Journey to the Galactic Center Bo Zhang

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 2018/11/12

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 2018/11/12

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 2018/11/12

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 2018/11/12

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, 504-515, 1996. 2018/11/12

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 2018/11/12

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 2018/11/12

Central Star Dynamics – Pistol Nebula and Star in Quintuplet Cluster 2018/11/12

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 2018/11/12

The Mini-Spiral The Mini-Spiral Northern Arm; Eastern Arm; Western Arc 2018/11/12

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 2018/11/12

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 2018/11/12

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* 2018/11/12

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 2018/11/12

Related Sites Infrared and Submillimeter Astronomy Group at MPE: http://www.mpe.mpg.de/ir/GC/index.php The UCLA Galactic Center Group: http://www.astro.ucla.edu/~jlu/gc/index.shtml Related Sites 2018/11/12