FCC 224 is an ultra-diffuse galaxy whose stars appear extremely dim, and nebulae are unimpressive. However, the Hubble Space Telescope has detected some very unusual globular clusters here.

Ultra-diffuse galaxy FCC 224
Using the FCC 224 space telescope, astronomers observe an ultra-diffuse galaxy whose stars appear extremely dim and whose nebulae are unimpressive. However, this is where the Hubble Space Telescope has discovered very unusual globular clusters.
Astronomers from the University of California, Santa Cruz (UCSC) and other institutions have observed an ultra-diffuse galaxy known as FCC 224. The observational results, published January 18 on the arXiv preprint server, provide important information about the properties of this galaxy and its globular cluster system.
Globular clusters (GCs) are clusters of tightly bound stars orbiting galaxies. Astronomers think of them as natural laboratories for studying the evolution of stars and galaxies. In particular, GCs can help researchers better understand the formation history and evolution of early-type galaxies, as the origin of GCs seems to be closely related to periods of intense star formation.
Found about 65 million light-years away in the Fornax galaxy cluster, FCC 224 is a calm, ultra-dispersed galaxy about 10 billion years old. It has an effective major axis radius of about 6,160 light-years, and its mass is estimated to be 200 million solar masses.
Bright globular clusters
Preliminary observations of FCC 224 found that it contains 12 extremely bright GCs. In particular, several clusters in this galaxy were found to have an absolute stellar magnitude of about -9.0 mag.
A team of astronomers led by Imeng Tang of the USCS decided to investigate the superluminous GC system FCC 224. They used the HST 3 wide-angle camera (WFC3) for this purpose. Their study was complemented by spectroscopic data from the Keck Cosmic Web Imager (KCWI) space telescope.
Observations have shown that the GCs in FCC 224 have colors very close to diffuse starlight with an extremely narrow color spread, and the galaxy itself does not show a significant color gradient. This indicates an intense single-spark history of star formation.
Unusual features of the galaxy FCC 224
The total mass of the GC in FCC 224 is 3.8 million solar masses, which is about 2% of the galaxy’s stellar mass. Consequently, FCC 224 has a more massive GC system than expected for its mass, indicating that globular clusters in this galaxy are supermassive on average. The average stellar age of FCC 224 was estimated to be 10.1 billion years, and the metallicity of the galaxy was measured to be -1.25 dex.
In addition, the globular clusters in FCC 224 were found to be relatively small, with half-light radii ranging from 7.8 to 15.6 light-years. It was also found that the galaxy FCC 224 shows radial segregation in mass, with clusters closer to the center brighter than the outer ones, and that the galaxy exhibits a very large galaxy luminosity function.
The authors of the paper conclude that FCC 224 has a very unusual globular cluster system, but more theoretical studies are needed to explain the origin of the special properties of this system.
According to phys.org