The Milky Way Galaxy

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positions interior to Sun’s orbit in Galaxy have some distance ambiguity

Less distance ambiguity outside of Solar orbit, and better evidence of arm-like morphology

Spiral Structure

Slide 35

The Galactic Nucleus

Inner 500pc of Galaxy

Extinction makes optical studies impossible - use radio or IR

Observe ionized gas, line emission, dust, star clusters

Stellar density is 107 stars per pc3 (compared to 0.1 in the solar neighborhood)

If the Sun were near the GC

Nearest star would be 1000AU away

A million stars brighter than Sirius in the night sky

Total starlight more than 200 times brightness of the full Moon

Slide 36

The Galactic Center

Optical vs Radio observations

Slide 37

75pc 150pc

The Galactic Center Radio schematic

Slide 38

Molecular material in inner 200pc relatively hot and dense: 104 per cm3 and 70 K

High velocity dispersion (50 km/s) of molecules

Mass: 108 Msun

High density helps star formation but high temps don’t

SF rate ~ 1Msun/year

The Galactic Center Star Formation

Slide 39

Radio emission shows bent arc of gas, filamentary structure

Also seen in IR

Thermal and synchrotron radiation

X-ray emission (produced when electrons from filaments collide with colder gas cloud) gives gas temperatures of T=107 to 108 K

Could result from past SN explosions

Slide 40

Supermassive Black Hole in the Galactic Center

Radio image (80 pc across) shows feature SgrA and radio filaments

Radio image (10 pc across) shows feature known as SgrA West – center of this is SgrA*

Investigate IR stellar motions in region about 1pc across (a few lightyears) to estimate BH mass

Slide 41

Measure proper motions of stars around Galactic Center

Adaptive optics at large telescopes improved ground-based resolution to 0.5” in IR (stellar positions measured to 0.002”)

90 stars identified and proper motions (largest at 1400 km/s!) centered about SgrA* to within 0.1”

Velocities consistent with Keplarian motion (all mass at center)

M = 2.6 +/- 0.2 x 106 Msun

Slide 42

Curvature of the paths near SgrA* constrain the volume of the mass to ~ Schwarzchild radius (few x 106 km)  Supermassive Black Hole