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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 l.jpg

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 l.jpg
(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 l.jpg
(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 l.jpg
(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 l.jpg
(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 l.jpg
(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 l.jpg
(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 l.jpg
(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 l.jpg
(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 l.jpg
(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 l.jpg
(7b) Spiral Arm Origin

  • Not simple windup  too tight

  • Self-propagating star formation +

  • differential rotation

  •  ragged arms

  • “flocculent” spirals


7c density waves l.jpg
(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 l.jpg

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 l.jpg
(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 l.jpg
(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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