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ASTROPHYSICS I: Lecture Notes, ETH Zurich 2019

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ASTROPHYSICS I: Lecture Notes, ETH Zurich 2019 ASTROPHYSICS I: lecture notes, ETH Zurich 2019 8 Galaxies covers Sections 9 in Choudhuri, except 9.3 and 9.6 which are treated in \cosmology" not covered at all: sections 9.4.2 and 9.7 additional topics: the local group, detailed model of steady accretion disks, and a few points on gravitational lensing History { 1784: catalogue of Messier ≈ 100 nebular object (avoid confusion with comets) { 1890: New General Catalogue (NGC): ≈ 8000 extended objects { around 1925: many extended nebula are galaxies like the Milky Way { today: > 108 galaxies cataloged SLIDES: Hubble types and galaxy pictures 8.1 Types of galaxies Hubble sequence: 4 morphological categories { \S"-galaxies (normal spirals): round central bulge and disk with spiral structure { \SB"-galaxies (barred spirals): spiral galaxy, where central bulge has a bar structure { \E"-galaxies (elliptical galaxies): homogeneous spherical or elliptical structure { \Irr"-galaxies (irregular galaxies): other galaxies (later added to the classification) Subclasses: { spiral galaxies: Sa/SBa { Sb/SBb { Sc/SBc (late type galaxies) { decreasing bulge size { increasing opening angle of spiral windings { elliptical galaxies: E0 { E7 (early type galaxies) { flattening of spheroid Ex (intrinsic ellipticity ≥ Ex) x = 10 (a-b)/a (a,b long and short axis) { S0: spheroidal galaxies { dominant spheroid (perhaps a small disk) Frequency of galaxies in NGC-catalog (nearby, bright galaxies) { S/SB: ≈ 70 %; E: ≈ 20 %; Irr: ≈ 7 % m 10 Typical luminosity MB:(MB = −20 is ≈ 10 L ) main types [mag] { S-galaxies: −17m to −23m { E-galaxies: gE: ≈ −21m E: ≈ −19m, dE: −14m to −18m dSph: −10m to −15m, cD: −22m to −25m (diffuce central cluster elliptical) { Irr-galaxies: Irr: −17m dIrr: −10m to −17m 61 Description of E and S galaxy types: E galaxies { very homogeneous appearance { no cold gas (no dust lanes, no HI emission, no HII-regions) { no star formation { integrated spectrum of cool stars (5000 K) { stellar orbits with random orientation ! \red and dead" spiral galaxies { central bulge (like small elliptical) { disk { contains cold gas (dust lanes, HI and CO emission), and HII-regions { spiral arms, more than average young stars and HII-regions { gas and stars follow predominant disk rotation { trailing spirals { integrated spectrum like hot star (HI-absorption) with emission lines (HII-regions) SLIDES: galaxy spectra and Sersic profiles surface brightness description: (not important for this lecture) Sersic surface brightness fitting functions: k: constant, h: radial scale { Sersic index n controls the curvature of the profile h r 1=ni I(r) / I exp −k 0 h A: elliptical galaxies: de Vaucouleurs fit h 0:25 i I(r) = Ieexp −7:67 r=re − 1 { n = 4, re: effective radius, Ie = I(re) !: luminosity distribution determined by the distribution of stars in gravitational potential B: disk galaxies: exponential fit −r=rd I(r) = I0e { n = 1, I0 central (extrapolated) disk surface brightness; rd characteristic radius 62 Mass estimates A: for elliptical galaxies { observation: widths of strong absorption lines { random motions of stars follows from virial theorem (2Ekin + Epot = 0) 1 2 mimj 2 Σi mivi − G Σi6=j = 0 2 jxi − xjj assumption: Nm = Mgal all N stars have mass m N 2 Gm2 Nmhv2i ≈ 2 hRi mean velocity and velocity dispersion GM hv2i ≈ 2hRi B: for disk galaxies: { HI-radial velocity measurements, vr = vc cos φ sin i { circular velocities (from FZ = FG) as for the Milky Way v2 GM(r) c = r r2 observed rotation curves are all flat vc ≈ const ! M(r) / r { as far out as rotation curves can be determined ! { strong evidence for the presence of dark matter in all disk galaxies SLIDES: HI map and rotation curves Velocity-luminosity relations: (empirical relations) A: for elliptical galaxies: Faber-Jackson relation: L 0:25 −1 σv ≈ 220 km s L∗ L∗: characteristic galaxy luminosity B: for disk galaxies: Tully-Fisher relation L 0:22 −1 vc ≈ 220 km s L∗ Interpretation: 2 { vc / M=rd (for given characteristic radius rd) 2 { disk mass scales like M / rd { luminosityp scales like L / M 2 1=4 ! vc / L= L or vc / L 63 8.2 The local group SLIDES: M31 and Magellanic Clouds Galaxies of the local group: { dominated by three disk galaxies (MW, M31, M33) { MW and M31 have > 5 faint satellite galaxies, type dIrr, dE, or dSph { local group contains > 50 galaxies { most are low luminosity objects { with low metallicity [Fe/H] ∼< − 1 (only few previous stellar generations) Many satellites are in interaction with MW or M31 { LMC and SMC show strong loss of HI-gas because of interaction { M32 and NGC 205 are so close to M31, interactions are unavoidable { Sgr dwarf galaxy collides just now with MW { interaction of dwarf galaxy with large spiral has only little effect