# Overall properties of the Gaia DR1 reference frame

## TL;DR

This paper evaluates the Gaia DR1 reference frame's properties through quasar position comparisons, Galactic aberration effects, proper motion system analysis, and Galactic kinematics, revealing minor biases and potential residual rotations.

## Contribution

It provides a comprehensive assessment of Gaia DR1's reference frame, identifying small biases and residual rotations, and explores their implications for Galactic kinematics.

## Key findings

- Negligible orientation difference between quasar positions and ICRF2.
- Galactic aberration causes a ~0.01 mas offset in Gaia DR1.
- Residual rotation components suggest possible issues in Gaia DR1 or Galactic models.

## Abstract

We compare quasar positions of the auxiliary quasar solution with ICRF2 sources using different samples and evaluate the influence on the {\it Gaia} DR1 reference frame owing to the Galactic aberration effect over the J2000.0-J20015.0 period. Then we estimate the global rotation between TGAS with {\it Tycho}-2 proper motion systems to investigate the property of the {\it Gaia} DR1 reference frame. Finally, the Galactic kinematics analysis using the K-M giant proper motions is performed to understand the property of {\it Gaia} DR1 reference frame. The positional comparison between the auxiliary quasar solution and ICRF2 shows negligible orientation and validates the declination bias of $\sim$$-0.1$\mas~in {\it Gaia} quasar positions with respect to ICRF2. Galactic aberration effect is thought to cause an offset $\sim$$0.01$\mas~of the $Z$ axis direction of {\it Gaia} DR1 reference frame. The global rotation between TGAS and {\it Tycho}-2 proper motion systems, obtained by different samples, shows a much smaller value than the claimed value $0.24$\masyr. For the Galactic kinematics analysis of the TGAS K-M giants, we find possible non-zero Galactic rotation components beyond the classical Oort constants: the rigid part $\omega_{Y_G} = -0.38 \pm 0.15$\masyr~and the differential part $\omega^\prime_{Y_G} = -0.29 \pm 0.19$\masyr~around the $Y_G$ axis of Galactic coordinates, which indicates possible residual rotation in {\it Gaia} DR1 reference frame or problems in the current Galactic kinematical model.