The Milky Way Galaxy 1/30/03

The Milky Way Brief History Morphology - what are the components of the Galaxy? The Galactic Rotation The Galactic Center

Brief History What is the Shape of the Milky Way? Late 1700s - Herschels counted stars in 683 regions of sky, assumed all are equally luminous. Concluded that Sun at center of a flattened system.

N(f '>f) = A f-3/2; where A = constant 1920 - Kapteyn used a greater number of star counts and came to roughly the same conclusion Star Counts: If stars are distributed uniformly in space, then in any patch of sky, the total number of stars with flux less than a limiting flux, f is: N(f '>f) = A f-3/2; where A = constant Log(N(m’<m)) = C + 0.6m; where C = constant (Note: These formulae were derived on the board in class)

logN m Actual star counts are less than would be predicted by this relationship at fainter flux levels (or larger magnitudes)

Conclusion: stellar density not uniform but decreases with distance from Sun; faster in direction perpendicular to Milky Way and slower in the direction of the Milky Way Milky Way is a highly flattened disk

1919 - Shapley studied globular clusters; used distance derived from pulsating stars to determine that Sun is not at center of Milky Way. These were found at great distances above and below the plane of the Galaxy, where extinction effects are much less than that found along the Milky Way

Globular clusters Definitely bound by gravity Contain large numbers of stars in a very small volume: 20,000-1,000,000 stars in a volume 20 pc in diameter very round and symmetrical in shape - very old -- among the first stellar complexes formed in the galaxy

Distances from Variable Stars

Morphology of Galaxy

Disk Young thin disk Old thin disk Thick disk

Thin disk Diameter ~ 50 kpc Young thin disk scale height = 50 pc Old thin disk scale height = 325 pc Contains youngest stars, dust, and gas Contains Sun, which is 30 pc above midplane M* = 6  1010 Msun Mdust+gas = 0.5  106 Msun (scale height 0.16) Average stellar mass ~ 0.7 Msun LB ~ 1.8  1010 Lsun Population I stars in the Galactic plane Contains ~ 95% of the disk stars [Fe/H] ~ -0.5 - +0.3 Age ~ < 12 Gyr Spiral structure seen in neutral H, HII regions, young O and B stars

Thick disk Diameter ~ 50 kpc Scale height = 1.4 kpc M* = 2-4  109 Msun LB ~ 2  108 Lsun [Fe/H] ~ -1.6 - -0.4 (less metal rich than thin disk) Age ~14-17 Gyrs

Gas and Dust

Spheroidal Components Central bulge Stellar Halo Dark Matter Halo

Central Bulge Diameter ~ 2 kpc Scale height = 0.4 kpc M* = 1  1010 Msun LB ~ 0.3  1010 Lsun [Fe/H] ~ -1.0 - +1.0 (less metal rich than thick and thin disk) Age ~10-17 Gyrs

Stellar Halo Diameter ~ 100 kpc Scale height = 3 kpc number density distribution ~  r-3.5 M* = 0.1  1010 Msun LB ~ 0.1  1010 Lsun [Fe/H] ~ -4.5 - -0.5 (metal poor) Age ~14-17 Gyrs

Dark Matter Halo? Rotation of Galaxy implies that there is a lot of mass in our Galaxy that we don’t see (ie, if we count up the mass from the stars that emit visible light, it’s much less than that implied by observing the motion of stars as a function of radius from the center of the Galaxy. How do we know that the stars in the disk rotate around the center of the Galaxy? How do we know the rotational velocity of the Sun? How do we know the rotation curve? (rotational velocity as a function of radius from the Galactic center?)

Determining the rotation when we are inside the disk rotating ourselves 23.5° 39.1° To determine the rotation curve of the Galaxy, we will introduce a more convenient coordinate system, called the Galactic coordinate system. Note that the plane of the solar system is not the same as the plane of the Milky Way disk, and the Earth itself is tipped with respect to the plane of the solar system. The Galactic midplane is inclined at an angle of 62.6 degrees from the celestial equator, as shown above.

Galactic longitute (l) is shown here The Galactic midplane is inclined 62.6° with the plane of the celestial equator. We will introduce the Galactic coordinate system. l=0° Galactic longitute (l) is shown here l l=90° l=270° l=180°

Galactic latitude(b) is shown here

Galactic Coordinate System: b l

Derivation of radial and tangential velocities for a rotating disk was done on the board.