because mass ratio is < 1 : 100 possible giant collision in the future between M31 and MW: { separation: 0.74 Mpc { relative velocity: ≈ −110 km/s { tangential velocity: < 10 km/s (HST-data from 2012) { expected collision in ≈ 4 Gyr The 3 brightest galaxies of the local group and their brightest satellite galaxies galaxy type MB [mag] Milky Way SBb −20:8 Large Magellanic Cloud Irr −17:9 Small Magellanic Cloud Irr −16:3 CMa dwarf galaxy dIrr −14:5 Sgr dwarf galaxy dSph −12:7 Andromeda galaxy (M31) Sb −21:6 NGC 205 (M110) dE6 −16:1 M32 E2 −16:0 IC10 dIrr −15:6 NGC 147 dE5 −14:9 Triangulum galaxy (M33) Sb −18:9 64 8.3 Galaxy interactions (only few qualitative points are given) Galaxy interactions are very frequent: { between spirals and small satellite galaxy (small + big) { collisions between two large spiral galaxies (big + big) { interactions in galaxy clusters and groups (one + many) ! interactions are important for galaxy evolution A: Collision between small galaxy and large spiral galaxy: { impact on spiral galaxy relatively small { tidal distortion: disk is warped vertically to disk plane { tidal distortion: deviation of axisymmetry of disk (loopsided disks) { spiral structure can be enhanced by corotating disturbance (M51) { density wave produces disk with ring structure instead of spirals (e.g. M31) { star formation may be triggered locally { impact on the smaller galaxy are strong { motion of stars is disturbed by potential of spiral galaxy { many stars leave potential of dwarf { gas of small galaxy collides with gas of spiral and is lost { only the more compact bulge remains ! formation of dE and dSph galaxies { alternative: small galaxy is totally disrupted B: Collision between two large spiral galaxies: { stars move through potential of other galaxy (star-star collisions are very rare) { stellar motions strongly disturbed { systematic disk rotation is reduced, { central bulge size increases (more stars with random orbit orientation) { merged galaxy (E-type), possibly with diffuse halo (cD-galaxy) { gas collides, { shock heating to T > 106 K, gas expands into intergalactic space { gas is removed from one or both galaxies (gas stripping) ! E-galaxy without cold gas { alternative: gas is compressed, burst of star formation ! very prominent, peculiear galaxies ! (ultra)-luminous IR-galaxies C: Interaction in galaxy clusters: { main processes: gas stripping by hot intercluster gas ! S-galaxies are distroyed { infalling galaxies merge with central galaxies ! formation of (diffuse) giant galaxies SLIDES and video: colliding galaxies 65 8.4 Active galactic nuclei (AGN) History: { Karl Seyfert 1947: describes spiral galaxies with very bright central regions ! Seyfert galaxies { around 1955: detection of extended radio emission which can be associated with galaxies ! Radio galaxies: e.g. Vir A, Her A, Cen A, Cyg A etc. { M. Schmidt (1963): radio source 3C273 is 13 mag point-like object at d ≈ 1 Gpc ! Quasars (quasi-stellar radio sources) estimaged energy output: 37 11 { Seyfert galaxies: L ≈ 10 W ≈ 10 L 39 13 { Quasars: L ≈ 10 W ≈ 10 L Size of source: { e.g. for NGC 4151 (20 Mpc): source not spatially resolved (e.g. < 100) ! rSource < 50 pc { e.g. for 3C273: luminosity variability ∆L=L ≈ 30 % on time scales ∆t ≈ days 5 5 11 ! rSource < c · ∆t = 3 · 10 km=s · 3 · 10 s = 10 km = 0:003pc 13 Big question: How to produce 10 L within 0.003 pc? Minimum mass from Eddington luminosity limit assumptions: { source is stable: radiation pressure force Fr < FG gravitational force { object is made of ordinary matter: { σe electron scattering dominates opacity { mH is the corresponding gas mass per electron 2 { Fr = L=(4πr ) · σe=c per electron 2 (photon-flux · photon-momentum · cross section: Nγ=4πr · hν=c · σe) 2 { FG = GMmp=r per proton Eddington limit Fr = FG 4πGMmpc 4:5 LE = ≈ 10 (M=M )L σe source is only stable, if mass 6:5 11 { M > 10 M for a 10 L -Seyfert galaxy 8:5 13 { M > 10 M for a 10 L -quasar 66 Energy source: disk accretion onto black hole Schwarzschild-radius for black hole 2GM M RS = 2 = 3000m c M 8 { for M8 = 10 M ! rS(M8) = 2AU Accretion onto black hole from infinity to RS: { change in potential energy ∆Epot per mass unit m: GMm mc2 ∆Epot = − = RS 2 ! ∆Epot is half of the rest mass! Energy production in an accretion disk: { steady flow of matter towards black hole { angular momentum must be transported away { energy in the disk must be radiated away to keep disk cool < 107 K energy production for a given mass accretion rate M_ { last stable orbit at 3 RS (typically) dE GMM_ Mc_ 2 pot = = dt 3RS 6 Assumption: Virial theorem is valid for quasi-Keplerian accretion disk { Ekin = −Epot=2 { ∆Epot=2 goes into orbital energy of the gas { ∆Epot=2 is dissipated and heats disk gas 7 { disk radiates L ≈ ∆Epot=2 and stays cool (< 10 K) { other energy loss mechanism may be important (gas jet) Result
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