## Reconsidering the Galactic coordinate system

^{1}
Department of AstronomyNanjing University,
Nanjing
210093,
PR China

e-mail: jcliu;zhuzi@nju.edu.cn; zhangh@nju.edu.cn

^{2}
Key Laboratory of Modern Astronomy and Astrophysics (Nanjing
University), Ministry of
Education, Nanjing
210093, PR
China

Received:
10
May
2010

Accepted:
18
October
2010

Initially defined by the IAU in 1958, the Galactic coordinate system was thereafter transformed in 1984 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 at that time. We consider that the present Galactic coordinates may be problematic because of the unrigorous transformation method from the FK4 to the FK5 and of the non-inertiality of the FK5 system with respect to the ICRS. This has led to some confusion when applying 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, but we could not find a satisfactory Galactic coordinate system that is connected steadily to the ICRS. To avoid unnecessary misunderstandings, we suggest reconsidering the definition of the Galactic coordinate system which should be directly connected with the ICRS for highly precise observations at the micro-arcsecond level.

Key words: astrometry / Galaxy: general / reference systems

*© ESO, 2010*