THE WFC3 GALACTIC BULGE TREASURY PROGRAM: A FIRST LOOK AT RESOLVED STELLAR POPULATION TOOLS

When the Wide Field Camera 3 (WFC3) is installed on the Hubble Space Telescope (HST), the astronomical community will have access to powerful new capabilities for investigating resolved stellar populations. The WFC3 Galactic Bulge Treasury program will obtain deep imaging in five photometric bands on four low-extinction fields. These data will have no proprietary period, and will enable a variety of science investigations not possible with previous data sets. To aid in planning for the use of these data and for future observing proposals, we provide an introduction to the Treasury program, its photometric system, and the associated calibration effort. The observing strategy is based upon a new photometric system employing five WFC3 bands spanning the UV, optical, and near-infrared: F390W, F555W, F814W, F110W, and F160W (analogous but not identical to the ground-based filters Washington C, V, I, J, and H). With these bands, one can construct reddening-free indices of temperature and metallicity. Using this photometric system, the program will target six fields in well studied star clusters, spanning a wide range of metallicity, and four fields in low-extinction windows of the Galactic bulge. The cluster data serve to calibrate the reddening-free indices, provide empirical population templates, and correct the transformation of theoretical isochrone libraries into the WFC3 photometric system. The bulge data will shed light on the bulge formation history, and will also serve as empirical population templates for other studies. One of the fields includes 12 candidate hosts of extrasolar planets. Color-magnitude diagrams (CMDs) are the most popular tool for analyzing resolved stellar populations. However, due to degeneracies among temperature, metallicity, and reddening in traditional CMDs, it can be difficult to draw robust conclusions from the data. The five-band system used for the bulge Treasury observations will provide reddening-free indices that are roughly orthogonal in temperature and metallicity, and we argue that model fitting in an index-index diagram will make better use of the information than fitting separate CMDs. We provide some results from simulations to show the expected quality of the data and their potential for differentiating between different star formation histories.

[1]  Extinction map of the Galactic centre: OGLE‐II Galactic bulge fields , 2003, astro-ph/0309206.

[2]  Shude Mao,et al.  Proper motion dispersions of red clump giants in the galactic bulge: observations and model comparisons , 2007, 0704.1619.

[3]  P. Jablonka,et al.  Stellar population gradients in bulges along the Hubble sequence - II. Relations with galaxy properties , 2007, 0707.0561.

[4]  David N. Spergel,et al.  Direct evidence for a bar at the Galactic center , 1991 .

[5]  S. Rabien,et al.  From Rings to Bulges: Evidence for Rapid Secular Galaxy Evolution at z ~ 2 from Integral Field Spectroscopy in the SINS Survey , 2008, 0807.1184.

[6]  Allan Sandage,et al.  The Oosterhoff period groups and the age of globular clusters. III. The age of the globular cluster system. , 1982 .

[7]  S. Cassisi,et al.  The Initial Mass Function of the Galactic Bulge down to ~0.15 M☉ , 1999, astro-ph/9906452.

[8]  M. Skrutskie,et al.  The Two Micron All Sky Survey (2MASS) , 2006 .

[9]  P. Stetson.  DAOPHOT: A COMPUTER PROGRAM FOR CROWDED-FIELD STELLAR PHOTOMETRY , 1987 .

[10]  P. Stetson,et al.  Measuring age differences among globular clusters having similar metallicities - A new method and first results , 1990 .

[11]  R. Wyse,et al.  Galactic Bulges , 1997, astro-ph/9701223.

[12]  M. S. Roberts.  Galactic astronomy. , 1981, Science.

[13]  D. Zaritsky,et al.  A Method for Determining the Star Formation History of a Mixed Stellar Population , 2001, astro-ph/0104202.

[14]  Laboratoire AIM,et al.  Bulge Formation by the Coalescence of Giant Clumps in Primordial Disk Galaxies , 2008, 0903.1937.

[15]  Edward L. Fitzpatrick,et al.  Correcting for the Effects of Interstellar Extinction , 1998, astro-ph/9809387.

[16]  J. Harris,et al.  The Star Formation History of the Small Magellanic Cloud , 2004 .

[17]  Formation and evolution of the Galactic bulge: constraints from stellar abundances , 2006, astro-ph/0611650.

[18]  J. Peacock,et al.  Simulations of the formation, evolution and clustering of galaxies and quasars , 2005, Nature.

[19]  Robert C. Kennicutt,et al.  Secular Evolution and the Formation of Pseudobulges in Disk Galaxies , 2004, astro-ph/0407343.

[20]  H. Ferguson,et al.  The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy* , 2006, astro-ph/0607637.

[21]  Leo A: A late-blooming survivor of the epoch of reionization in the local group , 2007, astro-ph/0702646.

[22]  Stefano Casertano,et al.  Transiting extrasolar planetary candidates in the Galactic bulge , 2006, Nature.

[23]  W. Freedman,et al.  The Star Formation History of the Local Group Dwarf Galaxy Leo I , 1998, astro-ph/9906121.

[24]  Samuel Harvey Moseley,et al.  Morphology, near infrared luminosity, and mass of the galactic bulge from Cobe dirbe observations , 1995 .

[25]  B. Strömgren,et al.  SPECTRAL CLASSIFICATION THROUGH PHOTOELECTRIC NARROW-BAND PHOTOMETRY. , 1966 .

[26]  I. Iben,et al.  Single star evolution I. Massive stars and early evolution of low and intermediate mass stars , 1984 .

[27]  William Wilson Morgan,et al.  Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas , 1953 .

[28]  L. Greggio,et al.  Age and Metallicity Distribution of the Galactic Bulge from Extensive Optical and Near-IR Stellar Photometry , 2003 .

[29]  S. Ortolani,et al.  The metal content of bulge field stars from FLAMES-GIRAFFE spectra - I. Stellar parameters and iron abundances , 2008, 0805.1218.

[30]  Don A. VandenBerg,et al.  The Victoria-Regina Stellar Models: Evolutionary Tracks and Isochrones for a Wide Range in Mass and Metallicity that Allow for Empirically Constrained Amounts of Convective Core Overshooting , 2006 .

[31]  W. Baade.  Evolution of stars and galaxies , 1963 .

[32]  C. J. Grillmair,et al.  Observations and Implications of the Star Formation History of the Large Magellanic Cloud , 1999 .

[33]  R. Rich,et al.  The First Detailed Abundances for M Giants in the Inner Bulge from Infrared Spectroscopy , 2007, 0707.1855.

[34]  J. Valenti,et al.  The Planet-Metallicity Correlation , 2005 .

[35]  Andrew E. Dolphin.  Numerical methods of star formation history measurement and applications to seven dwarf spheroidals , 2002 .

[36]  J. Silk,et al.  The formation history of the Galactic bulge , 2003, astro-ph/0308165.

[37]  G. Lucia,et al.  The hierarchical formation of the brightest cluster galaxies , 2006, astro-ph/0606519.

[38]  D. Lin,et al.  TIDAL DISSIPATION IN ROTATING SOLAR-TYPE STARS , 2007 .

[39]  A. Cimatti,et al.  The rapid formation of a large rotating disk galaxy three billion years after the Big Bang , 2006, Nature.

[40]  A. Cimatti,et al.  Dynamical Properties of z ~ 2 Star-forming Galaxies and a Universal Star Formation Relation , 2007, 0706.2656.

[41]  Puragra Guhathakurta,et al.  Stellar Cluster Fiducial Sequences with the Advanced Camera for Surveys , 2005 .

[42]  Antony A. Stark,et al.  Understanding the kinematics of Galactic Centre gas , 1991 .