Double Horizontal Branches in NGC 6440 and NGC 6569 unveiled by the VVV Survey

TL;DR

This paper reports the discovery of double horizontal branches in two massive, metal-rich globular clusters in the Galactic bulge, indicating possible multiple stellar populations or evolutionary effects.

Contribution

It uncovers a new feature of double horizontal branches in NGC 6440 and NGC 6569 using IR and optical data, suggesting complex stellar populations.

Findings

Double HBs detected in NGC 6440 and NGC 6569

Confirmed with multiple IR catalogs and HST data

Possible implications for stellar population complexity

Abstract

We report the discovery of a peculiar horizontal branch (HB) in NGC 6440 and NGC 6569, two massive and metal-rich Galactic globular clusters (GGCs) located in the Galactic bulge, within 4 kpc from the Galactic Center. In both clusters, two distinct clumps are detected at the level of the cluster HB, separated by only ~ 0.1 magnitudes in the Ks band. They were detected with IR photometric data collected with the "VISTA Variables in the Via Lactea" (VVV) Survey, and confirmed in independent IR catalogs available in the literature, and HST optical photometry. Our analysis demonstrates that these clumps are real cluster features, not a product of field contamination or interstellar reddening. The observed split HBs could be a signature of two stellar sub-populations with different chemical composition and/or age, as recently found in Terzan 5, but it cannot be excluded that they are caused by evolutionary effects, in particular for NGC 6440. This interpretation, however, requires an anomalously high helium content (Y > 0.30). Our discovery suggests that such a peculiar HB morphology could be a common feature of massive, metal-rich bulge GGCs.