Download Lectures 19-20 The Milky Way Galaxy

The Milky Way
Cerro Tololo InterAmerican Observatory
Large Magellanic Cloud
K. Don, NOAO/AURA/NSF
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Roger Smith/NOAO/AURA/NSF
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Panoramic Picture of Milky Way taken from Death Valley, CA,
Dan Duriscoe, US National Park Service
Monday, April 2, 12
Panoramic Picture of Milky Way taken from Death Valley, CA,
Dan Duriscoe, US National Park Service
Monday, April 2, 12
Panoramic Picture of Milky Way taken from Death Valley, CA,
Dan Duriscoe, US National Park Service
Monday, April 2, 12
Milky Way Galaxy
Our Galaxy is a collection of stars and interstellar
matter - stars, gas, dust, neutron stars, black holes held together by gravity
Composite near-IR (2 micron) Image from the Two Micron All
Sky Survey (IPAC/Caltech/UMass)
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Historical Models of the Milky Way Galaxy
Greeks: Γαλαξίας κύκλος Galaxias Kyklos "Milky Circle".
Roman: Via Lactea (Milky Way).
East Asia: “Silvery River” of Heaven (Chinese: 銀河; Korean: eunha; Japanese: Ginga)
Finno-Ugric (Finns, Estonians): “Pathway of the Birds”. Birds follow path for
migrations... some evidence this is true.
Austrailian Aboriginal: Wodliparri (house-river).
Galileo first suggested the Milky Way is a
vast collection of individual stars.
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Historical Models of the Milky Way Galaxy
In mid-1700s, Immanuel Kant (1724-1804) and Thomas Wright
(1711-1786) proposed the Galaxy must be a disk of stars to
explain the circular distribution in the sky. They went further
and suggested our Sun is one component in the Milky Way.
William Herschel (1738-1822)
In 1780 William Herschel produced the map below by counting stars in different
directions. He concluded that the Sun is near the center of the Galaxy, and that the
dimensions along the plane were five times greater than the vertical thickness.
Herschel assumed (1) all stars have same luminosity (Absolute Magnitude), (2) Number
density in space is roughly constant, and (3) there is nothing in space to obscure the Stars
(fainter stars are farther away)
Sun
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Historical Models of the Milky Way Galaxy
Jacobus Kapteyn (1851-1922) used star counting to confirm the
Herschel model, but with much-improved methods. Now
called the Kapteyn Universe.
Galaxy consists of a flattened Spheroidal system with a
decreasing stellar density with increasing distance from the
center. His published self-titled “attempt” to describe the
“Stellar system” (=Milky Way) appear in the year he died
(Kapteyn 1922, ApJ, 55, 302):
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Jacobus Kapteyn (1851-1922)
Historical Models of the Milky Way Galaxy
Kapteyn Universe
Picture of the Galaxy:
Sun, y=650 pc, x=38 pc
Numbers show where stellar density has declined by a factor of 2, 3, ... 10, from the central density.
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Historical Models of the Milky Way Galaxy
From 1915-1919, Harlow Shapley estimated the distances to
93 globular clusters using RR Lyrae and W Virginis variable stars
(like Cepheids). Shapley found they are not uniformly
distributed in the Galaxy, but are concentrated in the
constellation Sagittarius (where the center of Galaxy is). He
determined these were 15,000 pc (15 kpc) away.
Harlow Shapley (1885-1972)
The most distant clusters he could measure were 70 kpc away. Shapley argued our Galaxy
has a diameter of 100 kpc, close to 10x that of Kapteyn. Also as important, Shapley put
our Sun far from the center of the Galaxy. Kapteyn had the Sun near the center.
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Historical Models of the Milky Way Galaxy
Who was right, Kapteyn or Shapley ?
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Historical Models of the Milky Way Galaxy
Who was right, Kapteyn or Shapley ?
Neither ! They are both wrong, but for the same reason. They both ignored
the effects of dust, which causes the extinction of light.
