Revised Geometric Estimates of the North Galactic Pole and the Sun's Height Above the Galactic Midplane

Abstract

Astronomers are entering an era of μas-level astrometry utilizing the 5-decade-old IAU Galactic coordinate system that was only originally defined to $\sim$0°.1 accuracy, and where the dynamical centre of the Galaxy (Sgr A*) is located $\sim$0°.07 from the origin. We calculate new independent estimates of the North Galactic Pole (NGP) using recent catalogues of Galactic disc tracer objects such as embedded and open clusters, infrared bubbles, dark clouds, and young massive stars. Using these catalogues, we provide two new estimates of the NGP. Solution 1 is an "unconstrained" NGP determined by the galactic tracer sources, which does not take into account the location of Sgr A*, and which lies 90°.120$\pm$0°.029 from Sgr A*, and Solution 2 is a "constrained" NGP which lies exactly 90° from Sgr A*. The "unconstrained" NGP has ICRS position: ${\alpha }_{NGP}$ = 192°.729 $\pm$ 0°.035, ${\delta }_{NGP}$ = 27°.084 $\pm$ 0°.023 and $\theta$ = 122°.928 $\pm$ 0°.016. The "constrained" NGP which lies exactly 90° away from Sgr A* has ICRS position: ${\alpha }_{NGP}$ = 192°.728 $\pm$0°.010, ${\delta }_{NGP}$ = 26°.863 $\pm$ 0°.019 and $\theta$ = 122°.928 $\pm$ 0°.016. The difference between the solutions is likely due to the Sun lying above the Galactic midplane. Considering the angular separation between Sgr A* and our unconstrained NGP, and if one adopts the recent estimate of the Galactocentric distance for the Sun of $R_0$ = 8.2$\pm$0.1 kpc, then we estimate that the Sun lies $z_{\odot }$ $\simeq$ 17$\pm$5 pc above the Galactic midplane. Our value of $z_{\odot }$ is consistent with the true median of 55 previous estimates published over the past century of the Sun's height above the Galactic mid-plane ($z_{\odot }$ $\simeq$ 17$\pm$2 pc).

Accepted for publication in the MNRAS. 13 pages, 7 figures

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