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Lec. 22: Galactic Center and Galaxies
22 - 1
Lec 22: Galaxies 1
The Galactic Center
Lecture 22 APOD:
Lec 22: Galaxies 2
Prelim #2 Reminder
Prelim #1 on Wed. Oct 24 Normal class time & location
Format like Prelim #1
Will cover lectures 12 – 22 From Star Formation to today’s lecture
Will post practice exam
Lec. 22: Galactic Center and Galaxies
22 - 2
Lec 22: Galaxies 3
Lecture Topics
Milky Way Atomic and Molecular Gas
The Galactic Center
Mass of the Galaxy Dark Matter
Gas – Stars – Gas Cycle
Formation of the Milky Way
Public lecture tonight:
“Exploring the Universe from the South Pole”
by John Carstrom (Hans Bethe Lecturer)
7:30 pm in Schwartz Auditorium
Lec 22: Galaxies 4
Atomic Gas
Flattened pancake Radius > 20 kpc. Height ~ 250 pc in center, 1 kpc at 20 kpc
Mass ~ 3x109 Msun, ~2/3 outside the orbit of the Sun around the Galactic Center.
Distorted appearance at the fringes of the Galaxy. Interaction with LMC,SMC?
Lec. 22: Galactic Center and Galaxies
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Lec 22: Galaxies 5
Schematic of the atomic gas distribution
250 pc
Sun GalacticCenter 1 kpcwarp in
the disk
20 kpc
Mgas ~ 3x109 Msun
Lec 22: Galaxies 6
Molecular Gas Giant Molcular Clouds (GMCs):
Size ~ 10 - 50 pc, Mass ~ 103 - 106 Msun
Stars form in cores of GMCs.
Mass ~ 3x109 Msun, ~2/3 inside the orbit of the Sun around the Galactic Center
Lec. 22: Galactic Center and Galaxies
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Lec 22: Galaxies 7
The molecular gas distribution
Sun GalacticCenter
“Ring” of GMCs at 4-8 kpcw/ M ~ 2x109 Msun
Total mass of molecular gas ~ 3x109 Msun
M ~ 2x108 Msun for GC clouds
Molecular “Ring” from 4-8 kpc and concentration on GC
Thickness ~ 120 pc.
Lec 22: Galaxies 8
The Galactic Center What lies at the center of our Galaxy?
Dust obscures the visible light from us Use radio and infrared observations
Lec. 22: Galactic Center and Galaxies
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Lec 22: Galaxies 9
The Galactic Center
Unique place in our galaxy
Seems to contain a massive black hole.
Photo at right shows an optical image – the galactic center doesn’t stand out
Overlay shows some know optically identified nebulae and the location of the Galactic Center
Sagittarius region
Lec. 22: Galactic Center and Galaxies
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The Galactic Center
Optical (left) vs. Near – IR view of the galactic center region.
~0.5°
Visible Infrared
The Galactic Center
Optical (left) vs. Near – IR view of the galactic center region.
NGC 6334
NGC 6357
M6
M7
Galactic Center
Baade’s Window
~0.5°
Visible Infrared
Lec. 22: Galactic Center and Galaxies
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The Galactic Center
Optical (left) vs. Near – IR view of the galactic center region.
~0.5°
Visible InfraredNGC 6334
NGC 6357
M6
M7
Galactic Center
Baade’s Window
The Galactic Center 2Mass
Lec. 22: Galactic Center and Galaxies
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The Galactic Center Spitzer
The Galactic Center MSX
Lec. 22: Galactic Center and Galaxies
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The Galactic Center VLA
The Galactic Center VLA
Lec. 22: Galactic Center and Galaxies
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Galactic Center Region in the Radio
The Galactic Center Region
20 pc 2 pc
“Molecular ring” of gas rotating about the center.
Long linear structure tracing Galactic magnetic field
Star forming regions
Black hole?
Lec. 22: Galactic Center and Galaxies
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The Galactic Center
Dense star cluster peaks at the center ~2x106 Msun within 1 pc
(Black hole!?) Stars only 1000 AU apart A collision every 106
years!Infrared false color picture (1.65, 2.2, and 3.5 m. Stellar motion movie at 2.2 m.
Black hole accretion simulation
The center of the Milky Way might look something like this.
Lec. 22: Galactic Center and Galaxies
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Lec 22: Galaxies 23
NGC 1232 -- A galaxy like the Milky Way The stars and
gas rotate about the center of the Galaxy.
The rotation speed varies with distance from the center.
From the speed at a given point, we can deduce the mass!
Lec 22: Galaxies 24
Kepler’s Law for the Galaxy The total mass of the galaxy can be
computed from Newton’s laws. Like the mass of binary stars.
From Lecture 10 (Binary Stars), we have Newton’s version of Kepler’s third law
M
r
GP
322 4
M = total massP = periodr = distance from center
Lec. 22: Galactic Center and Galaxies
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Lec 22: Galaxies 25
Kepler modified by Newton
For a circular orbit
P
r2v
v = velocityP = periodr = distance from center
Combining this with Newton’s version of Kepler’s third law
G
rrM
2v)(
2
324
P
r
GM
22
23
2
4
v4
rr
G
Lec 22: Galaxies 26
Example “rotation curves”
Rotation curve for merry-go-round (left) and the solar system (right)
Lec. 22: Galactic Center and Galaxies
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Lec 22: Galaxies 27
Galaxy rotation curve
Milky Way rotation curve is more like that of a merry-go-round than that of the solar system. Thus there is no dominate central mass
Rotation curve for the Milky Way.
Relatively flat
Lec 22: Galaxies 28
The Mass of the Galaxy
For the Sun, v = 220 km/sec at a radius of 8.5 kpc. Orbital period = 240 million years.
Mass of MW = 1011 Msun within 8.5 kpc.
Going out to 30 kpc (previous slide) MW = 5x1011 Msun (v = 270 km/sec)
G
rrM
2v)( We use our
derived formula
Lec. 22: Galactic Center and Galaxies
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Lec 22: Galaxies 29
Dark Matter
The mass seen in stars is much less than that derived from Newton’s laws.
There must be some additional mass which is non-luminous!
The is unseen mass is call Dark Matter.
It is sometimes called missing massbecause it isn’t traced by the starlight.
Lec 22: Galaxies 30
The star-gas-star cycle The ISM provides the matter from which stars
form. Stars evolve and create “heavy” elements
Stellar nucleosynthesis and supernova
These elements are returned to the ISM Stellar winds, planetary nebula, and supernovae Not all material is returned resulting in the gas
being “used up”
The “enriched” gas is used by the next generation of stars.
Lec. 22: Galactic Center and Galaxies
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Star-gas-starcycle
atomic-hydrogen clouds
molecular clouds
star formation
stellar-burning/ heavy element formation
supernovae and stellar winds
hot bubbles
Lec 22: Galaxies 32
How did the Milky Way form? The Galaxy collapsed from a cloud of
gas and dust from its own self-gravity. Some (Pop II) stars formed first. Remaining gas collapses into a disk -
angular momentum conservation! First generation massive stars eject
metals into the disk so that Pop I stars have higher metallicities
Lec. 22: Galactic Center and Galaxies
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Formation of a galaxy system like Milky Way and M31 (Andromeda)
x-view z-view
Simulation by Rob Thacker
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