The Milky Way

The Milky Way

NucleusBulgeBarDiskHalo

Thick Disk

Thin Disk

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

Latitude (b), Longitude (l)

+b

-b

l=0l=90 l=270 l=180l=180

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

• 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