On the Origin of High-Altitude Open Clusters in the Milky Way

TL;DR

This study investigates how the Milky Way's spiral arms and bar influence the vertical distribution of open clusters, revealing that spiral arms significantly elevate clusters above the galactic plane more than the bar does.

Contribution

It demonstrates that spiral arms can raise a substantial fraction of open clusters to high altitudes, a novel insight into their dynamical role in galactic structure.

Findings

Spiral arms elevate about one-sixth of clusters above 200 pc from the plane.

Spiral arms have a greater vertical impact than the bar on open clusters.

The bar reduces the elevation effect caused by spiral arms by about half.

Abstract

We present a dynamical study of the effect of the bar and spiral arms on the simulated orbits of open clusters in the Galaxy. Specifically, this work is devoted to the puzzling presence of high-altitude open clusters in the Galaxy. For this purpose we employ a very detailed observationally motivated potential model for the Milky Way and a careful set of initial conditions representing the newly born open clusters in the thin disk. We find that the spiral arms are able to raise an important percentage of open clusters (about one-sixth of the total employed in our simulations, depending on the structural parameters of the arms) above the Galactic plane to heights beyond 200 pc, producing a bulge-shaped structure toward the center of the Galaxy. Contrary to what was expected, the spiral arms produce a much greater vertical effect on the clusters than the bar, both in quantity and height; this is due to the sharper concentration of the mass on the spiral arms, when compared to the bar. When a bar and spiral arms are included, spiral arms are still capable of raising an important percentage of the simulated open clusters through chaotic diffusion (as tested from classification analysis of the resultant high-z orbits), but the bar seems to restrain them, diminishing the elevation above the plane by a factor of about two.