Research investigates the environment of globular cluster NGC 6355

Using the Dark Energy Camera (DECam), Argentinian astronomers have investigated the environment of a galactic globular cluster known as NGC 6355. The study, presented in a paper published May 2 on the pre-print server arXiv, found that the cluster has several extra-tidal features.

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

Located some 27,800 light years away in the galactic bulge, NGC 6355 is a 13.2 billion-year-old GC with a chaotic orbit. It has an absolute magnitude of -8.07 mag and its mean metallicity is estimated to be at a level of -1.39 dex.

Recently, a team of astronomers led by Andrés E. Piatti of the National University of Cuyo in Mendoza, Argentina, has observed NGC 6355 and its environment, searching for extra-tidal structures. For this purpose, they used DECam mounted on the 4-m Blanco telescope at the Cerro Tololo Inter-American Observatory (CTIO).

“We analyzed wide-field DECam g,i imaging around the Milky Way bulge globular cluster NGC 6355, with the aim of searching for extra-tidal structures. The cluster was observed as part of our CTIO 4-m Blanco telescope 2023A-627924 observing program, thus obtaining relatively deep photometry of the cluster’s outskirts,” the researchers wrote in the paper.

DECam observations revealed the presence of extra-tidal overdensities above the mean stellar field density. These overdensities appear to be between four- and six-times larger than the star field density dispersion.

The identified overdensities turned out to consist of an extra-tidal extended envelope azimuthally not symmetric, a tidal tail, and scattered stellar debris alongside the tidal tail. It was noted that the tidal tail points toward the Milky Way center.

The astronomers noted that their discovery makes NGC 6355 the first Milky Way bulge GC with detected extra-tidal structures. However, they added that this finding is somehow surprising according to the theoretically expected scenario.

“These results contrast with previous theoretical speculations about the possibility to detect tidal tails of globular clusters with chaotic orbits because they would be washed out after they were generated,” the authors of the paper explained.

Therefore, based on their discovery, the astronomers concluded that tidal tails among bulge GCs seem to be a more common phenomenon than previously thought. However, more observations of the outskirts of such GCs are required in order to confirm this assumption.

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