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Hubble reveals unusual globular clusters in ultra-diffuse galaxy FCC 224

Using the Hubble Space Telescope (HST), astronomers from the University of California Santa Cruz (UCSC) and elsewhere have observed an ultra-diffuse galaxy known as FCC 224. Results of the observational campaign, Jan. 18 on the arXiv pre-print server, provide important insights into the properties of this galaxy and its globular cluster system.

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 65 million light years away in the Fornax galaxy cluster, FCC 224 is a quiescent ultra-diffuse galaxy about 10 billion years old. It has a major axis effective radius of approximately 6,160 light years and its mass is estimated to be at a level of 200 million solar masses.

Previous observations of FCC 224 have found that it hosts 12 unusually luminous GCs. In particular, several clusters in this galaxy were found to have absolute magnitudes of about -9.0 mag.

A team of astronomers led by Yimeng Tang of USCS decided to investigate the overluminous GC system of FCC 224. For this purpose, they employed HST's Wide Field Camera 3 (WFC3). Their study was complemented by spectroscopic data from the Keck Cosmic Web Imager (KCWI).

The observations found that GCs in FCC 224 have colors very close to the diffuse starlight with an extremely narrow color spread, and the galaxy itself does not show any significant color gradient. This suggests an intense, single-burst star formation history.

The total mass of GCs in FCC 224 was calculated to be 3.8 million solar masses, thus approximately 2% of the galaxy's stellar mass. Therefore, FCC 224 has a more massive GC system than expected at its mass, which suggests that GCs in this galaxy are overmassive on average. The mean stellar age of FCC 224 was estimated to be 10.1 billion years, while the galaxy's metallicity was measured to be at a level of -1.25 dex.

Moreover, it turned out that GCs in FCC 224 have relatively small sizes—with half-light radii between 7.8 and 15.6 light years. It was also found that the GC population of FCC 224 appears to exhibit radial mass segregation, with the GCs closer to the center being brighter than the outer ones, and that the galaxy showcases a top-heavy GC luminosity function.

The authors of the paper conclude that FCC 224 has a very unusual GC system; however, more theoretical studies are required in order to explain the origin of this system's peculiar properties.