Can we “slice” a spiral arm into its components and order the slices?
The Moon orbits around the Earth on a rough circle, the Earth orbits on a rough circle around the Sun, and the Sun orbits on a rough circle around the Center of our Galaxy known as the Milky Way. Gravitation is the force that keeps these circular orbits around a more massive object; the orbits are not quite a perfect circle, as local tides or other effects come into play.
Besides gravitation, there are density waves that affect the orbits of mass (stars, gas), slowing and accelerating their mass.
Density waves turn around the disk of our Milky Way at a constant angular speed, and any orbiting gas mass will be slowed when entering a spiral arm (by a deceleration in a shocked lane; some gas will be crushed into protostars and masers will erupt). Then, the remaining neutral gas will be accelerated and eventually leave the arm behind, and then continue in its roughly circular orbit around the Galactic Center. But the recently formed protostars will evolve and continue in their circular orbit, appearing later nearer the center of their spiral arm.
A look at spiral arms through different telescopes (radio, infrared, optical, ultraviolet) would show an ordered progression in time and space (dust lane, maser region, protostars, newly evolved stars, peak of the CO gas, old stars), with the dust lane being the closest to the direction of the Galactic Center. Thus, one could see an arm being “sliced” into its different components: red (hot dust, masers, protostars); orange (cold dust, ionized carbon and nitrogen, newly created visible stars); green (synchrotron electrons, hot hydrogen recombination); and blue (cold extended CO gas, neutral hydrogen, thermal electrons, old stars).
This “sliced” pattern is created by the higher speed of the orbiting gas as it encounters the slower speed of the density wave, interacting with the wave (deceleration, shock), and eventually leaving it behind.
Can we see a mirror-image across the Galactic Meridian?
As the gas goes on a circular orbit around the Galactic Center, this pattern reverses itself across the Galactic Meridian (the line from the Sun to the Galactic Center). Thus to the right of the Meridian (galactic longitudes from 0 to 90 degrees), the “sliced” pattern shows the dust lane first, while to the left of the Meridian (galactic longitudes from 270 to 360 degrees), the “sliced” pattern shows the dust lane last. No model, other than the density wave, can predict that mirror image.
What is the “co-rotation radius” of the Milky Way disk?
The circular speed vgas of the orbiting gas and stars (about 230 km/s) is roughly the same, as one goes radially from the Galactic Center to far out. Meanwhile, the angular speed of the density wave is always the same, so its circular speed vwave increases with increasing radial distance from the Galactic Center. Thus the two become equal somewhere (vgas = vwave), at a specific radial distance called the co-rotation distance (Rcoro) from the Galactic Center. But where is Rcoro in the Milky Way disk?
Past the co-rotation radius, the speed of the density wave (vwave) is higher than the slower speed of the orbiting gas and stars, thus the “slicing” pattern will be reversed. There, the dust lane would be seen last, not first, to the right of the galactic Meridian.
One can look for the masers’ slice and where it is located with respect to the CO gas’ slice beyond the galactic radius where the Sun is located near 8 kiloparsecs from the Galactic Center (1 parsec = 3.26 lightyears). The observational test for the Perseus arm, just beyond the Sun, shows the masers’ slice (black circles) to be closer to the Sun (big star) than the CO-gas’ slice (yellow curve). This is the same “slicing” pattern found in the Sagittarius arm (green curve), so both the Perseus arm and the Sagittarius arm are located inside the co-rotation radius, as is the Scutum arm (blue curve) and the Norma arm (red curve).
Thus, the observational data in our Milky Way indicates that the co-rotation radius is located beyond the Perseus arm, that is, Rcoro exceed 11 kiloparsecs from the Galactic Center.
To help locate it, one can look with other tracers in the direction away from the Galactic Center, called the anticenter (galactic longitude near 180 degrees). Preliminary statistics have been done on other data (not on masers), and these showed a mean Rcoro near 12 kiloparsecs (with a large error bar), thus located between the Perseus arm and the next (Cygnus) arm.
What is the local stuff, close to the Sun, made of?
The Sun is located in between the Sagittarius arm and the Perseus arm. Around the Sun, some other stuff has been found.
Length-wise, the materials (gas, stars) close to the Sun do not fit inside a normal, long spiral arm. It does not extend far from the Sun, not exceeding a few kiloparsecs.
Shape-wise, the local stuff has been called by other astronomers variously as “armlet,” “local arm,” “belt,” “blob,” “branch,” “bridge,” “feather,” “fork,” “finger,” “layer,” “ring,” “segment,” “spur,” “sub-arm,” “swath,” etc. The name seems to depend on the extent or twist of one’s imagination, or on one’s model to be fitted to the material.
Mass-wise, the mass of the “local arm” near the Sun is not well known, but it far exceeds what is known about the mass of the interarm region elsewhere in the Galactic disk.
The Sun (big star) is surrounded by stuff, such as recently-formed stars, old stars, ionized hydrogen regions, and masers, but the material is not placed in any order (there are no ordered “slices,” as seen elsewhere in long spiral arms).
These issues (odd shape, small length, large local mass, no ordered splices) cannot be the work of a density wave (requiring symmetry, mirror-image). It must be the work of something else.
Where does the local stuff, close to the Sun, come from?
This resembles the work of a local perturbation. Which or what type of model would cause such a perturbation: lindblad resonance, oscillating banana-shaped resonance, local differential rotation, supercloud, trail debris, tides, warps, etc?
Most of these different models come with some predictions that already contradict known observations. We thus focused on a few models, that of a possible molecular supercloud in the galactic halo, whose orbit is now crashing in the galactic disk near the Sun, or else that of possible molecular debris from the trail in the orbit of a dwarf galaxy’s successive past orbital passages around our Galactic Center.
In the supercloud model, the orbit of a halo supercloud crashing through the disk near the Perseus arm was previously computed and found to be braked, creating a shock and a collapse to become a part of Gould’s Belt near the Sun, while other parts of the supercloud were later deformed through our Galaxy’s differential rotation and formed new subsystems that approached the Sagittarius arm.
Other such halo superclouds coming in (infalling gas streams) were also predicted near the Sun, all adding new mass in the interarm region (importing it from the galactic halo).
In the tidal debris model, a few dwarf galaxies are known to orbit around the Milky Way galaxy, sometimes coming very close and crossing the Milky Way’s disk near the Sun. The tidal interaction would cause debris to be shed from the dwarf galaxy in its orbit (much like a comet coming too close to the Sun would be tidally disrupted, and cometary debris would be left behind in the orbit’s trail). The debris in the trail of a dwarf galaxy would show up better as seen from the Sun if they fall in the interarm region near the Sun. At each such orbital passage, more debris would be left behind and fall in our Milky Way later, to be then affected by our galaxy’s differential rotation.
Other such tidally-affected orbiting dwarf galaxies coming in (infalling trail debris ) were also predicted near the Sun, all adding new mass in the interarm region (importing it from nearby dwarf galaxies).
These findings are described in the following articles:
• Offsets of masers with respect to the middle of the Perseus arm, and the co-rotation radius of the Milky Way [Astrophysical Journal, 863, art.52]
• Meta-analysis from different tracers of the small Local Arm around the Sun – extent, shape, pitch, origin [Astrophysics and Space Science, 363, art.243]
• A substructure inside spiral arms, and a mirror image across the galactic Meridian [Astrophysical Journal, 821, art.53]
• A guided map to the spiral arms in the galactic disk [Astronomical review, 13, p.113]
This work was led by Jacques P Vallée from the National Research Council of Canada.