Download Dust Mapping Our Galaxy 1 [12.1]

Dust
•
[12.1]
Optical
Tiny grains ( ≤ 1µm)
Infra-red
•
Cores consisting of
No Reddening
• Graphite
• or Silicates
(MgxFe1-xSiO3; Mg2xFe2-2xSiO4; etc)
Ices can condense on
surface:
•
Optical
UBV
Near-IR
J H K
Galactic Pole
AV = 0.07 mag
C,O combined with H
• Absorb and scatter light
Galactic Center
AV = 30 mag
• Effect strongest in blue,
• less in red,
• zero in radio.
From star counts:
Baade’s Window
AV = 1.6 mag
Flux Æ
•
Ultraviolet
log λ (µm) Æ
Mapping Our Galaxy
Sun
Kapteyn (1922). Surfaces of constant star density.
From distribution of Globular Clusters:
• Use pulsating variables to find clusters’ distances.
• Clusters are out of MW disk Î little reddening.
Shapley (1920) Globular Clusters
(distances using RR-Lyraes)
< MV >
Period-Luminosity
Relation
[Fig. 14.5]
The Modern View
1
[Table 24.1]
Milky Way’s Morphology
Exponential disk
n( z , r ) = no ( e − z / zthin + 0.02e − z / zthick )e − r / hr
+ spheroids with various “forms”
n = no × form
[Fe/H] =log(Fe/H)
abundance ratio relative
to Sun
Usually see deVaucouleurs’
r1/4 surface brightness law:
Milky Way Bulge
• Elongated… now thought to be a bar
• From observations of Mira pulsating variables.
• Minor/major ~0.6
• Roughly follows DeV profile (r1/4 law)
• Baade’s window
• Moderate metallicity (-1 < [Fe/H] < +1)
~1010 M
SB0 isophotes [BM] Fig 4.57
• Expanding 3kpc arm
• H I feature
• vr = -50 km/s
• Elliptical orbit due to bar
ROUND BULGE
PEANUT-SHAPED
The Psychedelic Barred Spiral Movie
2
Milky Way Halo
• Globular clusters + field stars
• Field stars = high velocity stars
• ~150 globular clusters known, in 2 different systems:
• Older (~13 Gyr)
• Very metal-poor (-2.5 < [Fe/H] < -0.8)
• Spherical distribution around galactic center
• No net rotation
[Fig 24.12]
Warning!
• Younger
• (~11Gy, ~ same as thick disk)
• Moderate metallicity ([Fe/H]> -0.8)
• Compare to thick disk –0.6 < [Fe/H] < -0.4
• Flattened
• Show net rotation => part of thick disk??
These are
plots of
directions in
sky,
NOT crosssections of
MW
[Table 24.1]
3
The Gas Disk
25 kpc
8
dust
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:
•
•
•
•
Half-density
Thickness (pc)
<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) Æ
•
H2
Radius
(kpc)
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
Å Galactic Longitude
4