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The 3-kpc Arms

3-kpc Arms The Near 3-kpc Arm was discovered in the 1950's through 21-centimeter radio measurements of HI (atomic hydrogen). It was found to be expanding away from the center of the Milky Way at more than 50 km/s. This spiral arm contains about 10 million solar masses of gas, mostly hydrogen atoms and molecules. It is about 3.3 kpc from the galactic center and 5.2 kpc from the Sun.

The Far 3-kpc arm was discovered in 2008. It has properties like radius, expansion velocity, mass, and brightness that were mirror images of the near 3-kpc arm.

Figure 05-16f4 3-kpc Arms [view large image]

Figure 05-16f4 is a two dimansional H1 density map near the Milky Way center with longitude of the Galactic coordinates imposed on top.

Milky Way Bars These observations suggest different kinds of model to explain the origin of 3-kpc arms. Ultimately, they are related to the activities in the Galactic Center (GC). In barred spirals, there is a bar of stars runs through the central bulge. The arms of barred spirals usually start at the end of the bar. It is suggested that galactic bars develop when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. The tiny elongations in the stars' orbits grow and get locked into place, forming a bar. The bar becomes even more pronounced as it collects more and more stars in elliptical orbits. Eventually, a high fraction of the stars in the galaxy's inner region join the bar. This process is said to be demonstrated repeatedly with computer-based simulations.

Figure 05-16f6 MW Bars

Figure 05-16f6 shows the structure of the inner Milky Way including : the bulge, the long bar (origins of the spiral arms at its ends), an additional galactic bar, and the "3-kpc arms" - the near one is inside the Norman arm, both 3-kpc arms are devoid of stars.
Astronomers from the Sloan Digital Sky Survey III (SDSS-III) announced in December 19, 2012 that they have found hundreds of stars rapidly moving together in long, looping orbits around the center of the Milky Way. They think these stars are part of the Milky Way bar (see "So These Stars Orbit in a Bar...").

The following example illustrates how instability develops from perturbation and finally leads to a bar structure. It starts from a very thin disk of stars (viewed as particles in the mathematical formulation) uniformly distributed in the x-y plane. A perturbation in the z direction and propagating along the x-axis is investigated by mathematical formulation in the following. Figure 05-16f7 shows the development of such system as function of time (from t = 200 to t = 2000 in arbitrary unit) in computer simulation.

The formula for perturbation h as acceleration az in the z direction and moves in the x direction with velocity u is given by :

Figure 05-16f7 Instability

In another word, instability will develop with either or both high averaged stellar random velocity and low stellar surface density.
See "Firehose Instability".
In short, there are divergent data on the galactic center, making it difficult to explain by a single physical model. The next section on the black hole at the Milky Way center seems to be easier to understand (but more difficult to observe) as it has a well established theoretical foundation.

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