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The Milky Way

Cerro Tololo Inter-American Observatory

Large Magellanic Cloud

K. Don, NOAO/AURA/NSFSaturday, January 5, 2013

Roger Smith/NOAO/AURA/NSF

Saturday, January 5, 2013

Panoramic Picture of Milky Way taken from Death Valley, CA, Dan Duriscoe, US National Park Service


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)


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.



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

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)



Jacobus Kapteyn (1851-1922)

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):



Harlow Shapley (1885-1972)

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.

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.



Who was right, Kapteyn or Shapley ?



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.


Credit: Axel Aitoff


Morphology of the Milky Way

from Digital Sky LLC

Sun

R0 = 8 kpc


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.



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.



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.

Our Sun is a member of the thin disk, and lies about 30 pc above the midplane.



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.



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.


The Morphology of the GalaxyAge-Metallicity Relation

Rana 1991, ARAA, 29, 129

Time since formation of disk (Age - td, where td = 12 Gyr)

Solar Value



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 ?



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.


Spiral Structure



Evidence for Spiral Structure



http://www.youtube.com/watch?v=Suugn-p5C1M




http://www.atlasoftheuniverse.com/galaxy.html

http://www.atlasoftheuniverse.com/galaxy.html


Thin Diskcontains Spiral Arms

Thick Disk


Milky Way Galaxy

The Galactic Bulge

COBE Satellite image of Milky Way at 1.2-3.5 micron.


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.


The Galactic HaloGalactic (“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


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


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 !


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. 2002Saturday, January 5, 2013

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.


The Galactic Center


The Galactic CenterProf. Andrea Ghez’s UCLA group.


http://www.astro.ucla.edu/~ghezgroup/gc/pictures/orbitsMovie.shtml

http://www.astro.ucla.edu/~ghezgroup/gc/pictures/orbitsMovie.shtml

The Galactic Center


The Galactic Center

Nature, Vol. 419, p. 694 (2002)


The Galactic Center

Degeneracy between distance to center of Galaxy and Mass

of supermassive blackhole


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