Monthly Notices of the Royal Astronomical Society
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 × 105Lθ. 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 × 104Mθ. 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.
National Science Foundation
galaxies: individual: NGC 247, galaxies: star clusters: general, Hertzsprung-Russell and colour-magnitude diagrams
Physics and Astronomy
Aaron J. Romanowsky; Søren S. Larsen; Alexa Villaume; Jeffrey L. Carlin; Joachim Janz; David J. Sand; Jay Strader; Jean P. Brodie; Sukanya Chakrabarti; Chloe M. Cheng; Denija Crnojević; Duncan A. Forbes; Christopher T. Garling; Jonathan R. Hargis; Ananthan Karunakaran; Ignacio Martín-Navarro; Knut A.G. Olsen; Nicole Rider; Bitha Salimkumar; and For full author list, see comments below. "Low-density star cluster formation: Discovery of a young faint fuzzy on the outskirts of the low-mass spiral galaxy NGC 247" Monthly Notices of the Royal Astronomical Society (2023): 3164-3182. https://doi.org/10.1093/mnras/stac2898