1. Journey to the Galactic Center
Bo Zhang

2. Milky Way’s Structure The Structures of the Milky Way Halo Bulge
Thin Disk
Thick Disk: discovered in 1983; revolve around the center slower than ones in thin disk; metallicity between thin disk and the halo; stars as old as younger halo stars
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3. The Galactic Center
Located in the Constellation of Sagittarius at a distance of 26,000 light-years from Earth
Hidden behind dense molecular clouds
Can be studies by radio, infrared and high-energy astronomy
The Galactic Center
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4. The Molecular Ring
Indicators: Dark patches on the glowing background of Milky Way
Emitting radio and far-infrared radiation profusely
Distance: 10,000 to 16,000 ly from Earth
Observing Method: Analyzing the Doppler shift of the radiation to obtain the clouds’ velocities (But this lead to 2 different results, so the ring might be a spiral only)
Possible origin: Gravitational interaction between the nucleus and the outer materials
The Molecular Ring
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5. The Central Bar - Proof
The innermost HI clouds make their motions following a way observed in barred spiral galaxies
- de Vaucouleurs, 1964
Surface brightness distribution of the bulge: Brighter in the North and dimmer in the South
- Blitz and Spergel, 1991
de Vaucouleurs r1/4 Law:
Analysis of carbon stars, globular clusters, planetary nebulae, K-type stars
The Central Bar - Proof
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6. The Central Bar - Different Models
Different models of the bar projected onto the Galactic plane. Counters show the differences of surface distributions. And the galactic longitude increases counterclockwise.
Different Models of the Bar
From Kuijken, K. Observational Evidence for a Bar in the Milky Way. ASP Conference Series, Vol. 91, , 1996.
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7. Central Star Dynamics – Young Stars
Most central stars are old, but there are also young star populations.
They might be the byproduct of the central bar's formation.
The merging with Sagittarius Dwarf Galaxy also adds new stars to the central bulge from the thin disk.
These stars might not be so young as they looks.
Central Star Dynamics – Young Stars
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8. Central Star Dynamics - Superclusters
High density of O-type stars
Overabundance of Wolf-Rayet Stars
As big as 300 ly across and with 100 million solar masses
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9. Central Star Dynamics – Pistol Nebula and Star in Quintuplet Cluster
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10. Structures Around the Central Black Hole
Turbulent arches – indicators of the magnetic field
Circumnuclear Disk
Sagittarius A* ’s Mini-Spiral (considered as the infalling material from Circumnuclear Disk)
Fine Structures Around the Central Black Hole
Proposed by Martin Rees in 1974
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11. The Mini-Spiral The Mini-Spiral Northern Arm; Eastern Arm; Western Arc
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12. Central Star’s Orbits Observations by the UCLA Galactic Center Group
Left by Keck and Right by NTT, ESO
Observations by the UCLA Galactic Center Group
Observations by the MPI Group
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13. The ‘Starving’ Black Hole
The black hole only consumes 1/10000 of its available fuel.
Baganoff: Now the black hole is situated in a SN remnant formed tens of thousand years ago, whose blast waves can sweep the hole's fuel away.
Ghez: Infrared observation shows that the black hole is as active as other galaxies’.
The ‘Starving’ Black Hole
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14. Emission from Sgr A*
Spectral energy distribution of the emission from Sgr A* at radio and X-ray wavebands (by the MPI Group).
The first detection of infrared (3.8 micron) light from plasma falling onto the supermassive black hole at the galactic center, showing that violent events occur almost continually (by the UCLA Group).
Emission from Sgr A*
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15. The Black Hole’s Other Features
Might be a Kerr black hole according to a kind of quasi-periodicity of the flare near the hole
Radius: 14 million miles
Also in motion
The Black Hole’s Other Features
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16. Related Sites
Infrared and Submillimeter Astronomy Group at MPE:
The UCLA Galactic Center Group:
Related Sites
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