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The classical globular clusters found in all galaxy types have half-lightradii of $r_{\rm h} \sim$ 2-4 pc, which have been tied to formation in thedense cores of giant molecular clouds. Some old star clusters have largersizes, and it is unclear if these represent a fundamentally different mode oflow-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-massspiral galaxy NGC 247 in the nearby Sculptor group, and measure its radialvelocity using Keck spectroscopy. We use Hubble Space Telescope imaging tomeasure the cluster half-light radius of $r_{\rm h} \simeq 12$ pc and aluminosity of $L_V \simeq 4\times10^5 \mathrm{L}_\odot$. We produce acolour-magnitude diagram of cluster stars and compare to theoreticalisochrones, 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 isconsistent with hosting a single stellar population, modulo several unexplainedbright "red straggler" stars. SC1 appears to be surrounded by tidal debris, atthe 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 formationof these unusual clusters and an external perturbation of their host galaxy,illuminating a possible channel by which some clusters are born with largesizes.

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