Kapteyn missed stars he could not see, could not see the most distant regions
of the Milky Way.
Shapley’s variable stars were more luminous then he thought because their
apparent magnitudes were extincted.
Similar to being on a boat and trying to see land through fog.
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Credit: Axel Aitoff
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Infrared (3-8 micron) view of Center of Milky Way Galaxy
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Morphology of the Milky Way
R0 = 8 kpc
Sun
from Digital Sky LLC
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The Morphology of the Galaxy
The solar Galactocentric distance, R0, is still debated. In 1985 the
International Astronomical Union (IAU) adopted R0 = 8.5 kpc. Recent studies
find R0 = 8 kpc (Eisenhauer 2003). Your book uses this latter value.
The Galaxy is composed of a bulge,
a thin and thick disk, and a halo.
Most stars are in disk components.
Disk contains lots of gas and dust.
Halo has low density and it contains
many globular clusters.
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The Morphology of the Galaxy
Structure of Thin and Thick Disks
We define the size of the components using the scale height. (We don’t have
a way of defining the “edge” of the galaxy or its components ? )
If n is the number density of stars in the disk, and z is the vertical distance above
the Galactic midplane, then the number density scale height is
1/Hn = -(1/n) (dn/dz)
Take Hn to be a constant (OK assumption) then we can solve for n using
differential equations:
n = n0 exp( -z/Hn )
If Hn = z then n = n0 e-1, so H is the point where the number density has dropped
by a factor of e.
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The Morphology of the Galaxy
Structure of Thin and Thick Disks
Galactic Disk has two major components, the thin disk, and the thick disk.
Thin disk: composed of young stars, dust, and gas, with Hnthin = 350 pc
(youngest stars found with scale height of 35-90 pc).
Thick disk: older stars with a scale height of Hnthick = 1000 pc. The number
density of stars in the thick disk is ~8.5% that of the thin disk.
Total distribution of stars is given from current observations:
n(z,R) = n0 ( exp[-z/Hthin] + 0.085 exp[-z/Hthick] exp( -R/Hradial )
where z is the vertical height above the midplane, and R is the distance from the
Galactic center. Hradial = 2.25 kpc, n0 ≈ 0.02 stars/pc3 for 4.5 < MV < 9.5.
Note that these are all still uncertain....
Our Sun is a member of the thin disk, and lies about 30 pc above the midplane.
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The Morphology of the Galaxy
Age-Metallicity Relation
Thin and thick disks have different scale heights, stellar densities, and metal
fractions and ages !
Recall that stars have different metal fractions, different Populations.
Population I: high metal fractions, Z~0.02.
Population II: low metal fractions, Z~0.001.
Population III: zero metal fraction, Z~0. (hypothesized).
Astronomers commonly use the ratio of Iron (Fe) to Hydrogen (H) relative to
that in the Sun to quantify the metal fraction. We call this the metallicity:
Stars with [Fe/H] > 0 have a higher metal fraction than the Sun. Stars with [Fe/H] < 0
have a lower metal fraction.
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The Morphology of the Galaxy
Age-Metallicity Relation
Stars with [Fe/H] > 0 have a higher metal fraction than the Sun. Stars
with [Fe/H] < 0 have a lower metal fraction.
extremely metal-poor stars (Population II) have [Fe/H] ~ -5.4.
Highest values are [Fe/H] ~ 0.6.
Studying Globular Cluster “turn-off” masses, younger clusters have
high [Fe/H] then older clusters, which have low [Fe/H]. This is the
age-metallicity relation.
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The Morphology of the Galaxy
Age-Metallicity Relation
Solar Value
Rana 1991, ARAA, 29, 129
Time since formation of disk (Age - td, where td = 12 Gyr)
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The Morphology of the Galaxy
Thin Disk: typical iron-hydrogen ratios are -0.5 < [Fe/H] < 0.3.
