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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

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

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