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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 518; no. 2; pp. 3164 - 3182
• Main Authors: Romanowsky, Aaron J, Larsen, Søren S, Villaume, Alexa, Carlin, Jeffrey L, Janz, Joachim, Sand, David J, Strader, Jay, Brodie, Jean P, Chakrabarti, Sukanya, Cheng, Chloe M, Crnojević, Denija, Forbes, Duncan A, Garling, Christopher T, Hargis, Jonathan R, Karunakaran, Ananthan, Martín-Navarro, Ignacio, Olsen, Knut A G, Rider, Nicole, Salimkumar, Bitha, Santhanakrishnan, Vakini, Spekkens, Kristine, Tang, Yimeng, van Dokkum, Pieter G, Willman, Beth
• Format: Journal Article
• Language: English
• Published: 01.01.2023
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stac2898

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.