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 arm features nearer the Galactic Center (within 13o 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).