Connecting young star clusters to CO molecular gas in NGC 7793 with ALMA–LEGUS

Creators: Grasha, K.; Calzetti, D.; Bittle, L.; Johnson, K. E.; Donovan Meyer, J.; Kennicutt, R. C.; Elmegreen, B. G.; Adamo, A.; Krumholz, M. R.; Fumagalli, M.; Grebel, E. K.; Gouliermis, D. A.; Cook, D. O.; Gallagher, J. S., III; Aloisi, A.; Dale, D. A.; Linden, S.; Sacchi, E.; Thilker, D. A.; Walterbos, R. A. M.; Messa, M.; Wofford, A.; Smith, L. J.

Abstract

We present an investigation of the relationship between giant molecular cloud (GMC) properties and the associated stellar clusters in the nearby flocculent galaxy NGC 7793. We combine the star cluster catalogue from the HST LEGUS (Legacy ExtraGalactic UV Survey) programme with the 15 pc resolution ALMA CO(2–1) observations. We find a strong spatial correlation between young star clusters and GMCs such that all clusters still associated with a GMC are younger than 11 Myr and display a median age of 2 Myr. The age distribution increases gradually as the cluster–GMC distance increases, with star clusters that are spatially unassociated with molecular gas exhibiting a median age of 7 Myr. Thus, star clusters are able to emerge from their natal clouds long before the time-scale required for clouds to disperse. To investigate if the hierarchy observed in the stellar components is inherited from the GMCs, we quantify the amount of clustering in the spatial distributions of the components and find that the star clusters have a fractal dimension slope of −0.35 ± 0.03, significantly more clustered than the molecular cloud hierarchy with slope of −0.18 ± 0.04 over the range 40–800 pc. We find, however, that the spatial clustering becomes comparable in strength for GMCs and star clusters with slopes of −0.44 ± 0.03 and −0.45 ± 0.06, respectively, when we compare massive (>10^5 M_⊙) GMCs to massive and young star clusters. This shows that massive star clusters trace the same hierarchy as their parent GMCs, under the assumption that the star formation efficiency is a few per cent.

Additional Information

© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) Accepted 2018 August 6. Received 2018 August 2; in original form 2018 June 8, We are grateful for the valuable comments on this work by an anonymous referee that improved the scientific outcome and quality of the paper. The authors thank Stella Offner for helpful suggestions that improved the outcome of the scientific results. Based on observations made 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, under NASA Contract NAS 5-26555. These observations are associated with Program 13364 (LEGUS). Support for Program 13364 was provided by NASA through a grant from the Space Telescope Science Institute. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. This paper makes use of the following ALMA data: ADS/JAO.ALMA #2015.1.00782.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Parts of this research were supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in three Dimensions (ASTRO 3D), through project number CE170100013. AA acknowledges the support of the Swedish Research Council (Vetenskapsrådet) and the Swedish National Space Board (SNSB). MF acknowledges support by the Science and Technology Facilities Council [grant number ST/P000541/1]. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement number 757535). Software: ASTROPY (Astropy 2013), IPYTHON (Pérez & Granger 2007), MATPLOTLIB (Hunter 2007), NUMPY (van der Walt, Colbert & Varoquaux 2011), TREECORR (Jarvis, Bernstein & Jain 2004).

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