The Axial Zone of Avoidance in the Globular Cluster System and the Distance to the Galactic Center

TL;DR

This study investigates the existence and characteristics of an axial zone of avoidance in the Milky Way's globular cluster system, estimating the Galactic center distance and analyzing the zone's shape and potential external influences.

Contribution

It provides the first detailed parametrization of the axial zone of avoidance in the Galactic globular cluster system and refines the estimate of the Galactic center distance.

Findings

Identified a double cone-shaped zone of avoidance with a half-angle of about 15 degrees.

Estimated the distance to the Galactic center as approximately 7.2-7.3 kpc.

Found evidence of elongation possibly influenced by the Magellanic Clouds.

Abstract

We have checked the existence of a zone of avoidance oriented along the Galactic rotation axis in the globular cluster (GC) system of the Galaxy and performed a parametrization of this zone in the axisymmetric approximation. We show that an unambiguous conclusion about the existence of an axial zone of avoidance and its parameters cannot be reached based on the maximization of the formal cone of avoidance due to the discreteness of the GC system. The ambiguity allows the construction of the representation of voids in the GC system by a set of largest-radius meridional cylindrical voids to be overcome. As a result of our study, we have managed to identify northern and southern axial zones of avoidance with similar characteristics. The combined zone of avoidance is traceable at $|Z| \gtrsim 1$ kpc and is similar in shape to a double cone whose axis crosses the region of greatest GC number density. By modeling the distribution of Galactocentric latitudes for GCs, we have determined the half-angle of the cone of avoidance ${\alpha}_0 = 15.0^{+2.1}_{-4.1}$ deg and the distance to the Galactic center $R_0 = 7.3 \pm 0.5$ kpc (in the scale of the Harris (1996) catalog, the 2010 version) as the distance from the Sun to the point of intersection of the cone axis with the center--anticenter line. A correction to the calibration obtained from Galactic objects only leads to an estimate of $R_0 = 7.2 \pm 0.5|_{stat} \pm 0.3|_{calib}$ kpc. The probability that the zone of avoidance at the characteristics found is random in nature is $\le 2\%$. We have revealed evidence for the elongation of the zone of avoidance in the direction orthogonal to the center--anticenter axis, which, just as the north--south difference in this zone, may be attributable to the influence of the Magellanic Clouds. The detectability of similar zones of avoidance in the GC systems of external galaxies is discussed.