Investigating the origin of the Milky Way streams. A revised look at their orbital pole distribution in light of precession effects

TL;DR

This study revises the orbital pole distribution of Milky Way stellar streams, revealing that precession effects influence their orientations and suggesting a more complex origin history for these streams.

Contribution

It provides a new analysis of stream orbital poles considering precession, challenging previous assumptions of their isotropy and linking their distribution to distance and precession effects.

Findings

Streams farther from the Galactic center show a higher alignment with the DoS.

Precession effects likely displace orbital poles of closer streams.

Hydrodynamical simulations support the complex origin scenario.

Abstract

Stellar streams around the Milky Way (MW) can provide valuable insights into its history and substructure formation. Previous studies have suggested that several MW streams could have an origin related to that of the disc of satellite galaxies (DoS) and the young halo globular clusters of the MW, given that many of these structures present a similar orbital pole orientation. In this work we test the validity of this hypothesis by revising the orbital pole distribution of the MW streams with the latest stream dataset (galstreams). For a sample of 91 streams at Galactocentric distances of $d<100$ kpc we find that the pole distribution has no preferred orbital direction. However, as we subtract the streams closer to the Galactic centre, by imposing several lower distance cuts, we find that the larger the Galactocentric distance of the streams, the higher the fraction of stream poles pointing in a direction similar to the DoS. This trend could be explained if the stream pole distribution were originally anisotropic, but precession effects displaced the orbital poles of the streams closer to the Galactic centre. From the pole distribution and the estimated precession rates of the streams in the sample, we infer that the streams nearer the Galactic centre are indeed quite likely to be affected by precession. Finally, we corroborate with hydrodynamical simulations that, even in a scenario in which the MW substructures had a common origin, an overdensity in their orbital pole direction cannot be appreciated until the selected sample also includes material at $d \gtrsim 150$ kpc.