Transcript Document
Galactic coordinates in galactic plane in celestial equator plane NCP galactic equator tilted ~ 63 1/2 deg to CE galactic center is toward Sagitarrius RA~18h, Dec ~ -29deg
Makeup of Milky Way Galaxy Stars
- Disk (O, B stars, SG, young to old open clusters) Halo & Bulge (RR Lyr, globular clusters, MACHOs)
Gas
- some in disk, hot gas in halo
Dust
- in disk [results in reddening E(B-V), A v ]
DUST m-M = -5 + 5 log d + A E(B-V) = (B-V) observed - (B-V) normal A v ~ 3 E(B-V) [~ 1 mag/kpc roughly] Find E(B-V) from:
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spectral type of star and observed B-V
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H
/ H
ratio [normal = 3]
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HI maps N H /E(B-V) = 5x10 21 atoms/cm 2 /mag
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2200Å bump
0 Extinction = A(
)/E(B-V) 10 4 0 0 0 0 0
Wavelength (Å)
0
Stellar Populations
z(pc) Age (10 9 yrs) Z Distr Examples Extreme Pop I 120 <0.1 0.04 patchy O,B,SG, open clusters Older Pop I 160 0.1-10 0.03 patchy sun, A stars
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Disk Pop II 400 3-10 0.02 smooth planetaries, RR Lyr Intermed Pop II 700 10 0.01 smooth long P var Halo Pop II 2000 >10 0.003 smooth globular clusters
Interstellar Gas
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optical absorption lines CaI, CaII, NaI
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HII regions (recombination around hot star) T~10,000K, density ~ 5000 ions/m 3
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HI gas (21 cm) T~100K, density ~10 6 atoms/m 3
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molecular clouds (radio) H 2 , OH, NH 3 mol/m 3 T~10K, density ~10 9
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X-rays (hot coronal gas) T~ 10 6 K, density < 10 4 particles/m 3
Counting Stars D= #stars/unit volume Local luminosity function: #stars/unit V with given M v total sky = 4
steradians = 41,253 sq deg for solid angle
, area =
r 2 dV =
r 2 dr N(r) = D(r)dV=
Dr 2 dr = 1/3 Dr 3
log r = (m-M+5)/5 = 0.2 m + const (for given M) r = 10 (0.2m+c) and N(r) = 10 (0.6m+c) since 10 0.6
= 4, expect 4xmore at m+1 than m not observed r dr
r 2
Finding the mass of the Milky Way
Kepler’s law using sun’s orbit (P=250 million yrs, v=250 km/s, a=8kpc) m MW + m sun = 4
2 a 3 /GP 2 ~ 10 11 M
Halo mass: MACHOs, high vel stars Rotation curve: M = rv 2 /G
RV=0 at l=0, 180 RV=+ at l=45, 225 RV=- at l=135, 315
+ + sun GC + -
G
Spiral arms can be traced from the positions of clouds of atomic hydrogen
The Galactic Center (Sgr A * ) Evidence for a Supermassive BH at the center:
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stationary (located at dynamic center of MW)
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energetic X-ray source
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small size (radio shows smaller than solar system)
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no visible object at opt nor IR from Keck images
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motions of nearby stars (1000’s of km/s) imply 3 million M
How does Supermassive BH form?
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stars in center are < 1000AU apart (200,000AU near sun)
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SN chain reaction could produce many stellar BHs
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collisions between BHs cause monster supermassive BH
Galaxy Evolution
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Top Down: large concentration of matter (10 15 M
) fragment into galaxies of 10 12 M
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Bottom up: small structures merge into galaxies, then clusters 1) 2) 3) 4) 5) globulars formed ~ 13 billion yrs ago collapse to disk star formation continued in disk collisions with dwarf galaxies add to halo in ~ 5 billion yrs, collision with Andromeda could cause burst of star formation, uses up gas & dust and turns MW into an elliptical galaxy
Content Review of Astr 322- the Contents of the Milky Way Stars Gas & Dust Dark Matter Hot, cold; A v , E(B-V) MACHOs + ?
Single (sun), binary, clusters (open, globular) Properties (d, T, L, M v , spectra, mass, radius) Evolution - low mass (T Tauri, MS, giant, planetary, WD) - high mass (MS, SG, SN, pulsar or BH) Variables - geometric, eruptive, pulsating Structure Disk Bulge Halo Pop I Pop II Pop II Motion Disk - LSR Halo - high v, elliptical Viewing geometry Horizon (alt, azimuth) Celestial (RA, Dec) Galactice (b, l) Instrumentation: Telescopes (refractors, reflectors) CCDs, spectrographs, Space