Globular cluster NGC 6558 explored with the Gemini Observatory and the Hubble Space Telescope

Using the Gemini Observatory and the Hubble Space Telescope (HST), an international team of astronomers explored the galactic globular cluster known as NGC 6558. The results of the new study, published July 22 on the preprint server arXivprovide important insights into the properties of this cluster.

Globular clusters (GCs) are collections of tightly bound stars orbiting galaxies. Astronomers perceive them as natural laboratories enabling the study of the evolution of stars and galaxies. In particular, globular clusters could help researchers better understand the formation and evolution history of early-type galaxies, as the origin of GCs appears to be closely linked to periods of intense star formation.

NGC 6558 (also known as GCL 89 or ESO 456-SC62) is a galactic bulge globular cluster located about 24,100 light-years away in the constellation Sagittarius with an angular size of about 5.4 x 5.4 arcmin. Previous observations found NGC 6558 to have a metallicity of -1.32 dex and suggested that it may be one of the oldest objects in the Milky Way.

However, due to its high reddening, the NGC 6558 region presents problems in deriving precise parameters. A team of astronomers led by Stefano Souza of the University of São Paulo in Brazil has now performed near-infrared and optical photometry of the cluster. For this purpose, they used the Gemini-South telescope of the Gemini Observatory and HST’s Advanced Camera for Surveys (ACS).

“Our aim is to refine the fundamental parameters of NGC 6558 using high-quality Gemini-South/GSAOI and HST/ACS photometry. In addition, we intend to investigate its role in the formation of the galactic bulge,” the researchers explain.

The high-quality photometric data collected allowed the team to derive the fundamental parameters of NGC 6558. By removing as much contamination as possible from the stellar field of the galactic bulge, the astronomers were able to obtain the most accurate results than ever before.

The observations found that NGC 6558 is located about 27,400 light-years away. This is further than the distances obtained by previous studies, which suggested between 20,500 and 26,900 light years.

The reddening of NGC 6558 was measured at 0.34. Astronomers emphasize that this is the lowest value in the available literature, obtained with a differential reddening correction based on reference stars less contaminated by field stars.

Based on the obtained data, the authors of the paper estimate the age of NGC 6558 at some 13 billion years. They note that this age is compatible with other clusters of similar metallicity and the blue horizontal branch in the Galactic bulge.

Summarizing the results, the researchers concluded that NGC 6558, combined with other moderately metal-poor GCs, suggests that this population has an age of up to 13.62 billion years and a chemical enrichment 10 times faster than the rest of the Milky Way.

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