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 3 kpc, nor near 2 kpc, nor near 4 kpc from the Galactic Center (Figs. 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° 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; Figs. 1 and 2).