Photometry of the Globular Cluster M54 and the Sagittarius Dwarf Galaxy: The Age-Metallicity Relation

We present deep VI-band photometry of the globular cluster M54, a nearby field in the Sagittarius dwarf galaxy, and a control field. The color-magnitude diagrams reach well below the oldest main-sequence turnoffs, thus enabling an analysis of the galaxy's age-metallicity relation with unprecedented clarity. We also study the variable stars in the direction of M54. From 67 RR Lyrae variables, we confirm and improve on our previous estimates of the cluster horizontal branch magnitude, foreground reddening, and horizontal branch morphology. These values are used in determining the ages of M54 and the Sagittarius field populations. We confirm our previous result that M54 is the same age as Galactic globular clusters of similar metallicity. We also derive ages on a self-consistent scale for the other three globular clusters in Sagittarius. We find strong evidence for multiple episodes of star formation (or continuous star formation with a variable rate) in the field of Sagittarius. We characterize the principal episodes with the ages 11, 5, and 0.5 through 3 Gyr and with [Fe/H] values of -1.3, -0.7, and -0.4, respectively. On this scale, M54 has an age of 15 Gyr. Surprisingly, the age-metallicity relation we have derived for the galaxy as a whole is described quite well by a closed-box chemical evolution model. We also find that the populations associated with the Sgr field are clumped spatially around M54, and we consider several explanations for this phenomenon. We again speculate that Sagittarius is a nucleated dwarf elliptical galaxy with M54 as its nucleus.

[1]  A. Ferrara,et al.  Starburst-driven Mass Loss from Dwarf Galaxies: Efficiency and Metal Ejection , 1998, astro-ph/9801237.

[2]  P. Stetson,et al.  Ccd photometry of the globular cluster m5. I. the color-magnitude diagram and luminosity functions , 1996, astro-ph/9605101.

[3]  Peter B. Stetson,et al.  Robust variable star detection techniques suitable for automated searches: new results for NGC 1866 , 1993 .

[4]  B. Chaboyer.  GLOBULAR CLUSTER DISTANCE DETERMINATIONS , 1998, astro-ph/9808202.

[5]  K. Stanek,et al.  Discovery of a Tidal Extension of the Sagittarius Dwarf Spheroidal Galaxy , 1996 .

[6]  The Contribution of Late-Type/Irregulars to the Faint Galaxy Counts from Hubble Space Telescope Medium-Deep Survey Images , 1995, astro-ph/9511123.

[7]  R. Zinn,et al.  The horizontal-branch stars in globular clusters. 2: The second parameter phenomenon , 1994 .

[8]  H. Morrison,et al.  Formation of the galactic halo .... inside and out : a meeting held in honor of the 65th birthday of George Preston, Tucson, Arizona, 9-11 October 1995 , 1996 .

[9]  C. Alard.  Evidence for the Sagittarius Dwarf Galaxy at Low Galactic Latitudes , 1996 .

[10]  H. Richer,et al.  Terzan 7: A young metal-rich globular cluster in the Milky Way , 1995 .

[11]  C. Sturch.  Intrinsic UBV colors of RR Lyrae stars , 1966 .

[12]  J. Nemec,et al.  Period-luminosity-metallicity relations, pulsation modes, absolute magnitudes, and distances for population 2 variable stars , 1994 .

[13]  Young-Wook Lee,et al.  Ages for Globular Clusters with Predominantly Red Horizontal Branches , 1995 .

[14]  D. Schlegel,et al.  Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds , 1997, astro-ph/9710327.

[15]  Horace A. Smith,et al.  RR Lyrae Stars , 1995 .

[16]  H. Richer,et al.  Young globular clusters in the Milky Way: ARP 2 , 1995 .

[17]  The Sagittarius Dwarf Galaxy Survey (SDGS) — II. The stellar content and constraints on the star formation history , 1999, astro-ph/9903371.

[18]  COLOR-MAGNITUDE DIAGRAM DISTRIBUTION OF THE BULGE RED CLUMP STARS : EVIDENCE FOR THE GALACTIC BAR , 1994, astro-ph/9404026.

[19]  L. Marschall,et al.  CCD photometry of the old open cluster M67 , 1993 .

[20]  The Sagittarius Dwarf Galaxy Survey (SDGS) — I. Colour—magnitude diagrams, reddening and population gradients. First evidence of a very metal-poor population , 1998, astro-ph/9812344.

[21]  E. Friel,et al.  Deep CCD Photometry of Old Open Clusters , 1997 .

[22]  A. Sarajedini,et al.  Technique for the Simultaneous Determination of Globular Clusters Metallicity and reddening Using (V,V-I) Color-Magnitude Diagrams , 1994 .

[23]  M. Mateo,et al.  Variable Stars in the Sextans Dwarf Spheroidal Galaxy , 1995 .

[24]  A. Sivaramakrishnan,et al.  The Main-Sequence Stars of the Sagittarius Dwarf Galaxy , 1996 .

