The ACS survey of galactic globular clusters: M54 and young populations in the sagittarius dwarf spheroidal galaxy

Michael H. Siegel, Aaron Dotter, Steven R. Majewski, Ata Sarajedini, Brian Chaboyer, David L. Nidever, Jay Anderson, Antonio Marín-Franch, Alfred Rosenberg, Luigi R. Bedin, Antonio Aparicio, Ivan King, Giampaolo Piotto, I. Neill Reid

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Abstract

As part of the ACS Survey of Galactic Globular Clusters, we present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy. Our deep (F606W ∼ 26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. The distance and reddening to M54 are revised using both isochrone and main-sequence fitting to (m - M)0 = 17.27 and E(B - V) = 0.15. Preliminary assessment finds the M54+Sgr field to be dominated by the old metal-poor populations of Sgr and the globular cluster. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H] = -0.4 to -0.6. We also clearly show, for the first time, a prominent, ∼2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (∼0.1-0.8 Gyr old), more metal-rich ([Fe/H] ∼ 0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.

Original languageEnglish (US)
Pages (from-to)L57-L60
JournalAstrophysical Journal
Volume667
Issue number1
DOIs
StatePublished - Sep 20 2007

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Siegel, M. H., Dotter, A., Majewski, S. R., Sarajedini, A., Chaboyer, B., Nidever, D. L., Anderson, J., Marín-Franch, A., Rosenberg, A., Bedin, L. R., Aparicio, A., King, I., Piotto, G., & Reid, I. N. (2007). The ACS survey of galactic globular clusters: M54 and young populations in the sagittarius dwarf spheroidal galaxy. Astrophysical Journal, 667(1), L57-L60. https://doi.org/10.1086/522003