Download The Galactic Center

Lec. 22: Galactic Center and Galaxies
The Galactic Center
Lecture
Lec 22: Galaxies 22
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APOD:
Prelim #2 Reminder
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Prelim #1 on Wed. Oct 24
Normal class time & location
 Format like Prelim #1
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Will cover lectures 12 – 22
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From Star Formation to today’s lecture
Will post practice exam
Lec 22: Galaxies
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Lec. 22: Galactic Center and Galaxies
Public lecture tonight:
Lecture Topics

Milky Way

Atomic and Molecular Gas
The Galactic Center
 Mass of the Galaxy
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“Exploring the Universe
from the South Pole”
by John Carstrom (Hans
Bethe Lecturer)
7:30 pm in
Schwartz Auditorium
Dark Matter
Gas – Stars – Gas Cycle
 Formation of the Milky Way

Lec 22: Galaxies
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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?

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Lec. 22: Galactic Center and Galaxies
Schematic of the atomic gas distribution
250 pc
warp in
the disk
Galactic
Center
Sun
1 kpc
20 kpc
Mgas ~ 3x109 Msun
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Lec 22: Galaxies
Molecular Gas
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Giant Molcular Clouds (GMCs):
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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
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Lec. 22: Galactic Center and Galaxies
The molecular gas distribution

Molecular “Ring”
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from 4-8 kpc and concentration on GC
Thickness ~ 120 pc.
M ~ 2x108 Msun for GC clouds
Sun
Galactic
Center
“Ring” of GMCs at 4-8 kpc
w/ M ~ 2x109 Msun
Total mass of molecular gas ~ 3x109 Msun
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The Galactic Center

What lies at the center of our Galaxy?
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Dust obscures the visible light from us
Use radio and infrared observations
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Lec. 22: Galactic Center and Galaxies
The Galactic Center

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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
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Sagittarius region
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Lec. 22: Galactic Center and Galaxies
The Galactic Center
~0.5°
Visible

Infrared
Optical (left) vs. Near – IR view of the galactic center region.
The Galactic Center
~0.5°
Baade’s
Window
Galactic
Center
M6
NGC 6357
M7
Visible

NGC 6334
Infrared
Optical (left) vs. Near – IR view of the galactic center region.
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Lec. 22: Galactic Center and Galaxies
The Galactic Center
~0.5°
Baade’s
Window
Galactic
Center
M6
NGC 6357
M7
Visible

NGC 6334
Infrared
Optical (left) vs. Near – IR view of the galactic center region.
The Galactic Center
2Mass
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Lec. 22: Galactic Center and Galaxies
The Galactic Center
Spitzer
The Galactic Center
MSX
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Lec. 22: Galactic Center and Galaxies
The Galactic Center
VLA
The Galactic Center
VLA
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Lec. 22: Galactic Center and Galaxies
Galactic
Center
Region in
the Radio
The Galactic Center Region
pc
Long linear structure
tracing Galactic
magnetic field
2
pc
0
2
Star forming
regions
“Molecular ring” of
gas rotating about
the center.
Black hole?
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Lec. 22: Galactic Center and Galaxies
The Galactic Center

Dense star cluster
peaks at the center
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Infrared false color picture (1.65, 2.2,
and 3.5 m. Stellar motion movie at
2.2 m.
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~2x106 Msun within 1 pc
(Black hole!?)
Stars only 1000 AU apart
A collision every 106
years!
Black hole accretion simulation
The center of the Milky Way
might look something like this.
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Lec. 22: Galactic Center and Galaxies
NGC 1232 -- A galaxy like the Milky Way
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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!
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Lec 22: Galaxies
Kepler’s Law for the Galaxy
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The total mass of the galaxy can be
computed from Newton’s laws.
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Like the mass of binary stars.
From Lecture 10 (Binary Stars), we have
Newton’s version of Kepler’s third law
4 2 r 3
P 
G M
M = total mass
P = period
r = distance from center
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Lec. 22: Galactic Center and Galaxies
Kepler modified by Newton

For a circular orbit
v
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2 r
P
v = velocity
P = period
r = distance from center
Combining this with Newton’s version of
Kepler’s third law
4 2 3 v2
4 2 r 3
M

r
G P2
G
4 2 r 2
Lec 22: Galaxies
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r v2
M (r ) 
G
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Example “rotation curves”
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Rotation curve for merry-go-round (left) and
the solar system (right)
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Galaxy rotation curve
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Rotation curve for
the Milky Way.
Relatively flat
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
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The Mass of the Galaxy
r v2
M (r ) 
G
We use our
derived formula

For the Sun, v = 220 km/sec at a radius of 8.5
kpc.

Orbital period = 240 million years.
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Mass of MW = 1011 Msun within 8.5 kpc.
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Going out to 30 kpc (previous slide)
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MW = 5x1011 Msun
Lec 22: Galaxies
(v = 270 km/sec)
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Lec. 22: Galactic Center and Galaxies
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 mass
because it isn’t traced by the starlight.
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The star-gas-star cycle
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The ISM provides the matter from which stars
form.
Stars evolve and create “heavy” elements
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These elements are returned to the ISM
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Stellar nucleosynthesis and supernova
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.
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Lec. 22: Galactic Center and Galaxies
atomic-hydrogen
clouds
hot bubbles
molecular clouds
Star-gas-star
cycle
supernovae and
stellar winds
star formation
stellar-burning/
heavy element
formation
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
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Lec. 22: Galactic Center and Galaxies
Formation of a galaxy system like
Milky Way and M31 (Andromeda)
x-view
z-view
Simulation by Rob Thacker
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