Galaxies

The 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 contrary to the notion of everything falling into the black hole at the Milky Way center. A very simple explanation is by Newton's 3rd law of action/reaction, and it also occurs in 2 opposite directions to conserve linear momentum. 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.

Many problems arise on closer inspection of the 3-kpc arm. It is not really a spiral arm, and its origin is unknown. The enigma is summed up in the article "On the many "3-kpc arms" – shocked wave and nuclear rotation" :
• Figure 05-16f5 shows the radial velocity of the 3-kpc and other spiral arms near the Galactic Center (GC). The 3-kpc arms are confined within the space of -13 to +13 longitudes and a rather large radial velocity range from +120 to -120 km/sce (negative signifies toward the Sun, positive means points away). These features do not overlap with the other spiral arms.
• It doesn't look like a spiral arm as it lacks a continuous and curvaceous shape (not spiral-shaped).

	No OH maser is detected along the "3-kpc-arms", while these masers are seen in all spiral arms. It suggests that the 3-kpc arms contain mostly gas of hydrogen atoms, there is no stars, planets, dust, ... assorted astronomical objects within. It has no line-of-sight tangential movement, that is, it doesn't seem to be rotating.
Figure 05-16f5 Milky Way, Inner Region [view large image]

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.

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".

Anyway, there are many more models trying to explain the observations :


• An expanding ring structure - An expanding molecular ring (mostly H₂, but traced by CO emission) in the 100-200 pc region of

  GC has been observed back in late 1990 (see "Galactic Center Molecular Arms, ..."). There is also indication of young star forming at the outer edge of the Milky Way. Expansion rate of 500 m/sec has been measured for the galaxy NGC4565 (see "Are the Milky Way’s borders expanding?"). It seems that expansion is a feature for certain galaxies including the Milky Way contrary to the notion of everything tends to fall into the central black hole.

  Figure 05-16f8 CMZ

  Figure 05-16f8 is a velocity-declination map showing the 3-kpc arm and the Central Molecular Zone (CMZ) which is confined to a region of about 150 pc (1 pc ~ 10 times the size of the solar system out to the Oort cloud).

• Turbulence around a shock - The center of the Milky Way is an extreme environment which contains a massive black hole

  		surrounded by a very dense star cluster, two other adjacent star clusters, and a giant molecular cloud which would serve as an incubator to a new generation of stars. The gas and dust in its vicinity are denser by 2-3 orders of magnitude than in other locations in the Milky Way. This is also the case with the magnetic field. The kinematics of the gas is characterized by apparently random, supersonic flows. The paper on "Shock-Generated Turbulence In the Innermost 50 pc of the Galaxy Center" provides observational evidence for the existence of a supersonic turbulence, most likely generated by the shock waves.
  Figure 05-16f9 MW Center [view large image]	Figure 05-16f10 GC	Figure 05-16f9 shows the 21 cm radio wave observation with a few identified objects (the insert is in 6 cm radio); Figure 05-16f10 is an artist's rendition for a lot more possible objects and expolsive processes around the black hole (see more in "Black Hole at the Milky Way Center").



• Nuclear rotation - Another model notices that between 0 and 2 kpc from the Galactic Center region, billions of stars inhabit an elongated conglomeration called a stellar bar, which extends for several thousand light-years on either side of the galactic center. As that central bar rotates, it produces large-scale shock waves that likely sculpt the 3-kpc spiral arms and power their outward motions. In fact, theorists believe that those spiral arms connect to the ends of the bar.


• Flowing elliptical streamlines - This model involves gas flowing along concentric elliptical streamlines such as might be caused by a barlike perturbation in the gravitation field of the inner regions of the Galaxy. It is found that such a model can produce not only the so-called 3 kpc arm and the 135 km per sec arm features, but other high-velocity features as well. This model does not require a highly energetic explosion at the galactic nucleus as does the usual expanding spiral-arm model, because there is no net flow of matter out of the inner regions.


• Bar instability - A phenomenon where the rotational velocity of the bar in a barred spiral galaxy increases to the point of spiral spin-out. Under typical conditions, gravitational density waves would favor the creation of spiral arms. When bar instability occurs, these density waves are instead migrated out into a ring-structure by the pressure, force, and gravitational influence of the baryonic and dark matter furiously orbiting about the bar. This migration forces the stars, gas and dust found within the former arms into a torus-like region, forming a ring, and often igniting star formation.

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