The Milky Way. Cerro Tololo InterAmerican Observatory. Large Magellanic Cloud. K. Don, NOAO/AURA/NSF

Transcription

1 The Milky Way Large Magellanic Cloud Cerro Tololo InterAmerican Observatory K. Don, NOAO/AURA/NSF

2 Roger Smith/NOAO/AURA/NSF

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

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

5 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)

6 Milky Way Galaxy Our Galaxy is a collection of stars and interstellar matter - stars, gas, dust, neutron stars, black holes held together by gravity

7 Infrared (3-8 micron) view of Center of Milky Way Galaxy

8 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: ; 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.

9 Historical Models of the Milky Way Galaxy In mid-1700s, Immanuel Kant ( ) and Thomas Wright ( ) 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 ( ) 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

10 Historical Models of the Milky Way Galaxy Jacobus Kapteyn ( ) 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): Jacobus Kapteyn ( )

11 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.

12 Historical Models of the Milky Way Galaxy From , 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 ( ) 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.

13 Historical Models of the Milky Way Galaxy Who was right, Kapteyn or Shapley?

14 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.

15 Credit: Axel Aitoff

16 Infrared (3-8 micron) view of Center of Milky Way Galaxy

17 Star Counting

18 Morphology of the Milky Way R0 = 8 kpc Sun from Digital Sky LLC

19 from Digital Sky LLC

20 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.

21 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.

22 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 Hn thin = 350 pc (youngest stars found with scale height of pc). Thick disk: older stars with a scale height of Hn thick = 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/h thin ] exp[-z/h thick ] exp( -R/H radial ) where z is the vertical height above the midplane, and R is the distance from the Galactic center. H radial = 2.25 kpc, n stars/pc 3 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.

23 The Morphology of the Galaxy Structure of Thin and Thick Disks Neutral Gas Thin Disk Thick Disk Mass (10 10 M ) to 0.4 LB (10 10 L ) M/LB (M /L ) 3 Radius (kpc)

24 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~ 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.

25 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] ~ 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.

26 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)

27 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?

28 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 Gyr ago.

29 from Digital Sky LLC Spiral Structure Galaxy M 51

30 Spiral Structure from Digital Sky LLC

31 Evidence for Spiral Structure

32 Evidence for Spiral Structure

33 Evidence for the spiral structure of the Galaxy comes from nearby associations of hot, new stars. Also from observations of interstellar hydrogen gas.

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35 Evidence for Spiral Structure 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 Gyr ago.

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47 from Digital Sky LLC

48 Milky Way Galaxy Dust and gas sits in very narrow midplane. Fuel for new stars. Composite near-ir (2 micron) Image from the Two Micron All Sky Survey (IPAC/Caltech/UMass)

49 Credit: Axel Aitoff

50 The Galactic Bulge Galactic Bulge: Independent component from disk. Mass of the bulge is believed to be ~10 10 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 r 1/4 law distribution, discovered by Gerard de Vaucouleurs ( ), 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.

51 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].

52 The Galactic Bulge Gilmore et al 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).

53 The Morphology of the Galaxy Thin Disk contains Spiral Arms Thick Disk

54 Milky Way Galaxy The Galactic Bulge COBE Satellite image of Milky Way at micron.

55 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

56 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

57 The Components of the Galaxy Neutral Gas Thin Disk Thick Disk Bulge Halo Mass (10 10 M ) LB (10 10 L ) M/LB - 3 ~10 3 ~1-3 Radius (kpc) >100 Scale Height (kpc) < [Fe/H] > to to to 0.5 < -5.4 to -0.5 Age [Gyr] <~ <0.2 to to 13

58 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.

59 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: R0 r R with yields Solving for v, gives: for r < R For r > R, we have: Solving for v, gives: for r > R

60 Rotation Curves of Spiral Galaxies Observations! v ~ constant (r 0 )

61 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.

62 Rotation Curves of Spiral Galaxies Vera Rubin (b1928) Rubin, Thonnard, & Ford, 1978, ApJ, 225, L107 Responsible for most of the work on the galaxy rotation rate problem.

63 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 and differentiating Let the mass in the shell be Then this leads to:

64 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.

65 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 Carlos Frenk Simon White

66 Julio Navarro Carlos Frenk Simon White

67 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!

68 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

69 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 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 10 6 solar masses.

70 The Galactic Center

71 The Galactic Center

72 Prof. Andrea Ghez s UCLA group. The Galactic Center

73 The Galactic Center

74 The Galactic Center Nature, Vol. 419, p. 694 (2002)

75 The Galactic Center Nature, Vol. 419, p. 694 (2002)

76 The Galactic Center Degeneracy between distance to center of Galaxy and Mass of supermassive blackhole