On the many ‘3-kiloparsec arms’—shocked wave and nuclear rotation

Abstract

Many features near the Galactic Center have been called "3-kiloparsec arms". We reached a point of having too many divergent data, making it difficult to be constrained by a single physical model. Their differing characteristics suggest different physical and dynamical objects.Radial velocity data on the so-called '3-kpc arms' do not coincide with radial velocities of major spiral arms near 3kpc, nor near 2 kpc, nor near 4 kpc from the Galactic Center (Fig. 1 and 2).Different '3-kpc arm' features may require different models: turbulence around a shock in a Galactic density wave between 2 and 4 kpc from the Galactic Center (Table 1), or nuclear rotation between 0 and 2 kpc from the Galactic Center region (Table 2), or a putative radial expansion between 0 and 4 kpc from the Galactic Center.Despite their naming as 'Near 3-kiloparsec arms' or 'Far 3-kiloparsec arms', these features are not major arms. Those '3-kpc arms' features nearer the Galactic Center (within 13 o of Galactic longitude) may be different than those farther out (Table 2).Here we show that the plethora of observed '3-kpc arm' features can be separated in two: those with Galactic longitude of 13 degrees or more away from the Galactic Center (Table 1 -some of which are possibly associated with the observed major spiral arms), and those within 13 degrees from the Galactic Center (Table 2 -some of which are possibly associated with the observed central bars; Fig. 1 and Fig. 2).Previous efforts were made to identify and physically understand the '3-kpc arm' features. The identification of the '3-kpc arm' features dates back to van

A guided map to the spiral arms in the galactic disk of the Milky Way

“…These '3-kpcstreams' exist observationally, yet their ages and stability are unknown. They could be due to temporary turbulence, as sometimes predicted in some models [76].…”

“…Misnamed '3-kpc arm' features far from the Galactic Center (at l > +13° or at l < 347°) may be shocks associated with long spiral arms [76]. Table 3 shows for the Milky Way that each arm has a similar pitch angle, if one excludes the innermost and outermost regions (longitude 337° arm, and Cygnus + 1 arm).…”

“…In this figure, the observations showed, very near the arm tangents to 2 spiral arms, filaments going at high speeds (a misnamed '3-kpc arms' feature) which are potentially related to the spiral arms of Norma (at l=+20 o )and of Sagittarius (at l=-17 o ). Misnamed '3-kpc arm' features far from the Galactic Center (at l>+13 o or at l< 347 o ) may be shocks associated with long spiral arms (Vallée 2017c).…”

Superposing the Magnetic Spiral Structure of the Milky Way, on the Stellar Spiral Arms—Matching the Unique Galactic Magnetic Field Reversal Zone with Two Galactic Spiral Arm Segments

“…The magnetic field located very close to the Galactic Center (within 3 kpc) is difficult to measure (too few data yet). In addition, the origin of various '3-kpc features' emanating from the Galactic Center are still being debated - Vallée (2017c;2016b) proposed that some of these 3-kpc features are the starting point of long spiral arms (Norma, Sagittarius). The magnetic field in the Galactic nucleus and its boxy bar are still unknown.…”