Exploring the IMF of star clusters: a joint SLUG and LEGUS effort

Ashworth, G.; Fumagalli, M.; Krumholz, M.R.; Adamo, A.; Calzetti, D.; Chandar, R.; Cignoni, M.; Dale, D.; Elmegreen, B.G.; Gallagher, J.S.; Gouliermis, D.A.; Grasha, K.; Grebel, E.K.; Johnson, K.E.; Lee, J.; Tosi, M.; Wofford, A.

doi:10.1093/mnras/stx935

Exploring the IMF of star clusters: a joint SLUG and LEGUS effort

Ashworth, G.; Fumagalli, M.; Krumholz, M.R.; Adamo, A.; Calzetti, D.; Chandar, R.; Cignoni, M.; Dale, D.; Elmegreen, B.G.; Gallagher, J.S.; Gouliermis, D.A.; Grasha, K.; Grebel, E.K.; Johnson, K.E.; Lee, J.; Tosi, M.; Wofford, A.

Authors

G. Ashworth

Professor Michele Fumagalli michele.fumagalli@durham.ac.uk
Academic Visitor

M.R. Krumholz

A. Adamo

D. Calzetti

R. Chandar

M. Cignoni

D. Dale

B.G. Elmegreen

J.S. Gallagher

D.A. Gouliermis

K. Grasha

E.K. Grebel

K.E. Johnson

J. Lee

M. Tosi

A. Wofford

Abstract

We present the implementation of a Bayesian formalism within the Stochastically Lighting Up Galaxies (SLUG) stellar population synthesis code, which is designed to investigate variations in the initial mass function (IMF) of star clusters. By comparing observed cluster photometry to large libraries of clusters simulated with a continuously varying IMF, our formalism yields the posterior probability distribution function (PDF) of the cluster mass, age and extinction, jointly with the parameters describing the IMF. We apply this formalism to a sample of star clusters from the nearby galaxy NGC 628, for which broad-band photometry in five filters is available as part of the Legacy ExtraGalactic UV Survey (LEGUS). After allowing the upper-end slope of the IMF (α3) to vary, we recover PDFs for the mass, age and extinction that are broadly consistent with what is found when assuming an invariant Kroupa IMF. However, the posterior PDF for α3 is very broad due to a strong degeneracy with the cluster mass, and it is found to be sensitive to the choice of priors, particularly on the cluster mass. We find only a modest improvement in the constraining power of α3 when adding Hα photometry from the companion Hα-LEGUS survey. Conversely, Hα photometry significantly improves the age determination, reducing the frequency of multi-modal PDFs. With the aid of mock clusters, we quantify the degeneracy between physical parameters, showing how constraints on the cluster mass that are independent of photometry can be used to pin down the IMF properties of star clusters.

Citation

Ashworth, G., Fumagalli, M., Krumholz, M., Adamo, A., Calzetti, D., Chandar, R., …Wofford, A. (2017). Exploring the IMF of star clusters: a joint SLUG and LEGUS effort. Monthly Notices of the Royal Astronomical Society, 469(2), 2464-2480. https://doi.org/10.1093/mnras/stx935

Journal Article Type	Article
Acceptance Date	Apr 13, 2017
Online Publication Date	Apr 25, 2017
Publication Date	Aug 1, 2017
Deposit Date	Jun 19, 2017
Publicly Available Date	Jun 20, 2017
Journal	Monthly Notices of the Royal Astronomical Society
Print ISSN	0035-8711
Electronic ISSN	1365-2966
Publisher	Royal Astronomical Society
Peer Reviewed	Peer Reviewed
Volume	469
Issue	2
Pages	2464-2480
DOI	https://doi.org/10.1093/mnras/stx935

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This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.