 2. Spiral galaxies NGC 253 (Sc) M51 (Sc) NGC891 (Sb) M33 (Sc) NGC 3992 Barred spiral galaxies (SBa) NGC 1365 NGC 1300 (SBc) (SBb) Spiral galaxies: thin rotating discs of 1011 stars (90% of the visible mass) and interstellar gas (10%) Interstellar gas: >nN1 cm3, >T N104 K, 103 < n < 103 cm3, 10 < T < 106 K Milky Way parameters, Sbc Diameter 30 kpc Average thickness (stellar disc) 0.3 kpc Bulge radius 2 kpc 11 Visible mass 1.410 M Rotation period (near the Sun) 2.5108 yr Rotational velocity (near the Sun) V0 220 km/s Radius of the Solar orbit R0 8.5 kpc Age 1.21010 yr 3 Mean interstellar gas number density n0 1 cm 4 Mean gas temperature T0 10 K Mean speed of sound cs 10 km/s Thickness of the diffuse gas layer near the Sun 2h0 1 kpc Pitch angle of the spiral arms (trailing spiral) p0 10 The near-infrared sky (COBE, false-colour image): stars in the Milky Way (thin disk and the central bulge) http://lambda.gsfc.nasa.gov/product/cobe/slide_captions.cfm 2.1. Rotation: V(r) (rotation curve) measured for 1000’s of galaxies Sofue et al., Publ. Astron. Soc. Jpn. 51, 737, 1999 %Differential rotation: angular velocity varying with position. %Flat rotation curves at large radii (dark matter halo): V = r 200 km/s const; 1/r . Wait for the tutorials Rotational shear rate: G = r d /dr . Rotation curve and shear: Milky Way (solid) and a generic galaxy (dashed) Rotation curve V (r):balanceof the centrifugal force and gravity due to mass density ½g(~r) (neglecting pressure contributions from gas, magnetic ¯elds, etc.): V 2 @© = ; r2©=4¼G½ (~r). r @r g For a point-like mass distribution, ©=¡GM=r, V 2 GM GM = ) V = ; Kepler's rotation law r r2 r ©=graviational potential, r = spherical radius, applied in a thin disc z ¼ 0 (μ ¼ ¼=2). Solutions for V(r): tutorials Observed rotation curve V(r) ; galactic gravitating mass model (r) Rotation curve of the Milky Way, with contributions of various mass components (Kalberla, ApJ 588, 805, 2003) 2.2. Hydrostatic equilibrium of the gas layer A crude but physically realistic and useful description of the background gas distribution: d (P + P + P + P ) = ½g ; dz th turb mag c:r: z gravity pressure support P nkT P 1 ½v2 th = , thermal pressure, turb = 3 0 , turbulent pressure, 2 Pth = B =8¼, magnetic pressure, Pc:r:, cosmic ray pressure. Vertical gravitational acceleration near the Sun (disc+dark halo): z z gz = ¡ a1 + a2 ; 2 2 H z + H1 2 ¡9 ¡2 ¡9 ¡2 with a1 =4:4 £ 10 cm s , a2 =1:7 £ 10 cm s , H1 =0:2kpc and H2 =1kpc(Ferriμere 1998). Solutions for (z): tutorials Assumptions: %Energy (pressure) equipartition: Pth = Pturb = Pmag = Pc.r. %An isothermal atmosphere: ; ; v T(z) = const csound = const 0 = const. % For gz = const, 8v2H 1=2 ½ / exp(¡z=h); h = 0 2 ' 200 pc; 3jgzj csound = speed of sound, v0 = turbulent velocity, h = density scale height. Hydrostatic equilibrium at various radii: a flared gas disc d @© P = ½g ;g= ¡ ; r2©=4¼G½ (r;z): dz z z @z g ( = gravitating mass density) Oort (BAN, 15, 45, 1960): g @2©(r; z) 1 @ =4¼G½ (r; z)+ (rg ); @z2 g r @r r @© V 2(r; z) 1 centrifugal balance g = ¡ = / r @r r r + °at rotation curve z 0 0 gz(r; z)=¡4¼G ½g(r; z ) dz 0 z @g (r; z0) g (r; z0) + r + r dz0 0 @r r d ² 4 v2 ½ = ½g : 3 0 dz z z 0 0 ² gz(r; z)=¡4¼G ½g(r; z ) dz 0 z @g (r; z0) g (r; z0) + r + r dz0 : 0 @r r V 2 @g (r; z0) g (r; z0) ² g = ¡ ) r + r =0: r r @r r ¡1 ² ½g = ½0(z) exp(¡kgr) ) gz / exp(¡kgr); kg =3{5kpc 2 1=2 8v0 H2 h = ) h / exp(kgr=2); 3jgzj ² h grows with r,a°ared gas layer. Observations of the HI layer in the Milky Way (Kalberla & Dedes, A&A, 487, 951, 2008) 2.3. Interstellar gas and its multi-phase structure • Supernova explosions hot, tenuous gas slow cooling pervasive hot regions • Supernovae (SNe) transonic turbulence compression cooling cool clouds • Gravitational & thermal instabilities cold, dense clouds star formation supernovae The multi-layered interstellar medium (ISM): The warmer is the component of the ISM, the more it expands away from the Galactic midplane. Galactic fountain: Hot gas rises to the halo, cools, and returns to the disc in 109 yr Galactic gaseous halos: turbulent, rotating, hot, ionised, quasi-spherical gas envelopes of galactic discs, n 103 cm-3, T 106 K, cs 100 km/s, v0 ' 60 km=s; HI in NGC 891 Oosterloo et al., 2005 H several kpc Scale height of the halo gas: tutorials 2.4. Interstellar turbulence: driven mainly by SN explosions, 30% isolated, 70% in clusters (OB associations) Smaller contributions ( 25%): stellar winds and galactic differential rotation Starburst region NGC 604 in M33: 200 young massive stars (1560 M) in a region 300 pc across Supernova (SN): explosion of a massive star, M > 16 M , energy released E 1051 erg * ; a hot gas bubble expanding supersonically ; spherical shock wave SN remnant (SNR): a hot, expanding gas, X-ray emitting bubble (mostly heated interstellar gas), (bottom) X-rays and ROSAT MPIfR), top) %separated from the ISM by a dense shell of swept-up, shocked interstellar gas; %a site of cosmic ray acceleration. Tycho Brahe’s SN 1572: radio (W. Reich, Tycho Reich, (W. SN 1572: radio Brahe’s Kepler’s (bottom) X-rays and ROSAT (top) SN 1604: composite Stages of SNS expansion 1. Free expansion: the ejected mass ( 4M) dominates the swept-up mass, constant Cas A, VLA 5 GHz expansion velocity, lasts 200 yrs.
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