The Gaia inertial reference frame and the tilting of the Milky Way disk

TL;DR

This paper discusses how Gaia's precise measurements can establish an inertial reference frame to detect and analyze the tilting and dynamic behavior of the Milky Way disk relative to the dark matter halo and extragalactic objects.

Contribution

It demonstrates Gaia's capability to define a quasi-inertial frame with microarcsecond per year accuracy, enabling studies of the Milky Way's disk orientation and evolution.

Findings

Gaia can determine the inertial frame to better than 1 muas/yr.

The method allows detection of disk tilting and orientation changes over time.

Improves understanding of Milky Way dynamics and tests of gravity theories.

Abstract

While the precise relationship between the Milky Way disk and the symmetry planes of the dark matter halo remains somewhat uncertain, a time-varying disk orientation with respect to an inertial reference frame seems probable. Hierarchical structure formation models predict that the dark matter halo is triaxial and tumbles with a characteristic rate of ~2 rad/Hubble time (~30 muas/yr). These models also predict a time-dependent accretion of gas, such that the angular momentum vector of the disk should be misaligned with that of the halo. These effects, as well as tidal effects of the LMC, will result in the rotation of the angular momentum vector of the disk population with respect to the quasar reference frame. We assess the accuracy with which the positions and proper motions from Gaia can be referred to a kinematically non-rotating system, and show that the spin vector of the transformation from any rigid self-consistent catalog frame to the quasi-inertial system defined by quasars should be defined to better than 1 muas/yr. Determination of this inertial frame by Gaia will reveal any signature of the disk orientation varying with time, improve models of the potential and dynamics of the Milky Way, test theories of gravity, and provide new insights into the orbital evolution of the Sagittarius dwarf galaxy and the Magellanic Clouds.