The Milky Way

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The Milky Way
Nucleus Bulge Bar Disk Halo
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Thick Disk
Thin Disk
Thick 2 3 kpc Thin .6 - .8 kpc Where it
drops by a factor of e (2.72)
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Latitude (b), Longitude (l)
b
l0
l90
l270
l180
l180
-b
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Ecliptic
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Radio 74 cm
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Radio - 21 cm
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Infrared
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Near IR
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Visible
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UV
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X-ray
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Gamma-rays
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Disk

• Variations with distance from mid-plane
• Star density (decreases out)
• Fe/H (decreases out)
• Age (increases out)
• z-velocity (increases outward)
• Thin/Thick disk correlate with Populations

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• Thin disk
• Population I, solar Fe/H
• Spiral Arms (500 pc thick)
• Outside of spirals, stellar density lower
• Sun is in between arms, above midplane
• How do we know this?
• How do we find spiral arms?

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2 kpc
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Surveying the Galaxy

• Finding Disk, Spirals
• Look for gas
• H I for the disk
• CO, GMCs for the Spiral arms
• Other objects associated with spiral structure
• What do you actually see?
• What is actually measured?

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• Observe gas along a line of sight
• Measure radial velocities
• NOT DISTANCES!
• Need to assume a velocity pattern - rotation
  curve
• Need to know Suns velocity, distance

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Disk Sections

• Based upon how they move, and material
  concentration
• Within 3 kpc of center, Bulge (lt3kpc)
• From 3 kpc to solar distance (3-7.5 kpc)
• Beyond solar distance (gt7 kpc)
• Best velocity/material maps based on
• H I
• CO also indicate amount of H2

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• H I map
• Near l0º, very little H I lt 3 kpc (vel high, but
  not strong)
• Horns at l30º, vel150 km/s
• Curve not symmetric, galaxy is warped
• Rotation curve of galaxy is relatively flat

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• CO Map
• Not as spread out in velocity as H I
• Horns at l25, vel 130 km/s due to
  molecular ring
• Assymetric
• Rotation curve of galaxy is relatively flat
• H I 3x greater than CO in 3 kpcltRltsolar distance
• Very little CO beyond solar distance

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H I Map Fingers of God
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Overall gas distribution

• Molecular gas (CO) peaks at 4 kpc
• Within 3 kpc much less gas
• Almost all CO within solar circle
• Less than half of H I in solar circle
• H I remains high from 3 kpcltRltsolar circle
• At solar circle (½ density thickness)
• CO thickness 50 pc
• H I thickness 100 pc

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• Beyond solar circle
• Severe warping apparent (gt15 kpc)
• R24 kpc, warp 4 kpc
• H I peaks at 12-15 kpc
• H I density drops strongly after 16 kpc