Low-density star cluster formation: discovery of a young faint fuzzy on the outskirts of the low-mass spiral galaxy NGC 247
Janz, Joachim; van Dokkum, Pieter G.; Forbes, Duncan A.; Romanowsky, Aaron J.; Brodie, Jean P.; Strader, Jay; Sand, David J.; Larsen, Søren S.; Chakrabarti, Sukanya; Crnojević, Denija; Karunakaran, Ananthan; Spekkens, Kristine; Carlin, Jeffrey L.; Willman, Beth; Olsen, Knut A. G.; Martín-Navarro, Ignacio; Villaume, Alexa; Garling, Christopher T.; Hargis, Jonathan R.; Cheng, Chloe M.; Tang, Yimeng; Rider, Nicole; Salimkumar, Bitha; Santhanakrishnan, Vakini
United States, Netherlands, Canada, Finland, Australia, Spain
Abstract
The classical globular clusters found in all galaxy types have half-light radii of rh ~ 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 rh ≃ 12 pc and a luminosity of LV ≃ 4 × 105L⊙. We produce a colour-magnitude diagram of cluster stars and compare to theoretical isochrones, finding an age of ≃300 Myr, a metallicity of [Z/H] ~ -0.6 and an inferred mass of M⋆ ≃ 9 × 104M⊙. The narrow width of blue-loop star magnitudes implies an age spread of ≲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 an ~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.