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Dust • [12.1] Optical Tiny grains ( ≤ 1µm) Infra-red • Cores consisting of No Reddening • Graphite • or Silicates (MgxFe1-xSiO3; Mg2xFe2-2xSiO4; etc) Ices can condense on surface: • Optical UBV Near-IR J H K Galactic Pole AV = 0.07 mag C,O combined with H • Absorb and scatter light Galactic Center AV = 30 mag • Effect strongest in blue, • less in red, • zero in radio. From star counts: Baade’s Window AV = 1.6 mag Flux Æ • Ultraviolet log λ (µm) Æ Mapping Our Galaxy Sun Kapteyn (1922). Surfaces of constant star density. From distribution of Globular Clusters: • Use pulsating variables to find clusters’ distances. • Clusters are out of MW disk Î little reddening. Shapley (1920) Globular Clusters (distances using RR-Lyraes) < MV > Period-Luminosity Relation [Fig. 14.5] The Modern View 1 [Table 24.1] Milky Way’s Morphology Exponential disk n( z , r ) = no ( e − z / zthin + 0.02e − z / zthick )e − r / hr + spheroids with various “forms” n = no × form [Fe/H] =log(Fe/H) abundance ratio relative to Sun Usually see deVaucouleurs’ r1/4 surface brightness law: Milky Way Bulge • Elongated… now thought to be a bar • From observations of Mira pulsating variables. • Minor/major ~0.6 • Roughly follows DeV profile (r1/4 law) • Baade’s window • Moderate metallicity (-1 < [Fe/H] < +1) ~1010 M SB0 isophotes [BM] Fig 4.57 • Expanding 3kpc arm • H I feature • vr = -50 km/s • Elliptical orbit due to bar ROUND BULGE PEANUT-SHAPED The Psychedelic Barred Spiral Movie 2 Milky Way Halo • Globular clusters + field stars • Field stars = high velocity stars • ~150 globular clusters known, in 2 different systems: • Older (~13 Gyr) • Very metal-poor (-2.5 < [Fe/H] < -0.8) • Spherical distribution around galactic center • No net rotation [Fig 24.12] Warning! • Younger • (~11Gy, ~ same as thick disk) • Moderate metallicity ([Fe/H]> -0.8) • Compare to thick disk –0.6 < [Fe/H] < -0.4 • Flattened • Show net rotation => part of thick disk?? These are plots of directions in sky, NOT crosssections of MW [Table 24.1] 3 The Gas Disk 25 kpc 8 dust HI • • • • Increases outward Sun is ~ 30 pc above midplane. Disk is warped starting at R ~ 7 kpc Mass • • • Electron spin-flip transition in H ground state Galaxy essentially transparent at 21 cm Diameter: ~ 50 kpc Thickness: • • • • Half-density Thickness (pc) <4 100 4 - 10 250 11 300 15 650 20 1000 HI ( = H0) detected through 21 cm emission H I: 4 x 109 M H2: 1 x 109 M Average density: 0.04 M pc-3 z (kpc) Æ • H2 Radius (kpc) H I Maps Showing Warp R = distance from Galactic Center 6 4 2 0 -2 -4 -6 6 4 2 0 -2 -4 -6 100 0 R = 12 kpc R = 20 kpc R = 16 kpc R = 24 kpc -100 100 0 -100 Å Galactic Longitude 4