University of Oulu

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, Vakini Santhanakrishnan, Kristine Spekkens, Yimeng Tang, Pieter G van Dokkum, Beth Willman, 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, Volume 518, Issue 2, January 2023, Pages 3164–3182,

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

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Author: Romanowsky, Aaron J1,2,3; Larsen, Søren S4; Villaume, Alexa1,3,5;
Organizations: 1Department of Physics & Astronomy, San José State University, One Washington Square, San Jose, CA 95192, USA
2University of California Observatories, 1156 High Street, Santa Cruz, CA 95064, USA
3Department of Astronomy & Astrophysics, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
4Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, NL-6500 GL Nijmegen, the Netherlands
5Waterloo Centre for Astrophysics, Department of Physics & Astronomy, University of Waterloo, 200 University Ave. W., Waterloo, Ontario N2L 3G1, Canada
6Rubin Observatory Project Office, 950 N. Cherry Ave., Tucson, AZ 85719, USA
7University of Turku, Finnish Centre of Astronomy with ESO (FINCA), Vesilinnantie 5, FI-20014 Turku, Finland
8Space Physics and Astronomy Research Unit, University of Oulu, PO Box 3000, FI-90014 Oulun yliopisto, Finland
9Specim, Spectral Imaging Ltd., Elektroniikkatie 13, FI-90590 Oulu, Finland
10Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
11Center for Data Intensive and Time Domain Astronomy, Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
12Centre for Astrophysics and Supercomputing, Swinburne University, John Street, Hawthorn, VIC 3122, Australia
13School of Physics and Astronomy, RIT, Rochester, NY 14623, USA
14Institute for Advanced Study, 1 Einstein Drive, Princeton, NJ 08540, USA
15University of Tampa, 401 West Kennedy Boulevard, Tampa, FL 33606, USA
16CCAPP and Department of Astronomy, The Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
17Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
18Department of Physics, Engineering Physics and Astronomy, Queen’s University, Kingston, ON K7L 3N6, Canada
19Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
20Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
21NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
22Department of Physics & Astronomy, University of North Carolina at Chapel Hill, 120 E. Cameron Ave, Chapel Hill, NC 27599, USA
23Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, Ontario, K7K 7B4, Canada
24Department of Astronomy, Yale University, 52 Hillhouse Avenue, New Haven, CT 06511, USA
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 2.3 MB)
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Language: English
Published: Oxford University Press, 2023
Publish Date: 2023-09-29


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.

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Series: Monthly notices of the Royal Astronomical Society
ISSN: 0035-8711
ISSN-E: 1365-8711
ISSN-L: 0035-8711
Volume: 518
Issue: 2
Pages: 3164 - 3182
DOI: 10.1093/mnras/stac2898
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: This research is based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Support for Program number HST-GO-14748 was provided through a grant from the STScI under NASA contract NAS5-26555. AJR was supported by National Science Foundation (NSF) grant AST-1616710 and as a Research Corporation for Science Advancement Cottrell Scholar. JLC acknowledges support from NSF grant AST-1816196. DJS acknowledges support from NSF grants AST-1821967 and 1813708. SC was supported by the IBM Einstein Fellowship held at the Institute for Advanced Study, and by NSF AST 2009828. DC is supported by NSF grant AST1814208. CTG is supported by NSF grants AST-1615838 and AST-1813628. This research was supported in part by the National Science Foundation under Grant No. NSF PHY-1748958.
Copyright information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2022 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.