[25]  K. Ashman,et al.  Some Revised Observational Constraints on the Formation and Evolution of the Galactic Disk , 1997, astro-ph/9709122.

[26]  A. Walker.  BVI CCD Photometry of Galactic Globular Clusters. II. M68 , 1994 .

[27]  M. Irwin,et al.  A dwarf satellite galaxy in Sagittarius , 1994, Nature.

[28]  A. Sarajedini,et al.  The Globular Cluster M54 and the Star Formation History of the Sagittarius Dwarf Galaxy , 1997, astro-ph/9706168.

[29]  B. Carney,et al.  BV PHOTOMETRY OF RR LYRAE VARIABLES IN THE GLOBULAR CLUSTER M92 , 1992 .

[30]  A. Sarajedini,et al.  Reddenings, Metallicities, and Possible Abundance Anomalies in Young Globular Clusters , 1997 .

[31]  Patrick R. Durrell,et al.  Surface Photometry of Virgo Dwarf Ellipticals , 1997 .

[32]  A. Sandage.  The Oosterhoff Period Groups and the Age of Globular Clusters - Part Four - Field Rr-Lyrae Stars - Age of the Galactic Disk , 1982 .

[33]  P. Demarque,et al.  The revised Yale isochrones and luminosity functions : containing both the theoretical and photometric UBVRI quantities with HR diagrams and luminosity function plots , 1987 .

[34]  A. Sarajedini.  WIYN Open Cluster Study. III. The Observed Variation of the Red Clump Luminosity and Color with Metallicity and Age , 1999 .

[35]  S. Bergh.  Stellar Content of Local Group Galaxies - An Introduction , 1999 .

[36]  J. Neill,et al.  Placing the Fornax and Sagittarius Dwarf Spheroidal Globular Clusters in the Horizontal-Branch Type versus Metallicity Diagram , 1998 .

[37]  I. Shelton,et al.  The RR Lyrae Period-Amplitude Relation as a Clueto the Origin of the Oosterhoff Dichotomy , 1999, astro-ph/9902289.

[38]  Mario Mateo,et al.  DWARF GALAXIES OF THE LOCAL GROUP , 1998, astro-ph/9810070.

[39]  B. Chaboyer,et al.  Globular Cluster Ages and the Formation of the Galactic Halo , 1995, astro-ph/9509063.

[40]  Gerard Gilmore,et al.  The Kinematics, Orbit, and Survival of the Sagittarius Dwarf Spheroidal Galaxy , 1997 .

[41]  The Variable Stars and Blue Horizontal Branch of the Metal-rich Globular Cluster NGC 6441 , 1998, astro-ph/9808353.

[42]  Jennifer A. Johnson,et al.  VI Photometry of Nearby Globular Clusters: M3, M5, M13, and M92 , 1998 .

[43]  STScI,et al.  The Specific Globular Cluster Frequencies of Dwarf Elliptical Galaxiesfrom the Hubble Space Telescope , 1998 .

[44]  A. Layden,et al.  A Photometric Study of the Globular Cluster M54 and the Sagittarius Dwarf Galaxy: Evidence for Three Distinct Populations , 1995 .

[45]  C. Heiles,et al.  Reddenings derived from H I and galaxy counts : accuracy and maps. , 1982 .

[46]  Allan Sandage,et al.  Temperature mass, and luminosity of RR Lyrae stars as functions of metallicity at the blue fundamental edge. II , 1993 .

[47]  D. Schlegel,et al.  Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds , 1998 .

[48]  S. Rucinski.  A SIMPLE DESCRIPTION OF LIGHT CURVES OF W UMA SYSTEMS , 1993 .

[49]  Scott Trager,et al.  Catalogue of Galactic globular-cluster surface-brightness profiles , 1995 .

[50]  Capture of Field Stars by Globular Clusters in Dense Bulge Regions , 1997, astro-ph/9703166.

[51]  Michael J. West,et al.  The globular cluster system of the Galaxy. III: measurements of radial velocity and metallicity for 60 clusters and a compilation of metallicities for 121 clusters , 1984 .

[52]  A. Layden.  RR Lyrae Variables in the Inner Halo. I. Photometry , 1998 .

[53]  William E. Harris,et al.  A Catalog of Parameters for Globular Clusters in the Milky Way , 1996 .

[54]  P. R. Warren,et al.  Reddenings of Cepheids using BVI photometry , 1978 .

[55]  R. Zinn,et al.  Compositions of halo clusters and the formation of the galactic halo , 1978 .

[56]  Taft E. Armandroff,et al.  Abundances and Kinematics of the Globular Cluster Systems of the Galaxy and of the Sagittarius Dwarf , 1995 .

[57]  P. Schechter,et al.  DOPHOT, A CCD PHOTOMETRY PROGRAM: DESCRIPTION AND TESTS , 1993 .