Thick Disk: typical iron-hydrogen ratios are -0.6 < [Fe/H] < -0.4
(some as low as -1.6?!)
Which contains older stars ? Which “formed” first ?
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The Morphology of the Galaxy
Thin Disk: typical iron-hydrogen ratios are -0.5 < [Fe/H] < 0.3.
Thick Disk: typical iron-hydrogen ratios are -0.6 < [Fe/H] < -0.4
(some as low as -1.6?!)
Which contains older stars ? Which “formed” first ?
Appears that star formation began in thin disk about 8 Gyr ago, and is
continuing today. This is supported by the cooling times of white
dwarfs in the thin disk.
Thick disk predated most of that of the thin disk by 2-3 Gyr, probably
during the period 10-11 Gyr ago.
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from Digital Sky LLC
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Spiral Structure
Galaxy M 51
Spiral Structure
from Digital Sky LLC
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Evidence for Spiral Structure
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Evidence for Spiral Structure
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http://www.youtube.com/watch?v=Suugn-p5C1M
from Digital Sky LLC
Monday, April 2, 12
http://www.youtube.com/watch?v=Suugn-p5C1M
from Digital Sky LLC
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The Galactic Bulge
Galactic Bulge: Independent component from disk. Mass of the bulge is believed to
be ~1010 M⊙. Scale Height is ~100 to 500 pc, depending on whether younger
stars are used (smaller scale heights) than older stars (higher scale heights).
Surface brightness (units of L⊙ pc-2 ) follows the “r1/4 law” distribution,
discovered by Gerard de Vaucouleurs (1918-1995), also called the de
Vaucouleurs profile.
Our Bulge has an effective radius, re = ~0.7 kpc.
The Bulge is very difficult to observe because it is so centrally concentrated and
there is a lot of dust and gas in the Galactic center. Must look in “windows”
with lower extinction (one is the so-called “Baade’s window”).
Stars in the bulge have -2 < [Fe/H] < 0.5. Possibly multiple metallicity groupings
in bulge. One group is <200 Myr old, one is as old as 7-10 Gyr.
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The Galactic Bulge
McWilliam 1997
Bulge shows (at least) two populations. One with low [Fe/H] and high [α/Fe],
and one with high [Fe/H] and low [α/Fe].
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The Galactic Bulge
Gilmore et al. 1989
Stars with low [Fe/H] have high [α/Fe]. (Oxygen is an alpha element). Early metal
production occurred from core-collapse Supernovae, which produce more Oxygen (and
other α-elements) compared to Fe (which comes from Type Ia Supernovae).
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The Morphology of the Galaxy
Thin Disk
contains Spiral Arms
Thick Disk
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Milky Way Galaxy
The Galactic Bulge
COBE Satellite image of Milky Way at 1.2-3.5 micron.
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The Galactic Halo
Galactic (“Stellar”) Halo is composed of globular clusters (GCs) and field stars.
Shapley thought GCs were spherically distributed. There now appear to be two
populations.
Older, metal-poor globular clusters have [Fe/H] < -0.8, spherical distribution.
Younger clusters have [Fe/H] > -0.8, in Galactic plane
Zinn 1985, ApJ, 293, 424
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The Galactic Halo
Galactic (“Stellar”) Halo is composed of globular clusters (GCs) and field stars.
Older, metal-poor globular clusters have [Fe/H] < -0.8, spherical distribution.
These metal-poor GCs range from 500 pc to 120 kilo-pc ! Youngest is about 11
Gyr old and oldest are about 13 Gyr old.
