Systematic uncertainties in HB fitting for anomalous globular clusters

Characterize the unknown systematic errors introduced when the empirical horizontal-branch fitting methods are applied to very metal-poor globular clusters with red horizontal branches or to very metal-rich clusters with blue horizontal branches.

Background

The study derives globular-cluster distances by measuring the horizontal-branch magnitude and comparing it with an empirical metallicity–horizontal-branch-luminosity relation calibrated using Milky Way globular clusters. Different fitting procedures are used depending on whether red or blue horizontal-branch stars are present.

The calibration sample does not contain comparably metal-poor globular clusters with red horizontal branches. Consequently, applying the red-branch fitting method to very metal-poor clusters—such as the anomalously red-horizontal-branch clusters in NGC 6822—may introduce systematic errors in the inferred distances and related parameters.

References

As a result, in cases where it is necessary to apply the red' HB fitting method to very metal-poor GCs (or theblue' fitting method to very metal-rich GCs) there may be unknown systematics, since there are no comparably metal-poor red HB GCs are observed in the MW calibration sample (Mackey et al. in prep).

— Characterising the Globular Cluster Systems of Three Local Group Dwarf Galaxies: NGC6822, NGC147 and NGC185  (2608.28434 - McGill et al., 28 Aug 2026) in Section 3.1, subsection “Method”