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Evidence of a Non-universal Stellar Initial Mass Function. Insights from HST Optical Imaging of Six Ultra-faint Dwarf Milky Way Satellites

Gennaro, Mario and Tchemyshyov, Kirill and Brown, Thomas M. and Geha, Marla and Avila, Roberto J. and Guhathakurta, Puragra and Kalirai, Jason S. and Kirby, Evan N. and Renzini, Alvio and Simon, Joshua D. and Tumlinson, Jason and Vargas, Luis C. (2018) Evidence of a Non-universal Stellar Initial Mass Function. Insights from HST Optical Imaging of Six Ultra-faint Dwarf Milky Way Satellites. Astrophysical Journal, 855 (1). Art. No. 20. ISSN 1538-4357. http://resolver.caltech.edu/CaltechAUTHORS:20180228-094019082

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Abstract

Using deep observations obtained with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST), we demonstrate that the sub-solar stellar initial mass function (IMF) of six ultra-faint dwarf Milky Way satellites (UFDs) is more bottom light than the IMF of the Milky Way disk. Our data have a lower-mass limit of ~0.45 M_⊙, while the upper limit is ~0.8 M_⊙, set by the turnoff mass of these old, metal-poor systems. If formulated as a single power law, we obtain a shallower IMF slope than the Salpeter value of −2.3, ranging from −1.01 for Leo IV to −1.87 for Boötes I. The significance of these deviations depends on the galaxy and is typically 95% or more. When modeled as a log-normal, the IMF fit results in a higher peak mass than in the Milky Way disk, but a Milky Way disk value for the characteristic system mass (~0.22 M_⊙) is excluded at only 68% significance, and only for some UFDs in the sample. We find that the IMF slope correlates well with the galaxy mean metallicity, and to a lesser degree, with the velocity dispersion and the total mass. The strength of the observed correlations is limited by shot noise in the number of observed stars, but future space-based missions like the James Webb Space Telescope (JWST) and the Wide-Field Infrared Survey Telescope ( WFIRST) will enhance both the number of dwarf Milky Way satellites that can be studied in such detail and the observation depth for individual galaxies.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.3847/1538-4357/aaa973DOIArticle
http://iopscience.iop.org/article/10.3847/1538-4357/aaa973/metaPublisherArticle
https://arxiv.org/abs/1801.06195arXivDiscussion Paper
ORCID:
AuthorORCID
Gennaro, Mario0000-0002-5581-2896
Brown, Thomas M.0000-0002-1793-9968
Geha, Marla0000-0002-7007-9725
Guhathakurta, Puragra0000-0001-8867-4234
Kirby, Evan N.0000-0001-6196-5162
Tumlinson, Jason0000-0002-7982-412X
Vargas, Luis C.0000-0001-7155-0040
Additional Information:© 2018 American Astronomical Society. Received 2017 December 12. Accepted 2018 January 18. Published 2018 February 28. We would like to thank Professor Don Vandenberg (University of Victoria, British Columbia, Canada) for providing the oxygen-enhanced models used in this work, as well as the Fortran routines used for their interpolation. Support for program GO-12549 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. This work was supported by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute under RSA number 1474359. Data presented herein were obtained at the W.M. Keck Observatory from telescope time allocated to NASA through the agency's scientific partnership with the California Institute of Technology and the University of California. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. E.N.K. acknowledges support from NSF grant AST-1614081.
Group:Infrared Processing and Analysis Center (IPAC)
Funders:
Funding AgencyGrant Number
NASANAS 5-26555
NASA1474359
W. M. Keck FoundationUNSPECIFIED
NSFAST-1614081
Subject Keywords:galaxies: dwarf; galaxies: stellar content; Local Group; methods: statistical; stars: luminosity function, mass function
Record Number:CaltechAUTHORS:20180228-094019082
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20180228-094019082
Official Citation:Mario Gennaro et al 2018 ApJ 855 20
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:84996
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:28 Feb 2018 17:58
Last Modified:28 Feb 2018 17:58

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