Zinn 1985, ApJ, 293, 424
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The Components of the Galaxy
Neutral Gas
Thin Disk
Thick Disk
Bulge
Halo
Mass (1010 M⊙)
0.5
6
0.2-0.4
1
0.3
LB (1010 L⊙)
0
1.8
0.02
0.3
0.1
M/LB
-
3
~10
3
~1-3
Radius (kpc)
25
25
25
4
>100
Scale Height
(kpc)
<0.1
0.35
1
0.1-0.5
3
[Fe/H]
>+0.1
-0.5 to +0.3
-2.2 to -0.5
-2 to 0.5
< -5.4 to -0.5
Age [Gyr]
<~ 10
8
10
<0.2 to 10
11 to 13
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Rotation Curves of the Milky Way
Clemens 1985, ApJ, 295, 422
Rotation curve for our Galaxy. Strange thing is.... rotation
curve is flat beyond the Solar circle, R0 = 8.5 kpc.
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Rotation Curves of the Milky Way
Let Mass of Galaxy have a constant
surface density, Σ, for r < R. Velocity is
then just from Newton’s Laws:
with
Solving for v, gives:
R
R0
r
yields
for r < R
For r > R, we have:
Solving for v, gives:
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for r > R
Rotation Curves of Spiral Galaxies
Observations !
v ~ constant (r0)
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Rotation Curves of the Milky Way
Clemens 1985, ApJ, 295, 422
Rotation curve for our Galaxy. Strange thing is.... rotation
curve is flat beyond the Solar circle, R0 = 8.5 kpc.
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You can work out what the matter
density profile should be to match the
observed rotation curves of galaxies.
Assume it is spherical:
r
dr
Consider a spherical shell of radius r and thickness dr. The mass in the shell is dMr
Take Newton’s laws for the force acting on a particle (a star) in this shell.
rearranging
Let the mass in the shell be
Then this leads to:
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and differentiating
Solving for the density gives
A slight variation keeps the density from
diverging at r → 0 :
Julio Navarro, Carlos Frenk, and Simon
White in 1996 ran a series of cold-dark
matter simulations, and they came up with a
“Universal profile” used today:
This is the Dark Matter distribution in
galaxies. True for the Milky Way and others.
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This is the Dark Matter distribution in galaxies. True for the Milky Way and others.
Julio Navarro, Carlos Frenk, and Simon White in 1996 ran a series of cold-dark
matter compute simulations, and they came up with a “Universal profile” used today:
This seems valid over an very large range of a and ρ0. For the smallest galaxies to the
largest galaxy clusters.
Julio Navarro
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Carlos Frenk
Simon White
Julio Navarro
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Carlos Frenk
Simon White
The Galactic Center
Challenging to observe because of all the dust/gas !
But, in 15 million years, the Sun will be 85 pc above the Galactic midplane, we would
presumably have a much better view then !
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The Galactic Center
Astronomers use high angular
resolution images in the near-IR
(~2 micron) to help see through
the dust. This is helpful because
there are large number of K and M
giant stars (T ~ 4000 K) in the
central part of the galaxy, and
these are brightest in at 2-micron.
Note that the nearest star to the
Sun is ~1 pc away. The density of
stars is much higher in the Galactic
Center !
From Schödel et al. 2002
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The Galactic Center
Astronomers use high angular resolution images in the near-IR (~2 micron)
to help see through the dust. This is helpful because there are large
number of K and M giant stars (T ~ 4000 K) in the central part of the
galaxy, and these are brightest in at 2-micron.
Astronomer group led by Rainer Schödel and Reinhard Genzel followed
the orbits of K-giants near the Galactic center.
One star, S2, has a period of 15.2 yr with eccentricity e=0.87 and
perigalacticon distance of 1.8 x 1013 m = 120 AU (a few times bigger than
Pluto’s orbit).
You can work out from Kepler’s laws that the mass interior to S2’s orbit is
~3.5 x 106 solar masses.
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The Galactic Center
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The Galactic Center
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The Galactic Center
Prof. Andrea Ghez’s UCLA group.
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The Galactic Center
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The Galactic Center
Nature,Vol. 419, p. 694 (2002)
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The Galactic Center
Nature,Vol. 419, p. 694 (2002)
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The Galactic Center
Degeneracy between distance
to center of Galaxy and Mass
of supermassive blackhole
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