The Vast Polar Structure of the Milky Way Attains New Members

TL;DR

This study investigates the membership of new objects in the Milky Way's Vast Polar Structure (VPOS), confirming two new members and analyzing their orbital alignments, which challenges standard cosmological models.

Contribution

The paper identifies two new members of the VPOS and analyzes their orbital planes, providing evidence against the idea that the satellite distribution is a statistical outlier.

Findings

Crater is close to the VPOS and its inclusion improves the plane fit.

One stellar stream aligns with the VPOS and shares its orbital plane with Palomar 5.

Two streams outside the VPOS suggest the structure's extent and complexity.

Abstract

The satellite galaxies of the Milky Way (MW) align with and preferentially orbit in a vast polar structure (VPOS), which also contains globular clusters, stellar and gaseous streams. Similar alignments have been discovered around several other host galaxies. We test whether recently discovered objects in the MW halo, the satellite galaxy/globular cluster transition object named PSO J174.0675-10.8774 or Crater and three stellar streams, are part of the VPOS. Crater is situated close to the VPOS. Incorporating the new object in the VPOS-plane fit slightly improves the alignment of the plane with other features such as the Magellanic stream and the average orbital plane of the satellites co-orbiting in the VPOS. We predict Crater's proper motion by assuming that it, too, orbits in the VPOS. One of the three streams aligns well with the VPOS. Surprisingly, it appears to lie in the exact same orbital plane as the Palomar 5 stream and shares its distance, suggesting a direct connection between the two. The stream also crosses close to the Fornax dwarf galaxy and is oriented approximately along the galaxy's direction of motion. The two other streams cannot align closely with the VPOS because they were discovered in the direction of M31/M33, which is outside of the satellite structure. The VPOS thus attains two new members. This further emphasizes that the highly anisotropic and correlated distribution of satellite objects requires an explanation beyond the suggestion that the MW satellite system is an extreme statistical outlier of a $\Lambda$CDM sub-halo system.