Reconsidering the galactic coordinate system

TL;DR

This paper critically examines the outdated definition of the galactic coordinate system, highlighting issues due to transformations from older systems and proposing a redefinition directly linked to the ICRS for improved precision.

Contribution

It investigates the transformation issues of the galactic coordinate system and advocates for a new definition directly connected to the ICRS for higher accuracy.

Findings

Current galactic coordinates are inconsistent with ICRS.

Transformation from FK4/FK5 to ICRS is problematic.

A new, precise definition of galactic coordinates is needed.

Abstract

Initially defined by the IAU in 1958, the galactic coordinate system was thereafter in 1984 transformed from the B1950.0 FK4-based system to the J2000.0 FK5-based system. In 1994, the IAU recommended that the dynamical reference system FK5 be replaced by the ICRS, which is a kinematical non-rotating system defined by a set of remote radio sources. However the definition of the galactic coordinate system was not updated. We consider that the present galactic coordinates may be problematic due to the unrigorous transformation method from the FK4 to the FK5, and due to the non-inertiality of the FK5 system with respect to the ICRS. This has led to some confusions in applications of the galactic coordinates. We tried to find the transformation matrix in the framework of the ICRS after carefully investigating the definition of the galactic coordinate system and transformation procedures, however we could not find a satisfactory galactic coordinate system that is connected steadily to the ICRS. To avoid unnecessary misunderstandings, we suggest to re-consider the definition of the galactic coordinate system which should be directly connected with the ICRS for high precise observation at micro-arcsecond level.