Bulge and Nucleus of the Milky Way

The above shows the center of the Milky Way galaxy with the constellation Sagittarius superimposed on the image. The center of the Milky Way sits just above the spout of the teapot. The bulge sits 27,000 light years from the Earth with coordinates RA = 17h45m40.04s and δ = -29:00:28.1 (J2000). The bulge is not easily observed in the visible, UV, or soft x-rays. It is best observed in the γ-ray, hard x-rays, IR, sub-millimeter, and radio.

The bulge of the Milky Way is more spherical than the disk and is composed of stars which are more reddish than those in the disk (===>they are more evolved or low mass, but are still considered Pop I stars). The bulge is roughly 30,000 to 40,000 light years in diameter.

The central region of the Galactic Bulge (the nucleus) is interesting because it shows activity similar to that shown by Active Galactic Nuclei, AGNs (although at a considerably lower level).

• The core harbors ~ 1,600 stars per cubic light year. This is several 100,000 times as dense as the average stellar density of our Galaxy! Further, when one approaches the center of the Galaxy, one finds a dense cluster containing roughly 1 million stars with a stellar density around 10 million times as high as in the Solar neighborhood.

• Near the center of the Milky Way lies the bright radio source Sagittarius A. There is structure on scales ranging from 600 light years (the filamaents) to what appears to be a ring of material on scales of 10-15 light years to structure on scales of 10 Astronomical Units (~80 light minutes ~ 1.5 billion km)--an Astronomical Unit is the average distance of the Earth from the Sun.

• Further observations reveal a bright central object nearly at the exact center of the Galaxy, Sagittarius A^* . Just as one can measure galaxy rotation curves to gain a sense of the mass and mass distribution within galaxies, similar ideas can be applied to any gravitating system.
  • For the galactic center, one may infer the mass for Sgr A^* --(4.1-4.3)x10⁶ Solar masses--and that it is compact, 0.1 astronomical units, from study of stars (SO-2) near the Galactic center. SO-2 has orbital period - 15.2 years with pericenter = 17 light hours.

  • The shortest period star, SO-102, has period = 11.5 years.

  • Sgr A^*, typically has small Luminosity, L = 10³⁶ erg/s. If Sgr A^* is powered by accretion, we have
    L = GM_*S/R_*=4.45x10²¹S,
    where S is the mass accretion rate. For the observed L, we expect S = 10^14-15 g/s. Modeling the mass loss from the stars in the bulge leads to the estimate S_b = 6x10²⁰ g/s. This estimate is >> than that predicted from L. Suggested that this implies the existence of an event horizon.

  • VLBI sub-millimeter (1.3 mm) observations (JCMT on Mauna Kea, ARO/SMT on Mt Graham, AZ, and CARMA array at Cedar Flat, CA) observed Sgr A^* for three consecutive nights in 2009. Detected changes on the third night consistent with variability on length scales of a few Schwarzschild radius.

  • An exciting new instrumental project is the development of the Event Horizon Telescope. The Event Horizon Telescope will extend the above sub-mm (1.3 mm and 0.8 mm) VLBI experiment by adding the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the Large Millimeter Telescope (LMT) in Mexico, and the South Pole Telescope (SPT). Greatly increases the sensitivity of the array, and the N-S and E-W coverage to increase coverage in the UV plane.

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