Low-density star cluster formation: discovery of a young faint fuzzy on the outskirts of the low-mass spiral galaxy NGC 247

Abstract: The classical globular clusters found in all galaxy types have half-light radii of $r_{\rm h} \sim$ 2-4 pc, which have been tied to formation in the dense cores of giant molecular clouds. Some old star clusters have larger sizes, and it is unclear if these represent a fundamentally different mode of low-density star cluster formation. We report the discovery of a rare, young "faint fuzzy" star cluster, NGC 247-SC1, on the outskirts of the low-mass spiral galaxy NGC 247 in the nearby Sculptor group, and measure its radial velocity using Keck spectroscopy. We use Hubble Space Telescope imaging to measure the cluster half-light radius of $r_{\rm h} \simeq 12$ pc and a luminosity of $L_V \simeq 4\times10^5 \mathrm{L}\odot$. We produce a colour-magnitude diagram of cluster stars and compare to theoretical isochrones, finding an age of $\simeq$ 300 Myr, a metallicity of [$Z$/H] $\sim -0.6$ and an inferred mass of $M\star \simeq 9\times10^4 \mathrm{M}_\odot$. The narrow width of blue-loop star magnitudes implies an age spread of $\lesssim$ 50 Myr, while no old red-giant branch stars are found, so SC1 is consistent with hosting a single stellar population, modulo several unexplained bright "red straggler" stars. SC1 appears to be surrounded by tidal debris, at the end of a $\sim$ 2 kpc long stellar filament that also hosts two low-mass, low-density clusters of a similar age. We explore a link between the formation of these unusual clusters and an external perturbation of their host galaxy, illuminating a possible channel by which some clusters are born with large sizes.