Ch 15: Milky Way Galaxy

Ch 15: Milky Way Galaxy Milky Way = “Via Lactea”  spans many constellations Crosses the whole sky in a “great circle”  Our view, from the inside, of our galaxy. All Sky SH NH

(1) Optical/Infrared/Radio • Optical: dust important • IR & radio: true flatness revealed Optical + molecular (CO) All Sky Optical Near Infrared: stars Far Infrared: warm dust Hydrogen 21cm

(2) External View • Galaxy Anatomy: • Disk with arms • + ISM + young “open” clusters • Bulge with small bar • Nucleus @ very center • Halo w. old “globular” clusters 8 kpc = 25,000 l-yr Sun

(3) History: Slow Discovery • ~1610 Galileo: telescope  MW = many faint stars • ~1750 Herschel(s): count stars  small disk, sun @ center Sun • ~1900 Kapteyn: repeats & improves star counts •  10 kpc disk, sun @ center However, problem with star counts: dust prevents clear view Need tracer for galaxy outside dusty MW plane…..

(3b) ~1920 Shapley • Studied distribution of globular clusters • Used variable stars to get distances • (see later) •  larger system • sun not at center • center in Sagittarius Basically correct (though 3x too big; diagram labeled correctly)

(4) Star Motions • Disk : gas & stars in circular motion • Vorb(sun) = 220 km/s  Porb(sun) = 2 x 108 yr = 0.2 Gyr • ~ 50 orbits since formation  ~ 50 “years old” • Bulge & halo stars & GCs : • on “random” orbits • (range of oval shapes, • many inclinations & directions)

(4b) Rotation Curve & Galaxy Mass • Vrot roughly constant @ 200-250 km/s •  differential rotation (inner stars “overtake” outer ones) • Mass : a3/P2 ~ M ~ 1011 Msun inside sun • Note : as R↑ M↑ even beyond the visible edge ! •  Dark Matter is major component, MDM ~ 10 x Mstars “edge” All mass interior

(5) Star Populations • City metaphor: different age groups live in different places; • move differently; made of different materials (!) elliptical Metal Rich (strong lines) Metal Poor (weak lines)

(6) Galaxy History • Simple picture from populations (1960s) • Large, ~spherical, gas cloud  halo stars • Collapse, flattens, spins up  disk • (c.f. process of star/planet formation) • More evidence (1990s) complicates: • infall of smaller galaxies important • destroyed & add to halo & thicken disk • Gradual chemical enrichment from • stellar nucleosynthesis • ongoing star ↔ gas cycle in thin disk

(7) Spiral Arms • In other galaxies: • arms = regions ofstar formation • OB star clusters; DMCs; HII regions • Difficult to map in our galaxy: • OB stars & DMCs  nearby arms • global pattern not (yet) possible M51

(7b) Spiral Arm Origin • Not simple windup  too tight • Self-propagating star formation + • differential rotation •  ragged arms • “flocculent” spirals

(7c) Density Waves • orbit crowding makes a spiral pattern • in disk density moves slowly around • stars & gas move through arm • gas compressed  forms stars • density wave triggered/maintained by • passing neighbor/central bar •  strong armsgrand design spirals

Infrared MW plane (8) Galaxy Nucleus • Very well defined: < 1pc region • (= bacterium on 8½ x 11 sheet) • Very difficult to observe • 8 kpc away, behind dust/gas • use IR & radio to penetrate dust Optical

(8b) Galaxy Nucleus • Very high star density: ~106 stars/pc3 • Much star formation & associated supernova remnants • Anomalous radio source: Sgr A* • extremely small: < solar sys size @ center of star peak IR Radio

(8c) Nuclear Black Hole • Study orbits of stars near Sgr A* (using IR camera) • Keplerian ellipses & velocities  “point” mass at Sgr A* • orbits give mass  2.8 x 106 Msun • Star S2 approaches to 80AU • moves @ 5000 km/s !! • IR & X-ray “flashes” (mins) •  Black Hole • data excludes anything else. infrared

Ch 15: Milky Way Galaxy