The Milky Way Nucleus Bulge Bar Disk Halo
Feb 25, 2016
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