prelim #2 reminder - cornell universityhosting.astro.cornell.edu/academics/courses/astro... ·...

Lec. 22: Galactic Center and Galaxies

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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


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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?


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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


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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


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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


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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


NGC 6334

NGC 6357

M6

M7

Galactic Center

Baade’s Window

~0.5°

Visible Infrared


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The Galactic Center


~0.5°

Visible InfraredNGC 6334

NGC 6357

M6

M7

Galactic Center

Baade’s Window

The Galactic Center 2Mass


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The Galactic Center Spitzer

The Galactic Center MSX


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The Galactic Center VLA

The Galactic Center VLA


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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?


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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.


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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


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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)


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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


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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.


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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


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Formation of a galaxy system like Milky Way and M31 (Andromeda)

x-view z-view

Simulation by Rob Thacker

prelim #2 reminder - cornell universityhosting.astro.cornell.edu/academics/courses/astro... ·...

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