The Milky Way Nucleus Bulge Bar Disk Halo

Thick Disk Thin Disk Thick = 2 – 3 kpc Thin = .6 - .8 kpc Where it drops by a factor of e (2.72)

Latitude (b), Longitude (l)

Ecliptic

Radio – 74 cm

Radio - 21 cm

Infrared

Near IR

Visible

UV

X-ray

Gamma-rays

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

Outside of spirals, stellar density lower 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?

2 kpc

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?

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

Disk Sections Based upon how they move, and material concentration Within 3 kpc of center, Bulge (<3kpc) From 3 kpc to solar distance (3-7.5 kpc) Beyond solar distance (>7 kpc) Best velocity/material maps based on H I CO – also indicate amount of H2

H I map Near l=0º, very little H I < 3 kpc (vel high, but not strong) “Horns” at l=30º, vel=150 km/s Curve not symmetric, galaxy is warped Rotation curve of galaxy is relatively flat

CO Map Not as spread out in velocity as H I “Horns” at l=25, vel = 130 km/s – due to “molecular ring” Assymetric Rotation curve of galaxy is relatively flat H I 3x greater than CO in 3 kpc<R<solar distance Very little CO beyond solar distance

H I Map “Fingers of God”

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 kpc<R<solar circle At solar circle (½ density thickness) CO thickness = 50 pc H I thickness = 100 pc

Beyond solar circle Severe warping apparent (>15 kpc) R=24 kpc, warp = 4 kpc H I peaks at 12-15 kpc H I density drops strongly after 16 kpc