The Quenching of the Ultra-faint Dwarf Galaxies in the Reionization Era

Creators: Brown, Thomas M.; Tumlinson, Jason; Geha, Marla

Abstract

We present new constraints on the star formation histories of six ultra-faint dwarf galaxies: Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I. Our analysis employs a combination of high-precision photometry obtained with the Advanced Camera for Surveys on the Hubble Space Telescope, medium-resolution spectroscopy obtained with the DEep Imaging Multi-Object Spectrograph on the W. M. Keck Observatory, and updated Victoria-Regina isochrones tailored to the abundance patterns appropriate for these galaxies. The data for five of these Milky Way satellites are best fit by a star formation history where at least 75% of the stars formed by z ~ 10 (13.3 Gyr ago). All of the galaxies are consistent with 80% of the stars forming by z ~ 6 (12.8 Gyr ago) and 100% of the stars forming by z ~ 3 (11.6 Gyr ago). The similarly ancient populations of these galaxies support the hypothesis that star formation in the smallest dark-matter sub-halos was suppressed by a global outside influence, such as the reionization of the universe.

Additional Information

© 2014 American Astronomical Society. Received 2014 August 11; accepted 2014 October 2; published 2014 November 12. 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 Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We thank the anonymous referee who suggested revisions that improved the clarity of this work. We are grateful to P. Stetson for providing his DAOPHOT-II code and offering assistance with its use. D.A.V. acknowledges the support of a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada. L.C.V. was supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1122492. R.R.M. acknowledges partial support from CONICYT Anillo project ACT-1122 and project BASAL PFB-06, as well as FONDECYT project No.1120013. P.G. acknowledges support from NSF grant AST-1010039. 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, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO-12549.

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Submitted - 1410.0681v1.pdf

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