The initial mass function in the extended ultraviolet disc of M83

Creators: Bruzzese, S. M.; Thilker, David A.; Meurer, G. R.; Bianchi, Luciana; Watts, A. B.; Ferguson, A. M. N.; Gil de Paz, A.; Madore, B.; Martin, D. Christopher; Rich, R. Michael

Abstract

Using Hubble Space Telescope ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations of four fields within the extended ultraviolet disc (XUV disc) of M83. These observations show a clumpy distribution of main-sequence stars and a mostly smooth distribution of red giant branch stars. We constrain the upper end of the initial mass function (IMF) in the outer disc using the detected population of main-sequence stars and an assumed constant star formation rate (SFR) over the last 300 Myr. By comparing the observed main-sequence luminosity function to simulations, we determine the best-fitting IMF to have a power-law slope α = −2.35 ± 0.3 and an upper mass limit M_u = 25⁺¹⁷₋₃M⊙⁠. This IMF is consistent with the observed H α emission, which we use to provide additional constraints on the IMF. We explore the influence of deviations from the constant SFR assumption, finding that our IMF conclusions are robust against all but strong recent variations in SFR, but these are excluded by causality arguments. These results, along with our similar studies of other nearby galaxies, indicate that some XUV discs are deficient in high-mass stars compared to a Kroupa IMF. There are over one hundred galaxies within 5 Mpc, many already observed with HST, thus allowing a more comprehensive investigation of the IMF, and how it varies, using the techniques developed here.

Additional Information

© 2019 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 2019 November 4. Received 2019 November 4; in original form 2019 April 2. Published: 22 November 2019. We thank the anonymous referee and the MNRAS editors for comments that improved the quality of this paper. This study is based in part 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 10608 (PI: D. Thilker). Support for Program 10608 was provided by NASA through a grant from the Space Telescope Science Institute. This study is based in part on observations made with the Galaxy Evolution Explorer (GALEX). GALEX is a NASA Small Explorer, launched in 2003 April. We gratefully acknowledge NASA's support for construction, operation, and science analysis for the GALEX mission, developed in cooperation with the Centre National d'Etudes Spatiales of France and the Korean Ministry of Science and Technology. This study is also based in part on observations at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. This research has used the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. SMB was supported by an Australian Postgraduate Award, and this study formed part of her Ph.D. thesis (Bruzzese 2016). CL is funded by a Discovery Early Career Research Award (DE150100618). AGdP acknowledges support from the Spanish MCIUN project AYA2016-75808-R. We thank Alessandro Bressan for assistance with the Padova and Trieste Stellar Evolution Code (PARSEC) stellar evolutionary tracks and George Heald, Frank Bigiel, Ed Elson, and Daniel Dale for providing data and helpful discussions. We thank the Galaxy Halos, Outer disks, Substructure, Thick disks, and Star clusters (GHOSTS) team for making their data public. We thank Jin Koda for sending us details on the clusters observed in Koda et al. (2012). We thank Samuel Boissier and Mark Seibert for contributing to our application for the Hubble Space Telescope observations presented here and comments that improved the quality of this paper. SMB and GRM thank the Johns Hopkins University for their hospitality and accommodation during several visits while they worked on this project.

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