Detalles de publicación
PP 07026
The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy
(1) University of Texas, McDonald Observatory, 1 University Station, C1402, Austin TX, 78712
(2) Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755
(3) Dept. of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325
(4) Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL
(5) Department of Physics and Astronomy, Rice University MS-108, Houston, TX 77005
(6) Instituto de Astrof´?sica de Canarias, Via Lactea s/n, E-38200 La Laguna, Spain
(7) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218
(8) Dept. of Astronomy, Univ. of Washington, Box 351580, Seattle, WA 98195-1580
(9) Dipartimento di Astronomia, Universita di Padova, 35122 Padova, Italy
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 as part of the ACS Survey of Galactic Globular
Clusters. 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 usingboth 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]\sim0.6) 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.
cluster M54 (NGC 6715) and the superposed core of the tidally disrupted
Sagittarius (Sgr) dSph galaxy as part of the ACS Survey of Galactic Globular
Clusters. 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 usingboth 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]\sim0.6) 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.