Distances of the bulge globular clusters Terzan 5, Liller 1, UKS 1 and Terzan 4 based on HST NICMOS photometry

TL;DR

This study accurately determines the distances to several bulge globular clusters using HST NICMOS photometry, revealing their positions relative to the Galactic center and implications for pulsar populations.

Contribution

It introduces a method for deriving cluster distances using NICMOS photometry and comparisons with reference clusters, improving distance estimates for bulge globular clusters.

Findings

Terzan 5 is closer to the Sun at 5.5 kpc, affecting pulsar population estimates.

Liller 1 and Terzan 4 are at the bulge distance, UKS 1 is beyond the Galactic center.

Distance measurements impact understanding of cluster densities and pulsar formation.

Abstract

A large number of pulsars and X-rays sources are detected in globular clusters. To understand the structure and content of these clusters, accurate distances are required. We derive the distances of Terzan 5, Liller 1 and UKS 1 using as a reference a recent distance determination of NGC 6528, based on HST/NICMOS and NTT/SOFI infrared photometry. The distance of the metal-poor cluster Terzan 4 was derived from a comparison with M92 in NICMOS bands. Distances of the metal-rich clusters are obtained by comparison of the Horizontal Branch (HB) level of the clusters, relative to the reddening line passing through the HB of NGC 6528. We use methods based on NICMOS bands and transformations to J and H magnitudes with different assumptions. Liller~1 and Terzan~4 are found to be at the central bulge distance, UKS~1 is beyond the Galactic center, while Terzan 5 is closer to the Sun than the other four clusters. The distance of Terzan 5 is of paramount importance, given the impact of its population of 21 pulsars, which is related to the high cluster density. The distance of Terzan 5 is found to be d$_{\odot}$=5.5$\pm$0.9 kpc from the Sun, thus closer to us than values given in studies of pulsars in Terzan 5. As a consequence, the higher cluster density is even more favourable for formation of the millisecond pulsars recently detected in this cluster.