Download The Gas Disk Stellar halo Bulge (= bar) Disk

(dark matter)
(dark matter)
(dark matter)
Stellar halo
Bulge (= bar)
Sun
8 kpc
(dark matter)
(dark matter)
Disk
• thick
• thin
• gas
50 kpc
(dark matter)
(dark matter)
[see CO fig 24.5, 24.6]
The Gas Disk
Radius
(kpc)
25 kpc
8
dust
H2
HI
•
•
•
•
Increases outward
Sun is ~ 30 pc above midplane.
Disk is warped starting at R ~ 7 kpc
Mass
•
•
•
Electron spin-flip transition in H ground
state
Galaxy essentially transparent at 21 cm
Diameter: ~ 50 kpc
Thickness:
•
•
•
•
<4
100
4 - 10
250
11
300
15
650
20
1000
HI ( = H0) detected through 21 cm
emission
H I: 4 x 109 M
H2: 1 x 109 M
Average density: 0.04 M pc-3
z (kpc) 
•
Half-density
Thickness (pc)
H I Maps Showing Warp
R = distance from Galactic Center
6
4
2
0
-2
-4
-6
6
4
2
0
-2
-4
-6
100
0
R = 12 kpc
R = 20 kpc
R = 16 kpc
R = 24 kpc
-100
100
0
-100
 Galactic Longitude
1
The Interstellar Medium
• = the gas disk
• Includes ionized, neutral, molecular gas (H+, H0, H2) = (H II, H I, H2)
Molecular clouds
[12.1,12.2]
• Massive interstellar gas clouds
• Up to ~105 M
• 100’s of LY in diameter.
Fraction by mass
H
73%
He
25%
Metals 2%
• High density by interstellar medium standards
• Up to 105 atoms per cm3
• Shielded from UV radiation by dust, so atoms combined into
molecules.
• Mostly H2, but not easily detectable
• Also H2O, NH3, CO etc.
• form emission lines in observable passbands
• CO is usual tracer.
– mm wave observations ( low angular resolution)
• All stars form in molecular clouds.
CO map of Milky Way
Molecular clouds are in the disk and are associated with dust.
Optical (lII = +/- 90o)
Mm-wave map of CO within 2.5 kpc
Dame, Hartmann & Thaddeus (2000, ApJ)
2
M101
H I (21 cm)
traces spiral structure
M51
Molecular clouds occur in spiral arms
CO contours over
red image
CO contours over
21 cm map
Computer simulation of Star Formation in a
Molecular Cloud
By
Matthew Bate,
University of Exeter
UK
• Collapse and fragmentation of 50 solar-mass cloud.
• Initially 1 light-year in diameter.
3
Star formation thought to propagate in wave through
dense molecular clouds
• Photons from very luminous O stars heat and blow away surrounding
gas.
• So slightly older clusters no longer shrouded by dusty gas
• Compression of gas further inside cloud causes inward wave of star
formation (“triggered” star formation).
recombination
H II ( =
H+)
Regions
H0, H2
photoionization
H+
O5V 47,000K
Ne III
B0V 35,000K
O II
B5V 15,000K
A0V 9,500K
Flux 
He I
OI
H0 photoionization
cross section
O III
Wavelength (A) 
Black-Body Curves
representing hot stars
(but normalized to far UV)
S II Ar III
O II
H
H
S III
H
Wavelength 
4